MODULE 1

1. Write a note on cell cycle ?

Cell division is a very importantprocess in all living organisms.

Duringthedivisionofacell,DNAreplicationandcellgrowthalsotakeplace.Allthese
processes,i.e.,celldivision,DNAreplication,andcellgrowth,hence,have to take
place in a coordinated way to ensure correct division
andformationofprogenycellscontainingintactgenomes.Thesequenceofevents by
which a cell duplicates its genome, synthesises the
otherconstituentsofthecellandeventuallydividesintotwodaughtercellsistermed
cell cycle. Although cell growth (in terms of cytoplasmic increase)is a continuous
process, DNA synthesis occurs only during one specificstage in the cell cycle.
The replicated chromosomes (DNA) are
thendistributedtodaughternucleibyacomplexseriesofeventsduringcelldivision.T
heseeventsarethemselvesundergeneticcontrol.
Phases of cell cycle
  In both unicellular and multicellular eukaryotes, the cell reproduction is a cyclic process
of growth, nuclear division and usually cytoplasmic division called cell cycle.
 Cell cycle is a series of macro-molecular events that lead to cell division and the
production of two daughter cells, each containing chromosomes identical with
those of the parental cell.
 Two main molecular processes take place during the cell cycle are duplication of parental
chromosome during S phase and separation of chromosome equally to daughter cell
during M phase.
 In somatic cell, the cell cycle consists of following four phase;
 G1 (gap 1) phase
 S (synthesis) phase
 G2 (gap 2) phase
 M (mitosis) phase
1. G1 (gap1) phase:
 The first stage of interphase is called the G1 phase (first gap) because, from a
microscopic aspect, little change is visible. However, during the G1 stage, the cell is
quite active at the biochemical level.
 It is characterized by a change in chromosome from condensed state to more extended
state and series of metabolic events that leads to initiation of DNA replication. During
G1 phase, chromatin fibres become slender, less coiled and fully extended and more
active for transcription. The transcription results in synthesis of RNAs (tRNA, mRNA
and rRNA) ad series of proteins molecules required for initiation of DNA replication.
 The length of G1 phase varies from cell to cell and also the length of G1 phase is more
than other three phase in cell cycle.
 G1 phase represents 25-40% of generation time of a cell.
 G1 phase is very significant phase of cell cycle as the cell grows and accumulates the
building blocks of chromosomal DNA and the associated proteins as well as sufficient
energy reserves to complete the task of replicating each chromosome.
 Within G1 phase there is a definite check point at which DNA synthesis is initiated and
once the biochemical events associated with that point have occurred cell proceeds
towards division.
2. S (synthesis) phase:
 The synthesis phase of interphase is biochemically a phase of active DNA synthesis and
histone synthesis.
  In the S phase, chromosome numbers doubles which is accomplished by DNA
replication and associated proteins. Although some of the histone protein synthesis
occurs in G1 phase, most of it is synthesized during S phase.
 DNA replication is semi conservative and discontinuous type which results in the
formation of identical pairs of DNA molecules.
 After doubling of chromosome, sister chromatids are still firmly attached to the
centromeric region.
 At the center of each animal cell, the centrosomes of animal cells are associated with a
pair of rod-like objects, the centrioles, which are at right angles to each other. Centrioles
help organize cell division. Centrioles are absent in plants and most fungi.
 The centrosome (centriole) is also duplicated during the S phase. The two centrosomes
will give rise to the mitotic spindle, the apparatus that mediate the movement of
chromosomes during mitosis.
3. Gap2 (gap2) phase:
 G2 phase follows S phase. This phase represents 10-25% of generation time of cell.
 In G2 phase chromosome consists of two chromatids ie the cell has twice the amount of
DNA content.
 In the G2 phase, the cell restore its energy stores and synthesizes proteins necessary for
chromosome manipulation.
 Some cell organelles are duplicated, and the cytoskeleton is dismantled to provide
resources for the mitotic phase.
 There may be additional cell growth during G2. The final preparations for the mitotic
phase must be completed before the cell is able to enter the first stage of mitosis
4. M (mitotic) phase:
 M phase follows G2 phase. During this phase cell divides into two daughter cell with
equal distribution of chromosome among daughter cells. After M phase cell enter into
G1 phase and next cell cycle is repeated. However, some cell after completion of mitosis
do not enter into G1 phase, those cell are referred as G0 cells.
 M phase consists of following sub –phases;
 During prophase, the nuclear membrane disappears, spindle fibers form, and
DNA condenses into chromosomes (sister chromatids ).
 During metaphase, the sister chromatids align along the equator of the cell by
attaching their centromeres to the spindle fibers.
  During anaphase, sister chromatids are separated at the centromere and are pulled
towards opposite poles of the cell by the mitotic spindle.
 During telophase, chromosomes arrive at opposite poles and unwind into thin
strands of DNA, the spindle fibers disappear, and the nuclear membrane reappears.
 Cytokinesis is the actual splitting of the cell membrane; animal cells pinch apart, while
plant cells form a cell plate that becomes the new cell wall.
 Cells enter the G0 (inactive) phase after they exit the cell cycle when they are not
actively preparing to divide; some cells remain in G0 phase permanently.
2. Write about mitosis?
Mitosis is a type of cell division in which one cell (the mother) divides to produce two new
cells (the daughters) that are genetically identical to itself. In the context of the cell cycle,
mitosis is the part of the division process in which the DNA of the cell's nucleus is split into
two equal sets of chromosomes.
The great majority of the cell divisions that happen in your body involve mitosis. During
development and growth, mitosis populates an organism’s body with cells, and throughout an
organism’s life, it replaces old, worn-out cells with new ones. For single-celled eukaryotes like
yeast, mitotic divisions are actually a form of reproduction, adding new individuals to the
population.
In all of these cases, the “goal” of mitosis is to make sure that each daughter cell gets a perfect,
full set of chromosomes. Cells with too few or too many chromosomes usually don’t function
well: they may not survive, or they may even cause cancer. So, when cells undergo mitosis,
they don’t just divide their DNA at random and toss it into piles for the two daughter cells.
Instead, they split up their duplicated chromosomes in a carefully organized series of steps.
Phases of mitosis
Mitosis consists of four basic phases: prophase, metaphase, anaphase, and telophase. Some
textbooks list five, breaking prophase into an early phase (called prophase) and a late phase
(called prometaphase). These phases occur in strict sequential order, and cytokinesis - the
process of dividing the cell contents to make two new cells - starts in anaphase or telophase.
This cell is in interphase (late G22start subscript, 2, end subscript phase) and has already copied
its DNA, so the chromosomes in the nucleus each consist of two connected copies, called sister
chromatids. You can’t see the chromosomes very clearly at this point, because they are still in
their long, stringy, decondensed form.
This animal cell has also made a copy of its centrosome, an organelle that will play a key role
in orchestrating mitosis, so there are two centrosomes. (Plant cells generally don’t have
centrosomes with centrioles, but have a different type of microtubule organizing center that
plays a similar role.)
 In early prophase, the cell starts to break down some structures and build others up, setting the
stage for division of the chromosomes.
 The chromosomes start to condense (making them easier to pull apart later on).
 The mitotic spindle begins to form. The spindle is a structure made of microtubules, strong
fibers that are part of the cell’s “skeleton.” Its job is to organize the chromosomes and move
them around during mitosis. The spindle grows between the centrosomes as they move apart.
 The nucleolus (or nucleoli, plural), a part of the nucleus where ribosomes are made,
disappears. This is a sign that the nucleus is getting ready to break down.
In late prophase (sometimes also called prometaphase), the mitotic spindle begins to capture
and organize the chromosomes.
 The chromosomes become even more condensed, so they are very compact.
 The nuclear envelope breaks down, releasing the chromosomes.
 The mitotic spindle grows more, and some of the microtubules start to “capture” chromosomes.

In late prophase (sometimes also called prometaphase), the mitotic spindle begins to capture
and organize the chromosomes.
 The chromosomes become even more condensed, so they are very compact.
 The nuclear envelope breaks down, releasing the chromosomes.
 The mitotic spindle grows more, and some of the microtubules start to “capture” chromosomes.
 Microtubules can bind to chromosomes at the kinetochore, a patch of protein found on the
centromere of each sister chromatid. (Centromeres are the regions of DNA where the sister
chromatids are most tightly connected.)
Microtubules that bind a chromosome are called kinetochore microtubules. Microtubules that
don’t bind to kinetochores can grab on to microtubules from the opposite pole, stabilizing the
spindle. More microtubules extend from each centrosome towards the edge of the cell, forming
a structure called the aster.

In metaphase, the spindle has captured all the chromosomes and lined them up at the middle
of the cell, ready to divide.
 All the chromosomes align at the metaphase plate (not a physical structure, just a term for the
plane where the chromosomes line up).
 At this stage, the two kinetochores of each chromosome should be attached to microtubules
from opposite spindle poles.
Before proceeding to anaphase, the cell will check to make sure that all the chromosomes are
at the metaphase plate with their kinetochores correctly attached to microtubules. This is called
the spindle checkpoint and helps ensure that the sister chromatids will split evenly between
the two daughter cells when they separate in the next step. If a chromosome is not properly
aligned or attached, the cell will halt division until the problem is fixed.

In anaphase, the sister chromatids separate from each other and are pulled towards opposite
ends of the cell.
 The protein “glue” that holds the sister chromatids together is broken down, allowing them to
separate. Each is now its own chromosome. The chromosomes of each pair are pulled towards
opposite ends of the cell.
 Microtubules not attached to chromosomes elongate and push apart, separating the poles and
making the cell longer.
 In telophase, the cell is nearly done dividing, and it starts to re-establish its normal structures
as cytokinesis (division of the cell contents) takes place.
 The mitotic spindle is broken down into its building blocks.
 Two new nuclei form, one for each set of chromosomes. Nuclear membranes and nucleoli
reappear.
 Cytokinesis, the division of the cytoplasm to form two new cells, overlaps with the
final stages of mitosis. It may start in either anaphase or telophase, depending on the
cell, and finishes shortly after telophase.
 In animal cells, cytokinesis is contractile, pinching the cell in two like a coin purse with
a drawstring. The “drawstring” is a band of filaments made of a protein called actin,
and the pinch crease is known as the cleavage furrow. Plant cells can’t be divided like
this because they have a cell wall and are too stiff. Instead, a structure called the cell
plate forms down the middle of the cell, splitting it into two daughter cells separated
by a new wall.
When cytokinesis finishes, we end up with two new cells, each with a complete set of
chromosomes identical to those of the mother cell. The daughter cells can now begin their own
cellular “lives,” and – depending on what they decide to be when they grow up – may undergo
mitosis themselves, repeating the cycle.
3. Write about meiosis?
Mitosis is used for almost all of your body’s cell division needs. It adds new cells during
development and replaces old and worn-out cells throughout your life. The goal of mitosis
is to produce daughter cells that are genetically identical to their mothers, with not a single
chromosome more or less.
Meiosis, on the other hand, is used for just one purpose in the human body: the production
of gametes—sex cells, or sperm and eggs. Its goal is to make daughter cells with exactly
half as many chromosomes as the starting cell.
To put that another way, meiosis in humans is a division process that takes us from a
diploid cell—one with two sets of chromosomes—to haploid cells—ones with a single set
 of chromosomes. In humans, the haploid cells made in meiosis are sperm and eggs. When
a sperm and an egg join in fertilization, the two haploid sets of chromosomes form a
complete diploid set: a new genome.
Phases of meiosis
In many ways, meiosis is a lot like mitosis. The cell goes through similar stages and uses similar
strategies to organize and separate chromosomes. In meiosis, however, the cell has a more
complex task. It still needs to separate sister chromatids (the two halves of a duplicated
chromosome), as in mitosis. But it must also separate homologous chromosomes, the similar
but nonidentical chromosome pairs an organism receives from its two parents.
These goals are accomplished in meiosis using a two-step division process. Homologue
pairs separate during a first round of cell division, called meiosis I. Sister chromatids
separate during a second round, called meiosis II.
Since cell division occurs twice during meiosis, one starting cell can produce four gametes
(eggs or sperm). In each round of division, cells go through four stages: prophase,
metaphase, anaphase, and telophase.
Meiosis I
Before entering meiosis I, a cell must first go through interphase. As in mitosis, the cell
grows during G11start subscript, 1, end subscript phase, copies all of its chromosomes
during S phase, and prepares for division during G22start subscript, 2, end subscript phase.
During prophase I, differences from mitosis begin to appear. As in mitosis, the
chromosomes begin to condense, but in meiosis I, they also pair up. Each chromosome
carefully aligns with its homologue partner so that the two match up at corresponding
positions along their full length.
For instance, in the image below, the letters A, B, and C represent genes found at particular
spots on the chromosome, with capital and lowercase letters for different forms, or alleles,
of each gene. The DNA is broken at the same spot on each homologue—here, between
genes B and C—and reconnected in a criss-cross pattern so that the homologues exchange
part of their DNA.
 This process, in which homologous chromosomes trade parts, is called crossing over. It's
helped along by a protein structure called the synaptonemal complex that holds the
homologues together. The chromosomes would actually be positioned one on top of the
other—as in the image below—throughout crossing over; they're only shown side-by-side
in the image above so that it's easier to see the exchange of genetic material.

chiasmata, cross-shaped structures where homologues are linked together. Chiasmata keep the
homologues connected to each other after the synaptonemal complex breaks down, so each
homologous pair needs at least one.
The spots where crossovers happen are more or less random, leading to the formation of new,
"remixed" chromosomes with unique combinations of alleles.
After crossing over, the spindle begins to capture chromosomes and move them towards the
center of the cell (metaphase plate). This may seem familiar from mitosis, but there is a twist.
Each chromosome attaches to microtubules from just one pole of the spindle, and the two
homologues of a pair bind to microtubules from opposite poles. So, during metaphase I,
homologue pairs—not individual chromosomes—line up at the metaphase plate for separation.

When the homologous pairs line up at the metaphase plate, the orientation of each pair is
random. For instance, in the diagram above, the pink version of the big chromosome and the
purple version of the little chromosome happen to be positioned towards the same pole and go
into the same cell. But the orientation could have equally well been flipped, so that both purple
chromosomes went into the cell together. This allows for the formation of gametes with
different sets of homologues.
In anaphase I, the homologues are pulled apart and move apart to opposite ends of the cell.
The sister chromatids of each chromosome, however, remain attached to one another and don't
come apart.
Finally, in telophase I, the chromosomes arrive at opposite poles of the cell. In some
organisms, the nuclear membrane re-forms and the chromosomes decondense, although in
others, this step is skipped—since cells will soon go through another round of division, meiosis
II2,32,3start superscript, 2, comma, 3, end superscript. Cytokinesis usually occurs at the same
time as telophase I, forming two haploid daughter cells.
Meiosis II
Cells move from meiosis I to meiosis II without copying their DNA. Meiosis II is a shorter and
simpler process than meiosis I, and you may find it helpful to think of meiosis II as “mitosis
for haploid cells."
The cells that enter meiosis II are the ones made in meiosis I. These cells are haploid—have
just one chromosome from each homologue pair—but their chromosomes still consist of two
sister chromatids. In meiosis II, the sister chromatids separate, making haploid cells with non-
duplicated chromosomes.

During prophase II, chromosomes condense and the nuclear envelope breaks down, if needed.
The centrosomes move apart, the spindle forms between them, and the spindle microtubules
begin to capture chromosomes.
 The two sister chromatids of each chromosome are captured by microtubules from opposite
spindle poles. In metaphase II, the chromosomes line up individually along the metaphase
plate. In anaphase II, the sister chromatids separate and are pulled towards opposite poles of
the cell.
In telophase II, nuclear membranes form around each set of chromosomes, and the
chromosomes decondense. Cytokinesis splits the chromosome sets into new cells, forming the
final products of meiosis: four haploid cells in which each chromosome has just one chromatid.
In humans, the products of meiosis are sperm or egg cells.
How meiosis "mixes and matches" genes
The gametes produced in meiosis are all haploid, but they're not genetically identical. For
example, take a look the meiosis II diagram above, which shows the products of meiosis for a
cell with 2�=42n=42, n, equals, 4 chromosomes. Each gamete has a unique "sample" of the
genetic material present in the starting cell.
As it turns out, there are many more potential gamete types than just the four shown in the
diagram, even for a cell with only four chromosomes. The two main reasons we can get many
genetically different gametes are:
 Crossing over. The points where homologues cross over and exchange genetic material are
chosen more or less at random, and they will be different in each cell that goes through meiosis.
If meiosis happens many times, as in humans, crossovers will happen at many different points.
 Random orientation of homologue pairs. The random orientation of homologue pairs in
metaphase I allows for the production of gametes with many different assortments of
homologous chromosomes.
4. Write the differences between prokaryotic cell and eukaryotic cell?

Prokaryotic cells Eukaryotic cells

Eukaryotic cells are present as either unicellular or

This cells are always unicellular
multicellular.

The size of cell is generally range from 0.2 Eukaryotic cells range from 10 to 100 micrometers
micrometers to 2.0 micrometers in diameter in diameter.

In prokaryotic cells, the cell wall is present and it Eukaryotic cells have cell walls very rarely, if
is very complex in nature. present they have simple chemical nature.
 In this cells true nucleus absent, instead
True nucleus is present.
nucleotide is present

DNA is arranged in circular shape DNA is linear in shape

In prokaryotic cells, cytoplasm is present, but it is In eukaryotic cells, it consists of both cytoplasm
lacking in most cell organelles. and organelles, both are present.

Mitochondria is present and it is a powerhouse of

Mitochondria is absent
cells.

Ribosomes are present, and they are small in size Ribosomes are present but they are comparatively
and shape is spherical large and linear in shape.

Endoplasmic reticulum and lysosomes and Endoplasmic reticulum and lysosomes and
centromere’s all are absent centromere’s all are present.

Plasmids are commonly found in prokaryotes. Plasmids are very rarely found in eukaryotes

Cell division occur through binary fission Cell division occur through mitosis

Flagella is small in size If flagella are large in size.

In this cells only asexual reproduction occurs. Both sexual and asexual reproduction occurs.

Bacteria and Archaea are examples. Plant and animal cells are examples.

5. Write the differences between animal cell and plant cell?

Plant cell Animal cell

1. A plant cell is surrounded by a rigid

1. An animal cell does not have a cell wall.
cell wall.

2. Presence of a large vacuole is seen in 2. Whereas there are very small vacuoles as compared to plant cells
plant cells. are seen in animal cells.

3. Larger in size. 3. Smaller in size.

4. Plant cells have plastids. 4. Animal cells do not have plastids.

5. Centrosomes are absent in plant cells 5. Animal cells have centrosomes.

6. Plant cells do not have cilia. 6. Animal cells have cilia.

7. Lysosomes are very rare in plant cells. 7. Animal cells have lysosomes.

6. Write a note on cell structure and different types of cells ?

Types of Cells - Prokaryotic Cell and Eukaryotic Cell
Cells, which are the basic building blocks of all living species, are the basic building blocks of
all living organisms. The human body has thousands of cells. They help support the body's
structure, absorb nutrition, convert them to energy, and perform specialized functions. Cells
also contain the body's genetic material and have the potential to multiply.
Cells are made up of various portions that have a specific function. Organelles, for example,
are specialized structures within the cell that perform specific functions. Cells exist in various
sizes, shapes, and types in the human body. The concept of a "generalized cell" is proposed for
descriptive purposes. It incorporates characteristics from a variety of cells. The cell membrane,
the nucleus, and the cytoplasm, which sit in between the two, make up a cell. Dozens, if not
thousands, of tiny yet distinct structures known as organelles can be found in the cytoplasm.
Cytoplasm
It aids biochemical processes by acting as a catalyst. It acts as a platform for the cell's other
organelles to function. A cell's cytoplasm is responsible for all aspects of cell proliferation,
growth, and replication. Diffusion is a physical phenomenon that permits objects to move
across tiny distances within the cytoplasm.
Cytoskeleton
The cytoskeleton is a system of long fibers that sustain the cell's structural integrity. The
cytoskeleton is responsible for various functions, including establishing cell shape, facilitating
cell division, and allowing cells to move. It also features a track-like system for controlling the
movement of organelles and other substances within cells.
The cytoplasm contains organelles.
cytoplasmic organelles are "mini organelles" that float around in the cell's cytoplasm.
Organelles each have their own structure and function in the cell's operation. Organelles are,
by definition, membrane-bound structures in a cell. The nucleus is a nice example.

Types of Cells
Cells are categorized into two types – prokaryotic cell and eukaryotic cell depending on the
presence or absence of a true nucleus in the cell.

A usual cell contains cytoplasm which is surrounded by a thin membrane known as the cell
membrane. The key function of a cell membrane is to protect the constituents of the cell from
the outside environment. Selective materials are only permitted to enter the cell through the
cell membrane. It involves other cell organelles like mitochondria, nucleus, etc.

Cells are joined together to form tissues, tissues form organs, organs form organ systems such
as the circulatory system, digestive system, central nervous system, etc. They combine together
to form different forms of life in different shapes and sizes. Most of the organisms are
multicellular like humans. There are unicellular organisms like bacteria and protozoa and
multicellular organisms like human beings.
Cell Structure
There are several cells in an individual, and the different kinds of cells include - prokaryotic
cells, plant and animal cells. The size and the shape of the cell vary from millimeter to microns,
which are usually based on the type of function that it performs. Cells usually vary in their
shapes. They could either be rod-shaped, flat, curved, concave, spherical, rectangular, oval, etc.
These cells will only be visible under a microscope.

Prokaryotic Cell Structure

They are the first organisms to be existing on our planet earth. Organisms with this cell type,
are called prokaryotic organisms (or) prokaryotes. Prokaryotic cells are single-celled
organisms, with the deficiency of nucleus and comprise of a capsule, cytoplasm, cell wall, cell
membrane, ribosome, nucleoid, plasmids, pili, and flagella.

Prokaryotic Cell General Features

 The size of a cell varies from 1-10 microns. Few prokaryotic cells vary in their size.
 They are unicellular, which forms a colony.
 The shape of the cell contains rod spherical and flat-shaped organisms.
 Mode of nutrition - A few organisms are photosynthetic, and get nourished on living
things and dead things.
 They reproduce asexually by binary fission, transformation, conjugation, transduction.
Structure and Functions of a Prokaryotic Cell
Capsule: It is the greasy outer coating of the cell wall. The key function of the capsule is to
protect the cell from getting dry and also helps in protecting cells from outside pressures.

Cell Wall: It is the stronger and rigid structure, which provides the shape and protects the
inside organelles of a cell. It is the middle layer, which lies between the capsule and the cell
membrane.

Cell Membrane: It is the inner delicate structure, which plays an important role in regulating
the entry and exit of any materials in the cell. It acts as a permeable membrane. It is of about
5-10nm in width, which helps in the oozing of proteins and elimination of waste products.

Cytoplasm: It is the viscous membrane, which is lying in between the cell membrane and
nucleoid.

Nucleoid: It is the cytoplasm region covering genetic material. The DNA of a prokaryotic
organism is one big loop, which is situated inside the nucleoid. It plays an important role in
cell division.

Ribosome: It consists of both RNA and proteins. It supports protein synthesis in the cell. They
are the tiniest membrane present inside the cytoplasm.

Plasmids: They are the minute membrane of a cell with double-stranded DNA. Plasmids are
hardly present in prokaryotic organisms.

Pili: It is the thinnest tissue of a prokaryotic cell. They consist of a protein complex named
pilin and are mostly involved in sticking to the objects particularly during sexual reproduction.

Flagella: It is a membrane that is in a helical shape, and its sizes vary from 19-20nm in diameter
and play an important role in the mobility of an organism from one place to another place.

Eukaryotic Cell Structure

They are cells with a true nucleus. Organisms, with this cell kind, are identified by the term
eukaryotes. Animals, plants, and other organisms apart from bacteria, blue-green algae, and E.
coli have been grouped into this group. Eukaryotic cells are more complex in comparison to
prokaryotic cells. These organisms contain a membrane-bound nucleus with many cell
organelles to make several cellular functions within the system.

Eukaryotic Cell General Features

The size of a eukaryotic cell varies from 10-100 microns.
 They are multicellular and have membrane-bound organelles.
 They reproduce either sexually or asexually.
 Type of nutrition - Autotrophic and heterotrophic
 KingdomsProtozoa, Fungi, Algae, Plantae, and Animalia are eukaryotic organisms.

Structure and Functions of a Eukaryotic Cell

Plasma Membrane: They are semi-permeable membranes that behave as a boundary of a cell,
which protects and splits up the cell from the outside environment.

Nucleus: It is a double-layered wall of Phospholipid bilayer. They are the stockroom for the
cell’s genetic materials in the form of DNA and keep all the essential information, which are
vital for a cell to control.

Nuclear Membrane: It is the double membrane layer that surrounds the nucleus and it plays
a part in the entry and exit of resources within the nucleus.

Nucleolus: It is the non-membrane-bound organelle, which is located within the nucleus.

Mitochondria: They are the double smooth membrane, which is in all eukaryotic cells. They
are the powerhouse of the cell. It plays a vital part in the synthesis of ATP and transforms
glucose into ATP.

Endoplasmic Reticulum: Double membrane organelle, which splits the cell into sections. It
is linked to the nuclear membrane of the cell. It plays an important role in protein synthesis,
biosynthesis of lipids and steroids, stores and controls calcium, and digestion of carbohydrates.
The endoplasmic reticulum is of two type’s rough and smooth Endoplasmic reticula.
Ribosome: It is in the cytoplasm. They are the spot for cells protein synthesis, which is made
of ribosomal RNA and proteins.

Golgi Bodies: It is the compacted membrane, which is mostly used to preserve the materials
made by the cell. This membrane also helps in storing, carrying materials within the cell.
Therefore, it is also named the post office of a cell.

Lysosomes: They are membrane-bound organelles, which have digestive enzymes to break
down macromolecules. Lysosome plays a vital part in protecting the cell by destroying foreign
bodies entering the cell.

Cytoplasm: Jelly types of organelles, which are in the inner region of a cell. It plays a vital
part in keeping a cell stable and keeps the cell organelles distinct from each other.

Chromosomes: The rod-shaped structures, which are made of proteins and DNA.
Chromosomes also play a vital part in determining the sex of an individual.

Plant Cell Structure

Plant cells are eukaryotic cells, with a true nucleus, multicellular large, and progressive
membrane-bound organelles. These plant cells are relatively different from animal cells in
shape and other several organelles which are only found in animal cells but are absent in plant
cells. Depending on structure and functions, plant cells comprise of:
 Cell Wall: It is the outer layer of a plant cell, which helps in providing the form and
strength to the complete plant. A cell wall is made of cellulose that protects and helps
the plant to grow.
 Cell Membrane: A biological membrane that divides living cell organelles from
nonliving structures. This membrane plays a vital part in helping a cell to communicate
with the outside environment and in carrying proteins and other molecules throughout
the cell.
 Chloroplasts: Green-colored oval-shaped double-membrane organelles, which are the
spots of photosynthesis. The green-colored pigment (chlorophyll) located in the leaves
helps plants in absorbing solar energy to make food.
 Golgi Bodies (or) Golgi Complex: The sac-like structures, which are in a cell to
manufacture, store, packing, and shipping the substance throughout the cell.
  Mitochondria: Mitochondria play an important part in liberating energy and they are
the powerhouse of a cell. The metabolic processes of the cell are referred to as cellular
respiration. Many of the actions that occur during aerobic respiration occur in the
mitochondria. Mitochondria are the cell's energy-producing organelles, which keep it
alive. Mitochondria are little organelles that float freely within cells. They're digestive
organelles that take in nutrients, break them down, and transform them into energy-rich
molecules for the cell. There are dozens of mitochondria in specific cells, whereas in
others, there are none.
 Vacuole: They are the fluid bags, which are in great numbers in plant cells. The chief
function is this membrane is to preserve food and other waste substances. Vacuoles are
the biggest organelles present in plant cells.

Animal Cell Structure

Animal cells are eukaryotic cells, with the existence of a true nucleus; multicellular big and
advanced membrane-bound organelles. Similarly, like plant cells, animal cells have similar
organelles except for the cell wall, chloroplasts, number of vacuoles, and many more. Due to
the deficiency of the cell wall, the shape of an animal cell is uneven. Depending on structure
and functions, animal cells comprise of:
 Cell Membrane: They are semi-permeable membranes covering the cell. It supports
holding the cell together and controls the entry and exit of nutrients into the cell.
 Vacuole: They are the fluid bags, which are in fewer numbers in animal cells as
compared to plant cells. The key function is this membrane is to preserve food and other
waste substance.
 Golgi Bodies (or) Golgi Complex: The bag-like assemblies, which are existing in a
cell to manufacture, storage, packing and transport the selected particles throughout the
cell.
 Ribosome: It is situated in the cytoplasm. They are the place for protein synthesis,
which is made of ribosomal RNA and proteins.
 Mitochondria: They are bar-shaped organelles, play an important part in liberating
energy, and are known as the powerhouse of a cell.
7. What is Plant Tissues and describe them?
On this page we will be discussing Plant Tissues, its Types, Properties, Examples and
Classifications. But first learning about Plant Tissues in depth, let us talk about Cell and
Tissues.
Cell: The basic structural and functional units of life is called a Cell

Tissue: A group of cells performing the same function and are similar in structure is called
Tissues.

Plant Cell
Plants do not walk or move drastically; they are immobile. This is why they are given tissues
that are built from dead cells; these cells provide the structural strength.

These plants face serious natural challenges like cyclones, floods, and strong winds.

Types of Tissues
We can classify these tissues into two parts, Meristematic Tissues, and Permanent Tissues.

Meristematic Tissues
These tissues can further develop and have cells that keep on dividing.

These cell tissues exist in different regions of a plant structure. Based on the place where they
are situated, we can classify the meristematic tissues into three kinds:

 Lateral Meristem: This is found in the radial fractions or a root. Lateral Meristem
helps in developing a thick solid structure in a plant.
 Intercalary Meristem: it is found at the base or the internodes of a leaf structure
present in plants and trees. This meristem helps in strengthening the internode's size.
 Apical Meristem: We can find this on the tips of growing roots and stems; this helps
in the lengthening of a plant.

Permanent Tissues
The tissues whose cells cannot distribute themselves anymore but still can facilitate being
protective, strengthening, and flexible to a plant or a tree are called permanent tissues.
We can classify these tissues into two different types, Simple Permanent Tissue and Complex
Permanent Tissue.

Simple Permanent Tissue

Sclerenchyma: The tissues are dead and elongated and have lignin deposits on their cell wall.
They are found in the seed coverings, nuts, veins of leaves, vascular tissues of stems and
provide strength to a plant. They don't contain any intercellular gaps.

Collenchyma: These tissues are made up of pectin and cellulose and are intercellular living
cells with minuscule gaps between their structure. We can find them in the borderline regions
of the stems and leaves; they offer flexibility to plants by providing them a structural
framework and mechanical support.

Parenchyma: These tissues also contain living cells that are polygonal in shape with a large
central vacuole. They have intercellular spaces between them. They are the developers of the
pith and ground tissue in a plant. Their structure includes:

 Chloroplasts that are known as chlorenchyma help a plant in the photosynthesis process.
 Aerenchyma consists of huge air gaps which provide buoyancy to the plant.
 Some cells also act like storage cells for the plant where they store starch for the fruits
and vegetables.

Complex Permanent Tissue

These tissues include:

Phloem: This tissue is the primary reason for proper flow of food throughout the plant. Xylem:
This tissue is the primary reason for proper flow of water and other dissolved throughout the
plant.

Xylem Phloem

Xylos; wood Phlois; inner bark

Term coined and discovered by Nageli (1858) Term coined and discovered by
 Nageli (1858)

Water conducting tissue Food conducting tissue

Composed of four different types of cells called; Composed of four different types of cells
tracheid, trachea, xylemparenchyma and xylem called; sieve tubes, companion cells, Phloem
sclerenchyma. parenchyma and phloem fibers.

Except for Xylem parenchyma, all other cells Except phloem fiber, all phloem cells are
are dead. living.

Protective Tissues
Some tissues are there to provide stronghold support to a plant. The most well known protective
tissues are:

Cork: This tissue is entirely dead and is just there to protect the plants; their intercellular gaps
are not there anymore, and their cell walls are impenetrable to gas and water molecules.

Epidermis: This tissue creates the outer casting of the plant structure. It helps during the loss
of water and gaseous exchange.

Properties of Meristematic Tissues

 We also call them Meristems.
 They are constantly dividing the cells in tissues inside the plant.
 They have very small vacuoles.
 The tissues have cells with very dense protoplasms.
 These tissues have the power of healing the wounds of plants.
 They do not store food.
 They have a single, large and central nucleus.
 There is a high metabolic activity in them.

Properties of Permanent Tissues

 No vacuoles are present in the cells.
 They do not have the power of division.
  Their cells are suitably shaped and correctly assembled.
 Their cell wall is comparatively thicker.
 Their nucleus is more prominent.
 Their cytoplasm is thick.
 They do not store food.
 There are sometimes intercellular spaces present between the cells.

Summary:
 The meristematic cells keep on bifurcating.
 Meristematic tissues based on their location are of three types namely; apical, lateral,
and intercalary meristematic tissues.
 Permanent tissues are of two types: Simple and Complex.

Simple Tissues Complex Tissues

They are made up of only one type of They are made up of more than one type of cells
cells

Helps in storage of food and Helps in transportation of water, sugars, minerals, and
mechanical support. other metabolites.

Eg: Parenchyma, Collenchyma, Eg: Xylem and Phloem.

Sclerenchyma

 Dermal or protective tissues are simple tissues. They form the outer covering of plant
parts such as the stem, roots, fruits, flowers etc. The epidermis and cork are two types
of dermal tissues.
 The main function of the ground tissue is to provide support, strength, and flexibility to
plants. The three types of ground tissues are; Parenchyma, Collenchyma, and
Sclerenchyma.
 Vascular tissues are also called conducting tissues as they play an important role in the
transportation of water and food in plants.
 Xylem is a water conducting tissue.
 Phloem is a food conducting tissue.
8. Write a note on animal tissues?
Types of Animal Tissue
A tissue is a group of cells connected to each other that collectively perform similar functions
in an organism. All contents of the body including structures and various organs are made of
tissues.

The animal body comprises four basic types of tissues, all of which vary in their origin and
function. They are:
 Epithelial Tissues:

Made up of tightly packed cells layer together, epithelial tissues line the body surface.
Their functions include protection, absorption, and secretion. Epithelial tissues can be
found in the lining of the mouth and nose, digestive system lining, and the skin.

 Muscle Tissues:

These are of three types, smooth muscle tissue- found in inner linings of organs, skeletal
muscle tissue- found attached to the bone and helps in body movement and cardiac
muscle tissue- found in the heart. These tissues help in changing the size of a cell.

 Nervous Tissues:

Made up of neurons (nerve cells in the brain), these tissues form the entire nervous
system, including the spinal cord and the brain.
  Connective Tissues:

Made of various cells that are involved in lending support to the body, connective
tissues are namely the fat, bone, blood and cartilage in an animal body.

Epithelial Tissues
These tissues form the lining of body surfaces and also account for glands. The cells along
these tissues are tightly connected to each other. The epithelium does not contain blood vessels
and hence depends on the other connective tissues to derive its nutrients and other essentials.

It is found along the edges of the organs and has two prominent surfaces, namely, the apical
surface which is on the exterior and lies open to the body cavity, and the basal surface which
lies adjacent to the underlying tissue.

Epithelial cells can be either squamous, cuboidal, or columnar in shape. The number of cell
layers along with the combination of cell shapes decides the classifying features of epithelial
tissue.

Types of Epithelial Tissues and Their Function

Epithelial Tissues are always located on the inner or outer surfaces of organs, and their
functions largely depend on the exact position of their locations. These are of the following
types:

Squamous Epithelium

Closely packed with one another, these are thin and flat cells that mostly like the esophagus,
blood vessels, alveoli and the inner cavities of the mouth. The squamous epithelium tissue
lends protection against mechanical injuries, while also blocking any sort of germs from
entering.

The squamous epithelium may also be arranged in multiple layers, in which case it is known
as the stratified squamous epithelium tissue. These tissues are usually found in the lining of the
esophagus and the skin.

Cuboidal Epithelium Tissue

These are cuboidal in shape, hence rightfully deriving their name. Found in kidney tubules,
salivary glands, and sweat glands, the functions of the cuboidal epithelium tissue are secretion,
protection and absorption.

When the cuboidal epithelium is arranged in multiple layers, it is known as the stratified
cuboidal epithelium tissue, and found on the inner side of the salivary glands and pancreatic
ducts.

Columnar Epithelium Tissue

Mostly with column-like or pillar-like cells, these can be found in the intestine and lining of
the stomach. Important functions of the columnar epithelium tissue include secretion and
absorption.

Ciliated Epithelium Tissue

The columnar epithelium tissues often have cilia; this is when they come to be known as ciliated
epithelium tissues. These can be found in kidney tubules, the respiratory tract and lining of the
trachea. Their function is to help in the movement of material in a given direction.
Glandular Epithelium Tissue

These are majorly modified columnar epithelial tissues whose main function is secretion. They
can be found in the sweat glands and tear glands.

Muscle Tissues

Muscle tissues are specialised tissues found in animals, responsible for applying force to
various parts of the body by using the method of contraction. Thin and elongated cells called
muscle fibers make up the muscle tissues.

The structure of a muscle tissue contains three distinct elements- the cytoplasm in the muscle
fibers, called the sarcoplasm, a membrane network known as the sarcoplasmic reticulum, and
the surrounding membrane of the muscle fibers known as the sarcolemma.

Important properties of muscle tissues are as follows-

 Extensibility:

The ability of a muscle to stretch itself

 Contractibility:

The ability of the muscle cells to forcefully shorten themselves

 Excitability:
 The ability of a muscle tissue to respond to a stimulus given by any hormone or a motor
neuron

 Elasticity:

The ability of a muscle to recoil to it's usual length after being stretched

Types of Muscle Tissues and Their Functions

Mostly responsible for initiating movement of an organism, muscle tissues have a range of
other functions based on their types. These are:

Skeletal Muscle Tissues

Voluntary muscle and striated in nature, skeletal muscle tissues have neatly arranged bundles
and have tendons anchoring them. These have an impact on the skeletal movements of an
organism that include posture and locomotion.
Smooth Muscle Tissues

These are involuntary and non-striated in nature and have tapered ends. They are mostly located
in the blood vessel walls like arteries and veins, urinary tract, trachea and digestive system.
Smooth muscle tissues help in peristalsis to move food up and down the alimentary canal.

Cardiac Muscle Tissues

These majorly consist of making up the heart. Involuntary and striated, these are branched out
at irregular angles to help with coordinated contractions occurring inside the heart.

Nervous Tissues

Nervous tissues are the cells that form the central and peripheral nervous system. While in the
central nervous system, the nervous tissues form the spinal cord and the brain, in the peripheral
nervous system, the nervous tissues make up the cranial and spinal nerves, also including the
motor and sensory neurons.

The most important function of the nerve tissues is to transmit and carry nerve impulses in
various parts of the body. Impulses are often sent by axons and received by dendrites.

Nerve cells can be of three types- sensory neurons, motor neurons and interneurons.

Connective Tissues

These are tissues that support, connect or separate various other kinds of tissues and organs
inside the body. They are made up of cells, fibers like collagen and extracellular matrix.
Collective tissues can be found abundantly located inside the body in a freely arranged form or
in a matrix.
Types and Functions of Connective Tissues
The various types of connective tissues include areolar, adipose, bone, cartilage and fat. All
cells are involved in the secretion of collagen except for blood.

Areolar Connective Tissues

These are found under the skin, surrounding nerves and blood vessels. Their function is to
repair tissues and provide support.

Adipose Tissues

These can be found in the organs and skin. Composed of fat globules, their function is to
insulate the body with the fat presence.

Bones

These form the skeletal structure of the body and have a characteristic of being rich in calcium
and collagen fibers. They protect the body and are the location of blood cell production.

Cartilage

These can be found in the ear tips, vertebral column, bronchi and are made of chondrocytes
that are composed of flexible intercellular materials.

Blood

The functions of blood include putting up a defence system, transportation and most
importantly homeostasis. Blood is composed of blood cells that include platelets, RBC and
WBC along with plasma.
9. Write a note on five kingdom classification?

The basic two-kingdom classification that divides plants and animals into two groups was not
efficient enough to help study other organisms that did not fall in either of the categories. There
are organisms, with no similarities with plants or animals. Hence, to classify and study all
organisms, a broader system was needed to first classify and put all of them into different
groups.

The Five Kingdom System Differentiates Organisms based on:

 Cell Structure - it comprises of individual components needed by an organism to carry

out various life processes
 Body Organization - it is how the body of an organism is organized at various levels.
For instance, in humans, cells come together to form tissues, tissues come together to
form organs and organs organized into organ systems, and so on and so forth.
 Mode of Nutrition - it is the process through which an organism obtains and consumes
its nutrients. It can be autotrophic or heterotrophic. They are further divided into various
types. Refer to Vedantu's website to know more about Mode of Nutrition
 Mode of Reproduction - it is the way through which an organism reproduces. It can
be sexual or asexual. It can be through fragmentation, fission, budding regeneration,
vegetative propagation, etc. Vedantu can help you in understanding Modes of
Reproduction in detail.
 Phylogenetic Relationship (Evolutionary development and diversification) - in
simple words phylogenetic relationships are the relationships that help in tracing how
far are the two organisms related to each other via common ancestry.

Robert H Whittaker introduced the Five Kingdom Classification in 1969 for the study of
organisms.

The Five Kingdom System

The Five major groups as per this system are:

  Animalia
 Plantae
 Fungi
 Protista
 Monera

Kingdom Animalia

Eukaryotic and multicellular organisms with no cell wall or photosynthetic pigments come
under this group. Organisms in this group are heterotrophs and feed on external food (plants or
animals). Their mode of nutrition is holozoic. So, with such a mode of nutrition, they have to
ingest, digest, absorb, and assimilate the food in order to utilize it. Organisms of this group
reproduce sexually or asexually. The most distinguishing feature of this group of organisms is
the presence of sensory organs and a nervous system.

Examples- Porifera- Sponges, Cnidaria- Jellyfishes, Echinodermata- Starfishes, Arthropoda-

insects, Mammals, Birds, Amphibians.
Kingdom Animalia has a further sub-classification:

 Vertebrates- Those who have backbones.

 Invertebrates- Those who do not possess backbones.

Kingdom Plantae

As the name suggests, plants come under the kingdom Plantae. Organisms in this group are
Eukaryotes and multicellular with the presence of a cell wall made of cellulose. They also have
photosynthetic pigments present so that they can prepare their own food. Thus, they show the
autotrophic mode of nutrition mostly. Organisms in this group reproduce sexually or asexually.
Plants are further divided into two types:

 Flowering plants
 Non-Flowering plants

Plants with flowers reproduce sexually by pollination and the non-flowering plants reproduce
asexually by vegetative propagation.

Examples- Ferns, Mosses, Flowering plants, etc.

Kingdom Fungi

Fungi are Eukaryotes, can be unicellular, multicellular, or filamentous. They have a cell wall
made of chitin and polysaccharides. Their mode of nutrition is heterotrophic, which means they
cannot make their own food and rely on external sources. If you have noticed, most often, long
decomposing food or stale bread develops a furry growth on them, this growing organism is
nothing but fungi. Some fungi also survive as parasites and most of them are saprophytes, that
is, they survive on a dead or decaying matter. Some fungi are also found to be surviving in
symbiotic associations with other organisms, like plants or viruses. Fungi are an important class
of organisms for the ecosystem as they facilitate the decaying process. We also see commercial
uses of fungi in our day to day lives. Fungi can reproduce sexually or asexually by spore
formation.
Examples- Mushrooms, yeasts, etc.

Kingdom Protista

Eukaryotic and single-celled, also called unicellular organisms to come under this group.
Protists are both heterotrophs or autotrophs. The pant protists are autotrophs. Example: Algae-
Spirogyra, Euglena, etc. Animal and fungus-like protists are heterotrophs. Example- Amoeba,
Paramecium. Some Protists also possess flagella or cilia for locomotion. Protists can reproduce
asexually by Binary Fission, Nucleus division, budding, etc. Or sexually by gametes
formation.

Protists are further divided into three major groups:

Photosynthetic Protists

The protists which are capable of doing photosynthesis come under this group.

There are three types of photosynthetic protists:

 Chrysophytes are diatomic and desmids.

 Dinoflagellates are mostly marine planktons but are also found in freshwater bodies.
 Euglenoids have a protein layer called pellicle in place of the cell walls and are found
in freshwater bodies.

Saprophytic Protists

Are slime molds. They gain energy from dead and decaying organic matter, mostly twigs,
leaves, etc.

Parasitic Protists

Single-celled eukaryotic protozoans are under this group.

Kingdom Monera
Single-celled prokaryotic organisms with a cell wall are categorized under Kingdom Monera.
Their cell wall is made up of a polysaccharide and protein compound instead of cellulose like
other organisms. They lack many cell organelles like a cell membrane, Golgi apparatus,
mitochondria, endoplasmic reticulum, etc. Bacteria is the only organism in this group. They
are microscopic and are found in abundance. Life originated and evolved from bacteria.

Kingdom Monera is further divided into a) Archaebacteria- The most simple and primitive
form of bacteria that can survive in extreme conditions. b) Eubacteria- Organisms lacking
membrane-bound nucleus are often found in the intestines of the vertebrates and in soil.

Types of bacteria

Depending upon their shape, bacteria are divided into four groups

 Micrococcus- Are generally round or sphere-shaped. They are commonly found on the
skin, soil, meat, etc. These bacteria are harmless and are generally saprophytic in nature.
They need oxygen to grow and reproduce.
 Staphylococcus- Are also sphere-shaped bacteria but in some cases, they are infectious
and cause some major health problems like food poisoning, diarrhea, skin infection,
etc. They are also found on skin, hair, and other surfaces. They can thrive and grow
even in the absence of oxygen.
 Bacillus- Are rod-shaped bacteria. They produce endospores as a result of which they
are very tough. These are also saprophytes, found in soil water, dust.
 Pseudomonas- They are also rod-shaped bacteria. Pseudomonas can produce
exotoxins, which is why they infect individuals who have suppressed immunity or less
immunity. They hardly attack healthy individuals. They are found in soil, over the skin,
etc.

Advantages of Five Kingdom Classification

1. We can study the characteristics of organisms by only looking at a few members of a

particular kingdom.
 2. Classifying organisms makes it simpler and easier to understand their traits.
3. It helps trace origin and study growth patterns, reproduction, structure, and survival
needs.
4. The Five Kingdom System also divides unicellular and multicellular organisms into
different groups.

10. Write A Note On Chromosomes?

In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called
chromosomes. Each chromosome is made up of DNA tightly coiled many times around
proteins called histones that support its structure. Chromosomes were first described by
Strasburger (1815), and the term ‘chromosome’ was first used by Waldeyer in 1888. They
appear as rod-shaped dark stained bodies during the metaphase stage of mitosis when cells are
stained with a suitable basic dye and viewed under a light microscope.

 Chromosomes are the nuclear components of the special organization, individuality, and
function that are capable of self-reproduction and play a vital role in heredity, mutation,
variation and evolutionary development of the species.
 Each chromosome is made up of DNA tightly coiled many times around proteins that
support its structure.
 The proteins that bind to the DNA to form eukaryotic chromosomes are traditionally divided
into two classes: the histones and the non-histone chromosomal proteins.
 The complex of both classes of protein with the nuclear DNA of eukaryotic cells is known
as chromatin.
 Chromatin are a highly compacted structure consisting of packaged DNA and necessary so
as to fit DNA into the nucleus.
 The assembly of DNA into chromatin involves a range of events, beginning with the
formation of the basic unit, the nucleosome, and ultimately giving rise to a complex
organization of specific domains within the nucleus.
 In the first step of this process, DNA is condensed into an 11 nm fiber that represents an
approximate 6-fold level of compaction. This is achieved through nucleosome assembly.
 The nucleosome is the smallest structural component of chromatin and is produced through
interactions between DNA and histone proteins.
 Each nucleosome consists of histone octamer core, assembled from the histones H2A, H2B,
H3 and H4 (or other histone variants in some cases) and a segment of DNA that wraps around
the histone core. Adjacent nucleosomes are connected via “linker DNA”.

 In eukaryotes the chromosomes are multiple large, linear and are present in the nucleus of
the cell.
 Each chromosome typically has one centromere and one or two arms that project from the
centromere.
 Structurally, each chromosome is differentiated into three parts—
1. Pellicle
2. Matrix
3. Chromonemata
 Pellicle
 It is the outer envelope around the substance of chromosome.
 It is very thin and is formed of achromatic substances.
Matrix
 It is the ground substance of chromosome which contains the chromonemata.
 It is also formed of non-genic materials.
Chromonemata
 Embedded in the matrix of each chromosome are two identical, spirally coiled threads, the
chromonemata.
 The two chromonemata are also tightly coiled together that they appear as single thread of
about 800A thickness.
 Each chromonemata consists of about 8 microfibrils, each of which is formed of a double
helix of DNA.
In mitotic metaphase chromosomes, the following structural feature (except chromomere) can
be seen under the light microscope:

(1) Chromatid,

(2) Chromonema,

(3) Chromomeres,

(4) Centromere,

(5) Secondary constriction or Nucleolar organizer,

(6) Telomere and

(7) Satellite.

Centromere
 A small structure in the chromonema, marked by a constriction which is recognised as
permanent structure in the chromosome is termed as the centromere.
 At this point the two chromonemata are joined together.
 It is known as centromere or kinetochore or primary constriction.
 It divides the chromosome into two sections, or “arms.” The short arm of the chromosome is
labeled the “p arm.” The long arm of the chromosome is labeled the “q arm.”
 Its position is constant for a given type of chromosome and forms a feature of identification.
 In thin electron microscopic sections, the kinetochore shows a trilaminar structure, i.e., a 10
nm thick dense outer protein aceous layer, a middle layer of low density and a dense inner
layer tightly bound to the centromere.
 The chromosomes are attached to spindle fibres at this region during cell division.
Secondary Constriction or Nucleolar Organiser
 The chromosome besides having the primary constriction or the centromere possesses
secondary constriction at any point of the chromosome.
 Constant in their position and extent, these constrictions are useful in identifying particular
chromosomes in a set.
 The chromosome region distal to the secondary constriction i.e., the region between the
secondary constriction and the nearest telomere is known as satellite.
 Therefore, chromosomes having secondary constrictions are called satellite chromosomes or
sat-chromosomes.
 Nucleolus is always associated with the secondary constriction of sat-chromosomes. There-
fore, secondary constrictions are also called nucleolus organiser region (NOR) and sat-
chromosomes are often referred to as nucleolus organiser chromosomes.
Telomeres
 These are specialized ends of a chromosome which exhibits physiological differentiation and
polarity.
 Each extremity of the chromosome due to its polarity prevents other chromosomal segments
to be fused with it. The chromosomal ends are known as the telomeres.
 If a chromosome breaks, the broken ends can fuse with each other due to lack of telomere.
Types of Chromosomes
A. Autosomes and Sex Chromosomes
 Human chromosomes are of two types- autosomes and sex chromosomes.
 Genetic traits that are linked to the sex of the person are passed on through the
sex chromosomes. The rest of the genetic information is present in the autosomes.
 Humans have 23 pairs of chromosomes in their cells, of which 22 pairs are autosomes and
one pair of sex chromosomes, making a total of 46 chromosomes in each cell.
B. On the Basis of Number of Centromeres
1. Monocentric with one centromere.
2. Dicentric with two centromeres.
3. Polycentric with more than two centromeres
 4. Acentric without centromere. Such chromosomes represent freshly broken segments of
chromosomes which do not survive for long.
5. Diffused or non-located with indistinct centromere diffused throughout the length of
chromosome.
C. On the Basis of Location of Centromere

Figure: Types of Chromosomes

1. Telocentric are rod-shaped chromosomes with centromere occupying the terminal

position, so that the chromosome has just one arm.
2. Acrocentric are also rod-shaped chromosomes with centromere occupying a sub-
terminal position. One arm is very long and the other is very short.
3. Sub-metacentric chromosomes are with centromere slightly away from the mid-point so
that the two arms are unequal.
4. Metacentric are V-shaped chromosomes in which centromere lies in the middle of chro-
mosome so that the two arms are almost equal.
Prokaryotic Chromosomes
 The DNA of a bacterial cell, such as Escherichia coli, is a circular double-stranded molecule
often referred to as the bacterial chromosome.
 The circular DNA is packaged into a region of the cell called the nucleoid where it is
organized into 50 or so loops or domains that are bound to a central protein scaffold, attached
to the cell membrane.
 The DNA is negatively supercoiled, that is, it is twisted upon itself.
 It is complexed with several DNA-binding proteins, the most common of which are proteins
HU, HLP-1 and H-NS. These are histone-like proteins.
Eukaryotic Chromosomes
 The large amount of genomic DNA in a eukaryotic cell is tightly packaged in chromosomes
contained within a specialized organelle, the nucleus.
 With the exception of the sex chromosomes, diploid eukaryotic organisms such as humans
have two copies of each chromosome, one inherited from the father and one from the mother.
 Chromosomes contain both DNA and protein.
 Most of the protein on a weight basis is histones, but there are also many thousands of other
proteins found in far less abundance and these are collectively called non- histone proteins
(NHP).
 This nuclear DNA–protein complex is called chromatin.
 In the nucleus, each chromosome contains a single linear double-stranded DNA molecule.
 The length of the packaged DNA molecule varies. In humans, the shortest DNA molecule in
a chromosome is about 1.6 cm and the longest is about 8.4 cm.
 The extensive packaging of DNA in chromosomes results from three levels of folding
involving nucleosomes, 30 nm filaments and radial loops.
a. Nucleosomes
 The first level of packaging involves the binding of the chromosomal DNA to histones.
 Overall, in chromosomes, the ratio of DNA to histones on a weight basis is approximately
1:1.
 There are five main types of histones called H1, H2A, H2B, H3 and H4.
 Histones are very basic proteins; about 25% of their amino acids are lysine or arginine so
histones have a large number of positively charged amino acid side-chains.
 These positively charged groups therefore bind to the negatively charged phosphate groups
of DNA
b. 30 nm fiber
 If nuclei are lysed very gently, the chromatin is seen to exist as a 30 nm diameter fiber.
 The fiber is formed by a histone H1 molecule binding to the linker DNA of each nucleosome
at the point where it enters and leaves the nucleosome.
 The histone H1 molecules interact with each other, pulling the nucleosomes together.
c. Radial loops
 When chromosomes are depleted of histones, they are seen to have a central fibrous ‘protein
scaffold’ (or nuclear matrix) to which the DNA is attached in loops.
 Therefore, in vivo it seems likely that the next order of packaging involves the attachment of
the 30 nm fiber to multiple locations on this central protein scaffold in a series of radial loops.
 The mitochondria and chloroplasts of eukaryotic cells also contain DNA but, unlike the
nuclear DNA, this consists of double-stranded circular molecules resembling bacterial
chromosomes.
Nucleosome Model of Chromosome
 Nucleosome model is a scientific model which explains the organization of DNA and
associated proteins in the chromosome.
 It also further explains the exact mechanism of the folding of the DNA in the nucleus.
 The model was proposed by Roger Kornberg in 1974 and is the most accepted model of
chromatin organization.
 It was confirmed and christened by P. Oudet et al., (1975).
Nucleosome Model of Chromosome

Features of the Nucleosome Model of Chromosomes

 In eukaryotes, DNA is tightly bound to an equal mass of histones, which serve to form a
repeating array of DNA-protein particles, called nucleosomes.
 If it was stretched out, the DNA double-helix in each human chromosome would span the
cell nucleus thousands of time.
 Histones play a crucial role in packing this very long DNA molecule in an orderly way (i.e.,
nucleosome) into nucleus only a few micrometers in diameter.
 Thus, nucleosomes are the fundamental packing unit particles of the chromatin and give
chromatin a “beads-on-a-string” appearance in electron micrographs taken after treatments
that unfold higher-order packing.
 Each nucleosome is a disc-shaped particle with a diameter of about 11 nm and 5.7 nm in
height containing 2 copies of each 4 nucleosome histones–H2A, H2B, H3, and H4.
 This histone octamer forms a protein core [(i.e., a core of histone tetramer (H3, H4)2 and the
apolar regions of 2(H2A and H2B)] around which the double-stranded DNA helix is wound
1¾ time containing 146 base pairs.
 In chromatin, the DNA extends as a continuous thread from nucleosome to nucleosome.
 Each nucleosome bead is separated from the next by a region of linker DNA which is
generally 54 base pair long and contains single H1 histone protein molecule.
 Generally, DNA makes two complete turns around the histone octamers and these two turns
(200 bp long) are sealed off by H1 molecules.
 On average, nucleosomes repeat at intervals of about 200 nucleotides or base pairs. For
example, a eukaryotic gene of 10,000 nucleotide pairs will be associated with 50
nucleosomes and each human cell with 6 x 109 DNA nucleotide pairs contains 3 x 107
The Folding of the DNA
 The first step is the assembly of the DNA with a newly synthesized tetramer (H3-H4), are
specifically modified (e.g. H4 is acetylated at Lys5 and Lysl2 (H3-H4)), to form a sub-
nucleosomal particle, which is followed by the addition of two H2A-H2B dimers.
 This produces a nucleosomal core particle consisting of 146 base pairs of DNA bind around
the histone octamer. This core particle and the linker DNA together form the nucleosome.
 The next step is the maturation step that requires ATP to establish regular spacing of the
nucleosome cores to form the nucleo-filament.
 During this step the newly incorporated histones are de-acetylated.
 Next, the incorporation of linker histones is accompanied by folding of the nucleo-filament
into the 30 nm fiber, the structure of which remains to be elucidated.
 Two principal models exist- the solenoid model and the zig-zag.
 Finally, further successive folding events lead to a high level of organization and specific
domains in the nucleus.
Function and Significance of Chromosomes
 The number of the chromosomes is constant for a particular species. Therefore, these are of
great importance in the determination of the phylogeny and taxonomy of the species.
 Genetic Code Storage: Chromosome contains the genetic material that is required by the
organism to develop and grow. DNA molecules are made of chain of units called genes.
Genes are those sections of the DNA which code for specific proteins required by the cell for
its proper functioning.
 Sex Determination: Humans have 23 pairs of chromosomes out of which one pair is the sex
chromosome. Females have two X chromosomes and males have one X and one Y
chromosome. The sex of the child is determined by the chromosome passed down by the
male. If X chromosome is passed out of XY chromosome, the child will be a female and if a
Y chromosome is passed, a male child develops.
 Control of Cell Division: Chromosomes check successful division of cells during the
process of mitosis. The chromosomes of the parent cells insure that the correct information
is passed on to the daughter cells required by the cell to grow and develop correctly.
 Formation of Proteins and Storage: The chromosomes direct the sequences of proteins
formed in our body and also maintain the order of DNA. The proteins are also stored in the
coiled structure of the chromosomes. These proteins bound to the DNA help in proper
packaging of the DNA.
The Human Chromosomes
 Every normal human cell, except for sperm and egg cells, has 23 pairs of chromosomes for
a total of 46 chromosomes.
 Sperm and egg cells have only one of each pair of chromosomes for a total of 23. Each
chromosome contains hundreds to thousands of genes.
 The sex chromosomes are one of the 23 pairs of chromosomes. Normal people have 2 sex
chromosomes, and each is either an X or a Y chromosome. Normal females have two X
chromosomes (XX), and normal males have one X and one Y chromosome (XY).