BIOLOGY CLASS- IX th 8368723015

Ch 5

The Fundamental unit of Life

Introduction

• Cell is the structural and functional unit of life. It is the basic unit of life.
• It is discovered by Robert Hook in 1831 in cork slice with the help of
primitive microscope.

• Leeuwenhoek (1674), discovered the free living cells in pond water with the
improved microscope.

• Robert Brown discovered the nucleus in the cell in 1831.

• Purkinje coined the term ‘protoplasm’ for the fluid substance of the cell in
1839.
Discoveries about Cells – The Fundamental Unit of Life
Discovered By Period of What did they discover?
time
Robert Hooke 1665 noticed the presence of cells in a cork slice
Leeuwenhoek 1674 found the presence of living cells in the pond water
Robert Brown 1831 recognized the existence of a nucleus in the cell
Purkinje 1839 invented the term ‘Protoplasm’ which is the liquid
present in a cell
Schleiden and 1838, 1839 presented the cell theory that all organisms are actually
Schwann made up of cells
Virchow 1855 suggested that all cells come from cells that already
exist in nature

The cell theory

• The theory that all the plants and animals are composed of cells and the cell
is the basic unit of life, was presented by two biologists, Schleiden and
Schwann.

• The cell theory was further expanded by Virchow by suggesting that all cells
arise from pre-existing cells.

→ Types of organisms

• On the basis of no. of cells, organisms are of two types:

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(i) Unicellular Organism
(ii) Multicellular Organism

Unicellular Organism: These organisms are single celled which perform all
the functions. Example: Amoeba, paramecium, bacteria.
Multicellular Organism: Many cells grouped together to perform different
function in the body and also form various body parts. Example: fungi, plants,
animals.

• The shape and size of cell are different according to the kind of function they
perform. There is division of labour in cells.

• Each cell has certain kind of cell organelles to perform different type of
function like mitochondria for respiration.

→ Types of cells

• There are two types of cells:

(i) Prokaryotes
(ii) Eukaryotes

Prokaryotes Eukaryotes

Cells of organism lacks nuclear Cells of organism have nuclear

membrane. membrane.

Nucleolus is absent. Nucleolus is present.

Single chromosomes. Single or multi chromosomes

Reproduction is always Reproduction is both sexual and

asexual. asexual.

Always unicellular. Often multicellular.

Membrane bound cell Membrane bound organelles are

organelles are absent. present like mitochondria.

Centriole is absent. Centriole is present only in animals cell.

Cell division is by binary

fission. Cell division is by mitosis or meiosis.

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Example: Bacteria, Blue green Example: Fungi, Plant cell, Animal cell
algae, etc. etc.

Difference between Animal cell and Plant cell

Animal Cell Plant Cell

Cell wall is absent. Cell wall is present.

Plastids are absent. Plastids are present.

Centrioles are present. Centrioles are absent.

Golgi bodies are present and called

Golgi bodies are present. dictyosome.

Vacuoles are absent. If present, they Vacuoles are present and large in
are small. size.

Centrosome is present with one or

two centrioles. Centrosome is absent

Diffusion

• The spontaneous movement of a substance from a region of high

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concentration to the region of low concentration is called diffusion.

• Some substances like carbon dioxide or oxygen can move across the cell
membrane by a process called diffusion. Cell also obtains nutrition from the
environment.

Osmosis

• The movement of water molecules through selectively permeable membrane

along the concentration gradient is called osmosis.

• Plant cell tend to obtain water through osmosis.

Hypotonic or Hypertonic or Isotonic solution

Hypotonic Solutions
 If the concentration of water outside the cell is higher than the concentration of
water inside the cell gains water by the process of osmosis.
 Water can move into the cell from the cell membrane. In the case of hypotonic
solutions, more water enters the cells which result in swelling of the cells.

Figure 4 - Hypotonic Solution

Isotonic Solutions
 If the cells are put in an environment that has a similar concentration of
water as present inside. This state allo ws for the free movement of water

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across the membrane without changing the concentration of solutes on either
side.
 Therefore, the size of the cell does not vary in an isotonic solution because
there is no net movement of water.

Figure 5 - Isotonic Solution

Hypertonic Solutions
 If the cells are kept in an environment that has a lower concentration of water than
what is present inside the cells then due to the process of osmosis water moves out
of the cells.
 This results in a decrease in the size of the cells (they shrink) as more water comes
out of the cell.

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What happened to cell in sugar or salt solution?

Name of the
solution Condition Result

Medium surrounding cell Cell will gain water by

Hypotonic has higher water osmosis and likely to swell
solution concentration than cell. up.

Water crosses the cell

Medium has exactly same membrane in both
Isotonic water concentration as directions.
solution the cell. Cell will stay the same size.

Water crosses the cell in

Hypertonic Medium has lower both directions, but more
solution concentration of water water leaves the cell than
than the cell. enters it.

Plasma membrane or Cell membrane

• This is the outermost covering of the cell that separates the contents of the
cell from its external environment.

• The plasma membrane allows or permits the entry and exit of some
materials in and out of the cell.

• It also prevents movement of some other materials. The cell membrane is

called selectively permeable membrane.

• It is made up of lipid and protein.

Plasma Membrane
 It is just like an envelope that covers the whole cell. Therefore, a cell gets
separated from the external environment because it has a plasma membrane.
 The plasma membrane can decide which materials should enter or leave the cell
and which should not. That is why it is also called a ‘Selectively Permeable
Membrane’.

The Fluid Mosaic Model of Plasma Membrane

 The Fluid Mosaic model explains the structure of the plasma membrane.
According to it, the plasma membrane comprises 3 components - Lipids, Proteins
and Carbohydrates. These components can flow freely and fluidly inside the
plasma membrane.

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 There are two types of lipids (fats) in the plasma membrane –
 Phospholipid – It is a lipid made up of glycerol, two fatty acids, and
phosphate. It creates a semi-permeable membrane that allows the flow
of only certain materials inside/ outside the cell
 Cholesterol - It is a lipid that provides fluidity to the surface of the
plasma membrane.
 The proteins act as receptors of the cell and help in transportation across the cell
membrane. The carbohydrates attach themselves with the lipids and proteins and
are found on the extracellular side of the membrane.

Figure 3 -Structure of the Plasma Membrane

→ Properties of Plasma membrane

• It is flexible (made up of organic molecules called lipids and proteins).

• Its flexibility enables cell to engulf in food and other from the external
environment. This process is called endocytosis. Amoeba acquire food through
this process.

→ Functions of Plasma membrane

• It permits the entry and exit of some materials in and out of the cell.

• It prevents movement of some other materials not required for the cell as it
acts like selectively permeable membrane.

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Cell Wall

• Cell wall is another rigid outer covering in addition to the plasma membrane
found in plant cell. The cell wall lies outside the plasma membrane.

• The plant cell wall is mainly composed of cellulose. Cellulose is a complex

substance which provides structural strength to plants.

→ Function of Cell Wall

• Cell walls permit the cells of plants, fungi and bacteria to withstand very
dilute (hypotonic) external media without bursting.

• In such media the cells tend to take up water by osmosis. The cell swells,
building up pressure against the cell wall. The wall exerts an equal pressure
against the swollen cell.

• Because of cell wall, cells can withstand much greater changes in the
surrounding medium than animal cells.

Plasmolysis

• When a living plant cell loses water through osmosis there is shrinkage or
contraction of the contents of the cell away from the cell wall. This
phenomenon is known as plasmolysis.

The Nucleus
Nucleus is a prominent organelle present in the cell which is the controlling centre of
all activities of the cell.

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Figure 7 - Nucleus of a Cell

The Structure of the Nucleus

 A nucleus has a nuclear membrane that covers it all around.
 There are pores present on the nuclear membrane that allow the movement of
substances in and out of the nucleus.
 There are chromosomes, rod-shaped structures present in the nucleus which
contain genetic information.
The chromosomes contain two types of things -
1. DNA - This is responsible for organising and constructing new cells
2. Proteins - These help in the packaging and condensation of DNA.

Chromatin
Chromatin is thread-like material present in a cell. The chromatin organises itself into
chromosomes whenever the cell is about to divide.
DNA is also known as Deoxyribo Nucleic Acid. It was first discovered by Watson and
Crick in 1950s.
 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.

Figure 8 – Chromosomes and Chromatin

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 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.

Nucleolus
It is called the Brain of the Nucleus. It comprises 25% of the volume of the nucleus. It
consists of proteins and ribonucleic acids (RNA). It helps in the formation of
ribosomes which help in the formation of proteins inside the cell.

Figure 9 - Nucleolus inside a Nucleus

What is a nucleoid?
Sometimes cells do not have a well-defined nucleus because they lack a nuclear
membrane. Such a nucleus with no definite nuclear boundaries is called a Nucleoid.
What are the prokaryotes?
Organisms whose cells do not have a definite cell membrane are called Prokaryotes.
What are eukaryotes?
Organisms whose cells contain a well-defined nuclear membrane are
called Eukaryotes.

Are there any further differences between prokaryotes and eukaryotes?

Prokaryotes Eukaryotes
There is no presence of nucleus The nucleus exists in the cells
A single chromosome is present There are multiple chromosomes

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They undergo asexual reproduction They undergo sexual as well as a sexual
reproduction
They are generally unicellular They are generally multicellular organisms
organisms
There are no membrane-bound cell There are membrane-bound cell organelles
organelles present inside the cells
Example – Bacteria, Blue-green algae Example – Fungi, Plants and Animals
(Cyanobacteria)

Figure 10 - Eukaryotic and Prokaryotic Cells

Cytoplasm
 The plasma membrane has a fluid-like substance in it which is called the
cytoplasm.
 The cytoplasm contains several organelles that can perform distinct functions of
the cell

Functions of Cytoplasm
 It supports and suspends the cell organelles and molecules.
 The cellular processes occur in the cytoplasm such as the formation of proteins.
 It allows the movement of substances in the cell such as hormones.
 It dissolves cellular wastes.

The Cell Organelles

 In the case of Eukaryotic organisms, the cells contain organelles that have
their own membranes apart from the overall cell membrane of the cell.

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Figure 11 - Different Cell Organelles

What is the significance of membrane-bound organelles in a cell?

The cells perform several functions. The organelles are useful because they allow the
separation of different functions that are being performed by the cell.
Organelles which carry out important activities in a Cell –
1. Endoplasmic Reticulum
2. Golgi Apparatus
3. Lysosomes
4. Mitochondria
5. Plastids
6. Vacuoles
7. Centrioles
8. Ribosomes
9. Peroxisomes

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Endoplasmic Reticulum (ER)

Figure 12 - Endoplasmic Reticulum

 The structure of the ER is quite similar to that of the plasma membrane. It is a
network-like structure that consists of membrane-bound tubes and sheets.
 Two types of ER –
 Rough ER
 Smooth ER
 Rough ER contains ribosomes that are responsible for the manufacturing of
proteins in the cells. They give a rough texture to the cell.
 The smooth ER manufactures fats or lipids in the cell which allow the functioning
of the cell.
 What are the functions of lipids and proteins?
 Proteins and lipids synthesised on ER are used for making cell
membranes. The process is known as Membrane Biogenesis.
 Proteins can act as an enzyme
 Both protein and lipids can act as hormones
 Functions of ER
 Transportation of material between different parts of the cytoplasm and
also between the nucleus and cytoplasm
 Folding of proteins which are synthesised by ribosomes on RER.
 Detoxifying poisons and drugs out of the cell is the function of SER.

Golgi Apparatus

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Figure 12 – Golgi Apparatus
 Camillo Golgi discovered the Golgi Apparatus.
 It contains vesicles that are arranged parallel in stacks. These stacks are
called Cisterns. These vesicles have their own membranes. These
membranes are sometimes connected to those of the ER.
 Functions of Golgi Apparatus
 Golgi apparatus carries materials synthesised by the ER to different parts
of the cell. The material is stored and packaged in vesicles.
 Formation of complex sugar
 Formation of lysosomes.

Lysosomes

Figure 13 – Structure of Lysosome

 They are single-membrane vesicles that are responsible for cleaning the cell. They
can digest any foreign material such as food or bacteria and even the worn-out cell
organelles.
 How can lysosomes digest any foreign material that enters the cell?
 Lysosomes are capable of doing so because they have digestive enzymes
in them. These enzymes break the materials and digest them. These
enzymes are synthesised by RER and packaged into lysosomes by Golgi
bodies.
 Why lysosomes are called ‘suicide bags’?
 If the cell’s own material gets damaged or dead, there are chances that
lysosomes burst out, thus digesting its own cell.

Mitochondria
It is a double membrane organelle that has its own DNA and that is why often called
‘Semi-Autonomous Organelle’

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Figure 15 – Structure of Mitochondria

 The cell requires energy in order to carry out several activities. This energy
is generated by mitochondria which are often called the ‘Powerhouse’ of the
Cell. Mitochondria are the site of cellular respiration. They use oxygen from
the air to oxidise the carbohydrates and thereby release energy.
 What are the energy currencies of a cell?
 The Mitochondria generates ATP (Adenosine Triphosphate) which are
energy giving molecules of the cell that are often called their ‘Energy
Currency’.
 The two membranes of Mitochondria
 Outer Membrane – Porous in Nature
 Inner Membrane – Deeply Folded
 The Inner Membrane of Mitochondria called as Cristae Facilitates
Generation of ATP molecules as it has a larger surface area.

Plastids
Just like mitochondria, Plastids are also double membraned organelles that have their
own DNA and ribosome.
Plastids exist in plant cells only. Depending upon the type of function they play in the
cell they can be classified as –

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Figure 15 – Types of Plastids

Chromoplast Leucoplast
Coloured in nature, contain a Colourless in nature
pigment called chlorophyll
Cause photosynthesis in plants Act as storage spaces of the cells
Contain orange and yellow pigments Contain starch, proteins and oil
Can further be divided into Can further be divided into amyloplast, elaioplast
Chloroplasts and proteinoplast or aleuroplast.

Classification of Plastids
1. Amyloplast
 They are found in tubers, cotyledons and endosperm in plants.
 They are used to store starch.
2. Elaioplast
 They are found in epidermal cells of the plants
 They store oil.
3. Proteinoplast
 They are found in seeds and nuts.
 They store proteins.
Chloroplasts
 Chloroplasts are cell organelles that conduct photosynthesis in plants.

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 Chloroplast is derived from two Greek words Chloro and Plasts which
means green and plants respectively.
 Chloroplasts contain photosynthetic pigments called ‘Chlorophyll’ along
with lipids, carbohydrates, minerals, DNA, RNA, grana, thylakoids and
stroma.
 The main functions of chloroplasts are:
 Conducting photosynthesis in plants.
 Protein synthesis
 Releases oxygen
 Storage of Starch

Figure 16 – Chloroplast containing thylakoids, stroma and grana

Light-dependent Reactions in Photosynthesis – During photosynthesis chlorophyll
absorbs the light energy which is then used for two molecules ATP and NADPH.
Thylakoids – They are pillow-shaped compartments in the chloroplast. The light-
dependent reactions in photosynthesis take place in the thylakoids.
Stroma – It is a fluid-filled matrix in the chloroplasts. It is a colourless fluid that
contains all the enzymes that are needed for the light -dependent reactions in
Photosynthesis.
Grana – Stacks of thylakoids are called Grana. They are found in the stroma. They
provide a large surface area so that the reactions of photosynthesis can take place.

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Vacuoles
Vacuoles are the places where cells can store liquids and solids. They are present in
both plants and animals but the plant vacuoles are bigger in size than the animal
vacuoles.
Plant Cell Vacuoles Animal Cell Vacuoles
Plant cell vacuoles store all the material that is Animal cell vacuoles contain food
required for the plant to stay alive such as water items in unicellular organisms
Plant vacuoles maintain the turgidity of the plant Animal vacuoles can also expel
cell water and waste out of the cell
Plant cells generally contain a single large Animal cell contain several small
vacuole vacuoles
Plant vacuoles are present in the centre of the cell Animal vacuoles are scattered
throughout the cell

Types of Vacuoles
 Sap Vacuoles
 Contractile Vacuoles
 Food Vacuoles

Sap Vacuoles

Figure 17 - Sap Vacuoles

These vacuoles are filled with a fluid called Vascular Sap. The fluid contains Amino
Acids, Salt, Sugar, Proteins, Water, and Waste Materials. Sap vacuoles are separated
from the cytoplasm by a semipermeable membrane called Tonoplast. Their main
function is to allow rapid exchange between the cytoplasm and the surrounding
environment.

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Several sap vacuoles are found in young plant cells and animal cells. In mature plants,
the small sap vacuoles combine to form a single large central vacuole.
Contractile Vacuoles

Figure 18 – Osmoregulation in Amoeba through Contractile Vacuoles

They are found in protistan and algal cells in freshwater. The membrane of the
contractile vacuoles is highly extensible and collapses easily. These vacuoles are
responsible for osmoregulation (maintaining the water content of the cells) and
excretion in the cells.
Food Vacuoles

Figure 19 – Food Vacuoles and Digestion

They are found in the cells of protozoans and several lower animals. Food vacuoles
are responsible for the digestion of food in the cells as they contain food enzymes.
The digested food then passes into the cytoplasm. Found in single -celled organisms
like Amoeba.

Centrioles
 A centriole is a small set of microtubules arranged in a specific way.

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 Their main purpose is to help a cell in cell division.
 They are found near the nucleus but can be seen only during the cell division.
 They are found in pairs and form a special substance called Centrosome which
appears near the nucleus.
 When the cell divides, the centrosome divides into two parts and each part moves
to opposite sides of the cell.

Figure 20 - Centrioles

Ribosomes
 They are cell organelles responsible for protein synthesis.
 Ribosomes can be found in both prokaryotes and eukaryotes because the synthesis
of proteins is important in both of them.
 In prokaryotes, the ribosomes float freely in the cytoplasm.
 In eukaryotes, they can be found floating in the cytoplasm or they are often
attached to the endoplasmic reticulum.
 The ribosomes attached to the ER synthesise proteins that are to be exported out of
the cell while the ribosomes floating inside the cell synthesise proteins that are
used inside the cell.

Peroxisomes
 Peroxisomes are small vesicles found in the cells.
 These enzymes are used to break the toxic materials inside the cell.
 They digest the fatty acids of the cell as well as amino acids by carrying out
oxidation reactions in the cell.
 They are also responsible for the digestion of alcohol in the human body. Hence,
the liver contains a large number of Peroxisomes.

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Figure 21 - Peroxisomes in a cell

CELL DIVISION

1. Introduction: It is the process by which a mature cell divides and forms two
nearly equal daughter cells which resemble the parental cell in a number of
characters.

2. Discovery: Prevost and Dumas (1824) first to study cell division during the
cleavage of zygote of frog.

3. Nagelli (1846) was the first to propose that new cells are formed by the
division of pre-existing cells.

4. Rudolf virchow (1859) proposed “omnis cellula e cellula” and “cell lineage
theory”.

5. A cell divides when it has grown to a certain maximum size which disturb the
karyoplasmic index (KI)/Nucleoplasmic ratio (NP)/Kernplasm connection.

6. Two processes take place during cell reproduction.

 Cell growth: (Period of synthesis and duplication of various components of
cell).
 Cell division: (Mature cell divides into two cells).

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7. Cell cycle: Howard and Pelc (1953) first time described it. The sequence of
events which occur during cell growth and cell division are collectively called
cell cycle. Cell cycle completes in two steps:
 Interphase
 M-phase/Dividing phase

(i) Interphase : It is the period between the end of one cell

division to the beginning of next cell division. It is also
called resting phase or not dividing phase. But, it is
actually highly metabolic active phase, in which cell
prepares itself for next cell division. In case of human
beings it will take approx 25 hours. Interphase is completed
in to three successive stages.
(a) G 1 phase/Post mitotic/Pre-DNA synthetic phase/Gap I st
(b) S-phase/Synthetic phase
(c) G 2 -phase/Pre mitotic/Post synthetic phase/gap-II nd
(ii) M-phase/Dividing phase/Mitotic phase
(a) Nuclear division i.e. karyokinesis occurs in 4 phases – prophase, metaphase,
anaphase and telophase. It takes 5-10% (shortest phase) time of whole division.
(b) Cytokinesis : Division of cytoplasm into 2 equal parts. In animal cell, it
takes place by cell furrow method and in plant cells by cell plate method.

8. Duration of cell cycle: It depends on the type of cell and external factors such
as temperature, food and oxygen. Time period for G 1 , S, G 2 and M-phase is
species specific under specific environmental conditions. e.g. 20 minutes for
bacterial cell, 8-10 hours for intestional epithelial cell, and onion root tip cells
may take 20 hours.

9. Regulation of cell cycle: Stage of regulation of cell cycle is G 1 phase during

which a cell may follow one of the three options.
 It may start a new cycle, enter the S-phase and finally divide.
 It may be arrested at a specific point of G 1 phase.
 It may stop division and enter G 0 quiscent stage. But when conditions
change, cell in G 0 phase can resume the growth and reenter the G 1 phase.
10. Cell division is of three types, Amitosis, Mitosis and Meiosis.
11. Difference between cell Mitosis and Meiosis

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S.No Characters Mitosis Meiosis
I. General
(1) Site of occurrence Somatic cells and during the Reproductive germ cells of gonads.
multiplicative phase of
gametogenesis in germ cells.
(2) Period of occurrence Throughout life. During sexual reproduction.
(3) Nature of cells Haploid or diploid. Always diploid.
(4) Number of divisions Parental cell divides once. Parent cell divides twice.
(5) Number of daughter Two. Four.
cells
(6) Nature of daughter Genetically similar to parental Genetically different from parental
cells cell. Amount of DNA and cell. Amount of DNA and
chromosome number is same as chromosome number is half to that
in parental cell. of parent cell.
II. Prophase
(7) Duration Shorter (of a few hours) and Prophase-I is very long (may be in
simple. days or months or years) and
complex.
(8) Subphases Formed of 3 subphases : early- Prophase-I is formed of 5
prophase, mid-prophase and late- subphases: leptotene, zygotene,
prophase. pachytene, diplotene and
diakinesis.
(9) Bouquet stage Absent. Present in leptotene stage.
(10) Synapsis Absent. Pairing of homologous
chromosomes in zygotene stage.
(11) Chiasma formation and Absent. Occurs during pachytene stage of
crossing over. prophase-I.
(12) Disappearance of Comparatively in earlier part. Comparatively in later part of
nucleolus and nuclear prophase-I.
membrane
(13) Nature of coiling Plectonemic. Paranemic.
III. Metaphase
(14) Metaphase plates Only one equatorial plate Two plates in metaphase-I but one
plate in metaphase-II.
(15) Position of centromeres Lie at the equator. Arms are Lie equidistant from equator and
generally directed towards the towards poles in metaphase-I while
poles. lie at the equator in metaphase-II.
(16) Number of Two chromosomal fibre join at Single in metaphase-I while two in
chromosomal fibres centromere. metaphase-II.
IV. Anaphase

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(17) Nature of separating Daughter chromosomes Homologous chromosomes
chromosomes (chromatids with independent separate in anaphase-I while
centromeres) separate. chromatids separate in anaphase in
anaphase-II.
(18) Splitting of Occurs in anaphase. No splitting of centromeres. Inter-
centromeres and zonal fibres are developed in
development of inter- metaphase-I.
zonal fibres
V. Telophase
(19) Occurrence Always occurs Telophase-I may be absent but
telophase-II is always present.
VI. Cytokinesis
(20) Occurrence Always occurs Cytokinesis-I may be absent but
cytokinesis-II is always present.
(21) Nature of daughter 2N amount of DNA than 4N 1 N amount of DNA than 4 N
cells amount of DNA in parental cell. amount of DNA in parental cell.
(22) Fate of daughter cells Divide again after interphase. Do not divide and act as gametes.
VII. Significance
(23) Functions Helps in growth, healing, repair Produces gametes which help in
and multiplication of somatic sexual reproduction.
cells.
Occurs in both asexually and
sexually reproducing organisms.
(24) Variations Variations are not produced as it Produces variations due to crossing
keeps quality and quantity of over and chance arrangement of
genes same. bivalents at metaphase-I.
(25) In evolution No role in evolution. It plays an important role in
speciation and evolution.

Types of Mitosis
 Anastral mitosis: It is found in plants in which spindle has no aster.
 Amphiastral mitosis: It is found in animals in which spindle has two asters,
one at each pole of the spindle. Spindle is barrel-like.
 Intranuclear or Promitosis: In this nuclear membrane is not lost and
spindle is formed inside the nuclear membrane e.g. Protozoans (Amoeba)
and yeast. It is so as centriole is present within the nucleus.
 Extranuclear or Eumitosis: In this nuclear membrane is lost and spindle is
formed outside nuclear membrane e.g. in plants and animals.
 Endomitosis: Chromosomes and their DNA duplicate but fail to separate
which lead to polyploidy e.g. in liver of man, both diploid (2N) and

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BIOLOGY CLASS- IX th 8368723015
polyploid cells (4N) have been reported. It is also called endoduplication
and endopolyploidy.
 Dinomitosis: In which nuclear envelope persists and microtubular spindle is
not formed. During movement the chromosomes are attached with nuclear
membrane.

Types of meiosis:
On the basis of time and place, meiosis is of three types
 Gametic/Terminal meiosis: In many protozoans, all animals and some
lower plants; meiosis takes place before fertilization during the formation of
gametes. Such meiosis is described as gametic or terminal.
 Zygotic or Initial Meiosis: In fungi, certain protozoan groups, and some
algae fertilization is immediately followed by meiosis in the zygote, and the
resulting adult organisms are haploid. Such a meiosis is said to be zygotic or
initial. This type of life cycle with haploid adult and zygotic meiosis is
termed the haplontic cycle.
 Sporogenetic Meiosis

(a) Diploid sporocytes or spore mother cells of sporophytic plant, undergo

meiosis to form the haploid spores in the sporangia.

(b) Haploid spore germinates to form haploid gametophyte which produces the
haploid gametes by mitosis.

(c) Haploid gametes fuse to form diploid zygote which develops into diploid
sporophyte by mitotic divisions. e.g. in higher plants like pteridophytes,
gymnosperms and angiosperms.

SCIENCE & BIOLOGY BY NITISH SIR (8368723015)