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Microscopes

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Contribution of microscope to the expansion of knowledge on cells and cellular organization
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Advancement of the cytology is mostly based on the microscopy.
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microscope.
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The discovery and early study or cells progressed with the invention of

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…………………………………………………………………………………….. Anton Van Leeuwenhoek

Light microscope

 Visible light is passed through the specimen and then through glass lenses.
 The lenses refract the light in such a way that the image of the specimen is magnified as it is projected into the
eye.
 The simplest microscope is a single lens.

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Clifford Dobel: plate showing the drawings of bacteria from the human
mouth

2006 A/L MCQ (40)

Microorganisms were first observed and recorded by
1) Louis Pasteur in France
2) Robert Coch in Germany
3) Anton Van Leeuwenhoek of Holland
4) Robert Hook in England
5) Paul Ehrlich in Germany

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 Compound light microscopes are commonly used in school laboratories and it is used in medical laboratories as a
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diagnostic tool.

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The compound light microscope

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 Resolution power and magnification are important parameters which can be seen in a microscope.

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Magnification is the ratio of an object's image size to its actual size.
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of the specimen
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The maximum magnification of light microscope is 1000 times, the actual size

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Light from an object (specimen on the slide) passes first through objective lens ,then produce a magnified image.
Above image then acts as an object for the second lens (the eye piece lens) which further magnifies it.
 The total magnification is hence the product of the magnification of each lens.

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 Eg: If magnification of objective lens = x 40, eyepiece lens = x 15

 Total magnification is = 15  40 = x 600

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Maximum effective magnification of a light microscope is 1000.
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Resolution power is minimum distance between two points that can be distinguished as separate points.
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resolution power of light microscope is 0.2 .
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resolution is a measure of the clarity of the image.
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Due to the limitation in resolution, the magnification cannot be increased above  1000.

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j Staphylococcus bacteria under the high power of

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light microscope

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Staining
Most biological structures are colourless , transparent ,hence some means of obtaining contrast between different
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structures must be employed. The most common method is staining.
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Stain
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Structure 7 Final colour
Haemotoxylene
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Nucleus Blue
Eosin a
Cytoplasm/cellulose Pink/red
Methyline blue
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Saffranin Lignin/suberin Red
Aneline sulphate Lignin Yellow
Feulgens DNA Red/purple
Iodine solution Starch Dark blue / purple

Questions

1. An electron micrograph shows this scale bar that measures 2 cm 1µm

What is the magnification of the electron microscope?

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1) 2x10-2 2) 2x102 3) 2x103 4) 2x104 5) 2x10-4

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2. In an electron micrograph , a mitochondrion measures 36mm long by 21 mm wide.
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If the magnification of the micrograph is x 30 000 what are the actual dimensions of this organelle?

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1) 0.12 x 0.07 µm 2) 0.36 x 0.21 µm 3) 1.20 x 0.70 µm
4) 2.60 x 2.10 µm 5) 3.60 x 2.10 µm

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3. How many times is the resolution power of compound light microscope greater than that of the unaided human
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eye?
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4. How many times is the maximum resolution of transmission electron microscope greater than that of compound
light microscope?

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The Electron Microscope

 The limitation imposed upon the resolving power of the light microscope by the wavelength of light.
 The resolution power is inversely proportional to the wavelength.
 Due to this scientists considered the use of other forms of radiations with comparatively shorter wavelengths.
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As a result, electron microscopes were developed. In electron microscopy, a beam of electrons is focused through

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the specimen or on to its surface.
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practice it magnifies just over
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This means that in theory the electron microscope should be able to magnify objects up to
times.
times. In

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Electron microscopes have revealed many organelles and other sub cellular structure that were impossible to
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resolve with the light microscopes.
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There are two types of electron microscopes.

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1. Transmission electron microscopes (TEM).
2. Scanning electron microscopes (SEM)
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Transmission electron microscopes

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It is used to study the internal structures of cells.
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In this microscope, a beam of electrons is passed through a thin, especially prepared slice of material.
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A very thin specimen is used. Specimens stained with heavy metals which attach more to certain cellular

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structures than other areas.

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Image reflects the pattern of electrons passed through the specimen, displays on a screen.
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While electrons pass through the specimen, more electrons may get displayed in regions where structures were
densely stained.
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Scanning electron microscopes

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Specimen is mostly coated with gold prior to observation.
In this instrument a fine beam of electrons is reflected from the surface of specimen.
 Here the specimen scatters many electrons whereas others are absorbed.
 This instrument is ideal to observe the surface view in three dimensional appearance.

Differences between light and electron microscope

Light Microscope Electron Microscope

Glass lenses are used to focus the light rays. Powerful magnets are used to focus beam of electrons.

Image is directly detected by naked eye.

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Not directly detected by naked eye, micrographs are
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Living and non-living objects can be observed.

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Actual colour of the object can be observed. un Only non-living objects are observed.
Actual color cannot be observed.

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Dyes used to stain the object.
a j Heavy metals are used to stain the object.

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R Transmission electron microscope (TEM)

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Scanning electron microscope (SEM)

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Scanning electron microscope

Historical background of cell and the structure and functions of the sub cellular units

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Anton Van Leeuwenhook (1650), a contemporary of Robert Hooke, Was the first to describe and record living

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single celled organisms, Euglena & bacteria. He invented the compound light microscope.
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Robert Hooke (1665) examined a cork tissue using compound microscope and gave the term "CELL" to describe

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the basic units.
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Matthias Schleiden (1831), a botanist, studying plant tissue concluded that all plants are made up of cells.
Theodore Schwann a zoologist (1839) concluded that animal tissues are also made up of cells.

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Rudolf Virchow (1855) showed that all cell arise from pre-existing cells by cell division,

Cell theory

 Schleiden, Schwann and Virchow presented the 'Cell Theory' which included the following.
1. All organisms are composed of one or more cells.
2. The basic structural and functional unit of organisms is the cell.
3. All cells arise from pre-existing cells.

 All organisms are composed of cells.

 Recall the hierarchy of life, the levels of organization mentioned earlier.
 The basic unit which can be called "living" is the cell, which may from a single celled organism
(e.g., Chlamydomonas, Yeast) or a multicellular plant or animal.
 The cell is the basic structural and functional unit of life.
 The level of organization of matter represented by a cell show all the characteristics of life.
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Any stage below level of a cell cannot be considered living, whether it is a single celled organisms or multicellular
plant or animal.

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Organization of cells

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Cells are the basic structural and functional unit of all organisms.
There are two kinds of cellular organization – Prokaryotic and Eukaryotic
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All cells share certain basic features. They are; 7
 All cells are bounded by a plasma membrane which is a selective barrier.
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 Within the cell have, a semifluid, jelly like substance which is called cytosol.
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 Subcellular components are suspended within the cytosol.
 They have carry DNA as genetic materials.

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 Ribosomes are found in all cells.

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Bacteria and Archaea are prokaryotic cells. all the other organisms have eukaryotic cells.
Use the electron micrographs of plants cells and animal cells to study the above characteristics.

Prokaryotic Cellular Organization

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Eukaryotic Cellular Organization
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Structures and functions of organelles and other sub cellular components

Plasma membrane
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 Plasma-membrane is the outer limit of cytoplasm.

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All cellular membranes resemble the ultra-structure of plasma membrane.
In 1972, Singer and Nicolson put forward the fluid mosaic model of cell membrane.
 It is mainly composed of;
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2. Protein
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1. Phospholipids (most abundant type of lipid in plasma membrane)

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The Plasma membrane has the following features.
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It is about 7nm thick.
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 It is mainly made up of a phospholipid bilayer.
 Phospholipids are amphipathic molecules.
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The hydrophilic heads of the phospholipids face outwards into the aqueous environment of both inside and outside
of the cell.
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 The hydrophobic hydrocarbon tails face inwards and create a hydrophobic interior.
 Plasma-membrane is compared to the fluid mosaic model.
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Since phospholipid molecules are moveable, they provide the fluid nature to the membrane.
Protein molecules embedded randomly contribute to its mosaic nature.
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Some of the protein molecules penetrate all the way through the membrane, called trans- membrane proteins and

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some others penetrate only part of the way into the membrane. These are called integral proteins.
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Most of the integral proteins are trans-membrane proteins which have hydrophilic channels.
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These act as pores through which ions and certain polar molecules can pass.
Some proteins are not embedded in the lipid bilayer at all, and are loosely bound to the inner surface of the

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membrane, called peripheral proteins.
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Some proteins and lipids have short branching carbohydrate chains like antennae, forming glycoprotein and

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glycolipids, respectively.
Animal’s cell membrane may contain few cholesterol molecules randomly integrated into the lipid bilayer.
These cholesterol molecules provide flexibility and stability to the membrane by reducing membrane fluidity at
moderate temperatures and prevent membrane solidification at low temperatures.
 The two sides of the membrane may differ in composition and function.

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Functions
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1. The cell membrane surrounds the cytoplasm of living cell physically separating the intracellular components from
the extracellular environment.
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2. Cell membrane is selectively permeable and able to regulate the exchange of material needed for survival.
3. Proteins embedded in the cell membrane identify the cell, enabling nearby cells to communicate with each other

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(involved in cell recognition).

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4. Some protein molecules act as receptor molecules for
interacting with specific bio-chemicals. Such as hormones
neurotransmitters and immune proteins.
5. Some proteins in the cell membrane attaching to some
cytoskeletal fibres and helps to maintain the shape of the
cell.
6. Some proteins in the membrane act as enzymes.
e.g: Microvilli on epithelial cell lining of some parts of
the gut contain digestive enzymes in their cell surface
membrane.

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Subcellular components
There are many sub-cellular components in the cell. Some of them are organelles, which are bound by membranes and

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suspended in the cytosol of eukaryotic cell to perform specialized functions.

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Nucleus
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 Most prominent organelle, consist most of the genes, having an average diameter of 5µm and enclosed by a double
membrane cover called nuclear envelope.

Nuclear envelope
 composed of two membranes, inner and outer membranes,
 separated by a space of 20-40 nm.
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 Nuclear envelope is perforated by nuclear pores which has pore complex to regulate the entry and exit of
substances.
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 It has nuclear lamina, made up of protein fi laments which line the interior side of the nuclear envelope.
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Nuclear matrix
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 made up of protein filaments and extended throughout the interior of the nucleus.
 Chromatin and nucleolus are embedded in the nuclear matrix.

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Nucleolus
 appears as darkly stained granules with fibers adjoining part of the chromatin.
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Chromatin
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 appears as a diffused mass in electron micrographs of non-dividing
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 cells. It is a complex of DNA and proteins. During nuclear divisions, chromatin
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 has a constant number of chromosomes. a
 condenses, tightly coils and form threads, called chromosomes. Each species

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 (e.g. typical human cell has 46 chromosomes).

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Functions
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Control all cellular activities.
2. Synthesize DNA to produce new nuclei for cell divisions.
3. Synthesize r-RNAs and ribosomal subunits required for protein synthesis, through nucleolus.
4. Synthesize mRNA and t-RNA according to the information present on the DNA.
5. Store and transport genetic information.

Ribosomes

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There are sub cellular components which carryout protein synthesis.
They consist of two subunits; larger subunit and smaller subunit.
 They are composed of rRNA and protein.
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According to the size, ribosome are found in two types; 70S and 80S.
70S ribosomes are found freely on the cytoplasam of prokaryotes, Matrix of mitochondria and stroma of

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chloroplasts.
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80S ribosomes are found only in eukaryotes.
Based on their nature of presence, they are categorized as two types; free ribosomes and bound ribosomes.
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Free ribosomes: freely available as group in cytoplasam.
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Bound ribosomes are attached to the membrane surface of rough endoplasmic reticulum.

Functions

 Protein synthesis

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Endoplasmic reticulum

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It is a network of internal membranes forming flattered or tubular sacs separating cytosol from ER lumen.
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It is continuous with the outer membrane of nuclear envelope.

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There are two types of ER; Rough ER and Smooth ER.

Rough ER
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RER is a network of flattened sacs , and ribosomes bound to surface.
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proteins synthesized by ribosomes move in to lumen of ER.

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 Transport protein synthesized by ribosomes
 Synthesizing glycoproteins
 Produce transport vesicles
 Facilitate the growth of own membrane by adding phospholipids proteins and carbohydrates. Therefore called as
membrane factory

Smooth ER

 Smooth ER is a network to tubular sacs without ribosomes.

 Membrane bound enzymes are present.

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Functions
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• Metabolism of carbohydrates. r un
• It synthesizes lipids including oils, steroids and phospholipids.

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• Produce transport vesicles to transport within cell.
• Involves in detoxification.
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• Stores Ca2+ ions.
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Golgi apparatus

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 Golgi complex is s stacks of flattened sacs or Cisternae.

 Inner and outer surfaces can be identified as cis face and trans face.
 Cis face is located near the E.R. to receive vesicles from E.R.
 Trans face give rise to secretor vesicles which budded off and travel other side.
 Golgi complex is abundant in secretary cells.
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Functions
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• Collecting, packaging and distribution of materials

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• Manufacturing cellulose and non cellulose cell wall components such as pectin
• Produce lysosomes
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Lysosomes R
 They are single membrane bounded vesicles contributing to digestive activity.
 They contain hydrolytic enzymes which catalyze breakdown of carbohydrates, proteins, lipids and nucleic acids.

Functions
 Digest food particles received by phagocytosis.
 Transport residue material out of cell by exocytosis.
 Digest worm out organelles.
 Autolysis causing cell death.

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Peroxisome
 They are single membrane bounded vesicles with oxidizing enzymes.
 They are present in both plants and animals.
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 Enzymes in peroxysome catalyze the break-down of
-4 7 .

Functions
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 Detoxification of peroxides
 Photorespiration in plants
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Specialized peroxysomes called glyoxysomes are found in fat storing tissues in


plants.
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Glyoxysomes converts fatty acids into sugar.

Mitochondria

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 It is one of the most common organelles in eukaryotic cells.

 it is an elongated organelle with two enclosing membranes.
 Outer membrane is smooth but the inner membrane is convoluted to form cristae.
 Cristae increase the surface area and they contain stalk particles.
 The gap/space in between inner and outer membranes of the mitochondrion is called inter-membrane space.
 The inner most part of the organelle is known as mitochondrial matrix, which consists of 70s ribosomes circular
DNA molecule (mitochondrial DNA), phosphate granules and enzymes.
 The matrix carries enzymes for the reactions in Krebs cycle (in cellular respiration).
 Further, cristae composed of proteins and enzymes essential for electron transport chain and oxidative
phosphorylation.
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……………………………………………………………………………………………………………………………

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Functions
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 Synthesize ATP in aerobic respiration.
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 Involve in Photorespiration.

Chloroplast
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
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It is a biconvex lens shaped organelle with two membranes which s found in plants and some protists.


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The outer and inner membranes are smooth and are separated by a very narrow inter-membrane space.
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Inside the chloroplast there is another membrane system.


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This membrane produces flattened and interconnected sacks called thylakoids.
Thylakoids contain complexes called photosystems which are made up of photosynthetic pigments.

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Thylakoids stacked to form a granum.

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The grana are interconnected by inter-granal lamellae.

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The fluid outside the thylakoid is stroma which contain circular DNA (chloroplast DNA), 70s ribosomes, many
enzymes, starch granules and lipid droplets.

Functions
 Photosynthesis

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Cytoskeleton

 Cytoskeleton is the supporting structure of the cell and maintain its shape.
 It is more important for animal cells which lack cell walls. Cytoskeleton is made out of microtubules and protein
filaments. Additionally, it is Dynamic hence, has the ability to break and reform as needed.
 There are three types of component in the Cytoskeleton as follow;
Microtubules, Actin filaments of Microfilaments, Intermediate filaments

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Functions
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• Provide strength to the cytoplasm

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• Anchorage organelles and cytosolic enzymes of the cell
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• Movement of cytoplasm, cytoplasmic streaming, positioned organelles and move chromosomes when necessary.
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• Maintain the shape of the cell (mainly in animal cells)

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Cilia and Flagella

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 Cilia and flagella share a common internal structure.

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

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Flagella are long elongated structures and Cilia are short cellular projections that are often organized in rows.
Cilia are more numerous than flagella on the cell surface,they are made of microtubules, with a 9+2 structure

 un
(Nine doublets of microtubules are arranged in ring, with two single microtubules in its center).
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They are covered by plasma membrane and bound to a basal body which anchors the cilium or flagellum to the
cell. The Basal body has 9+0 arrangement (no microtubules in its center)
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Functions
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 Act as locomotor appendages.
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 Can move fluid over the surface of the tissue.
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 Cilia lining in oviducts help move an egg toward the uterus.
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Centrioles

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Centriole is made up of cylindrically arranged microtubules which are non membrane bounded subcellular

 R
component present only in animal cells.
Each centriole composed of nine sets of triplet microtubules arranged in a ring (9+0).
 A pair of centrioles which arranged perpendicular to each other are located in a region called centrosome near the
 nucleus.

Functions
 Produce aster and spindle in cell division.

Central Vacuole

 central vacuole is a large structure, bound by tonoplast, filled with liquid called cell sap found in plant cells.
 the composition of sap differs from cytosol and it contains water, ions such as Potassium and Chloride and
sometimes water soluble coloured pigments such as anthocyanin.

Functions
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 Stores waster and other materials such as sugars, ions and pigments.
 Maintains water balance of the cell.
 Gives turgidity and support to cell. -4 7
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 Produce colours in some plants with sap pigments.
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 Stores soluble substances needed for cellular activities.
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Extracellular components

Cell wall
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Cell wall is an extracellular structure of plant cells.
Animal cells do not have cell walls.
 However, prokaryotes, fungi and some protists also have a thin and flexible cell wall.
 The chemical composition of the wall greatly varies from species to species and even from one cell type to
another even in the same plant.
 Nevertheless in Plants, cell wall is generally made up of cellulose, pectin, hemicelluloses, lignin and suberin
(in some plant cells only).

 Plants generate two types of cell walls: primary and secondary walls.
 Young cells first secrete primary cell wall: it is the wall laid down during plant cell division.
 Just out side the primary wall there is a thin layer (middle lamella) which is rich in sticky polysaccharides called
pectins (magnesium and calcium pectate).
 Middle lamella glues adjacent cells together.
 Due to the deposition of hardening substances on the primary wall a secondary cell wall is generated secondarily.
Primary cell wall is permeable, relatively thin, flexible, composed mainly of cellulose fibers which are laid

3 1
unevenly running through the extracellular matrix (middle lamella) , water can move freely through the free
spaces of cell wall.
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
 un
Secondary cell wall lies between plasma membrane and primary cell wall.
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
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It contains several layers of hard materials, forming a rigid structure.
in addition to cellulose, impermeable substances such as lignin and suberin are also incorporated in to the
secondary wall.
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


support.
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Lignin cement anchors cellulose fibers together providing hard and rigid matrix, giving the cell wall an extra

Cell wall has pits through which cytoplasm of adjoining cells join through plasmodesmata.

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Functions
 Protection and support.
 Allow development of turgidity when water enters the cell.
 Prevents bursting during turgidity.
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 Limits and control cell growth.
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 Components of apoplast pathway.
 Maintaining cell shape.
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 Hold the plant up against the force of gravity.
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Cell junctions
 Cell junctions are structures at which neighbouring plasma membranes are joined.
 They are also interact and communicate via sites of direct physical contacts.

Functions
 Connects the internal chemical environment of adjacent cells.
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
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There are three types of cell junctions in animal cells.

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1. Tight junctions
 connect the plasma membranes of adjacent cells tightly bound by specific proteins forming continuous seals
around the cells.
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 prevent leakages of extracellular fluids through intercellular space
 e.g. skin epithelium.
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2. Desmosomes / Anchor junctions
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 E.g. muscle tissue.
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 mechanically attach the cytoskeletons of adjoining cells by intermediate filaments for strong binding.

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3. Gap junctions / communicating junctions
 provide cytoplasmic channels from one cell to an adjacent cell.
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 Gap junctions consists of special membrane proteins that surround the pore through which ions, sugars amino
acids may pas.
 They allow signal and material exchange between adjacent cells through direct connections.
 e.g. heart muscles, animal embryo.

Plasmodesmata

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 Microscopic channels which runs through plant cell walls.
 They are cytoplasmic living connections between cytoplasam of adjoining cells.
 these are membrane lined channels filled with cytoplasam.

Extracellular matrix of animal cells.

 Although animal cells lack cell walls they do have elaborate extracellular matrix (ECM).
 Main components of the ECM are glycoproteins and other carbohydrates containing molecules secreted by the
cells.
 most abundant glycoprotein in the ECM of most animal cell is collagen which forms strong fibres outside the cell.

 Elastin fibers are also embedded in proteoglycan.
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The collagen fibres are embedded in a network woven out of proteoglycan secreted by cells.

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Functions
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 Forms a protective layer over the cell surface.
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 Linking extra cellular matrix and cytoskeleton.
 Influence the cell behaviour by involving in the mechanical and chemical signaling.
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Cell cycle and the process of the Cell division

Cell cycle
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 The sequence of events that takes place in the life of a cell from the end of one cell division to the end of the
next cell division is referred to as cell cycle.
 At the end of the cell division, two genetically identical daughter cells resembling the parent cell are produced in
mitosis.

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Eukaryotic cell cycle

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 Eukaryotic cell cycle may divide into two major phases.
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1. Interphase
2. Mitotic phase/ M-phase
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
 t h
Interphase is the longer phase of cell division.
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It covers about 90% of the cell cycle.

1.
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Interphase could be divided into three phases;
G1 phase (first gap phase)
2. S phase (synthetic phase)
3. G2 phase (second gap phase)

G1 phase

 In this phase synthesis of proteins and production of cellular organelles leading to cell growth occur.
 Proteins essential for S phase are produced during this phase.

S phase

 DNA replication occurs and synthesis of histone proteins takes place.

 DNA wind around histone beads and form chromatin.
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G2 phase
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
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Cells continue to grow through protein synthesis as well as cellular organelles.


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Proteins essential for mitotic phase will be synthesized.
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Duplication of centrosomes takes place in animal cells.

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Cell cycle control system

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 There are cell cycle-controlling checkpoints available at G1, G2 and M phases to ensure that the cell is
ready for moving into upcoming phases of cell division.
 Some cells receive a go-head signal at the G1
 check point, it will usually complete the G1, S, G2 and M phases and divide.
 If it does not receive a go head signal at that point it may exit the cycle,
 entering into a non dividing stage called the Go phase.
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 The most cells of the human body are actually in the Go phase. e.g. nerve cells and muscle cells.

r un Mitotic phase/ M phase


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M phase covers only about 10% of cell cycle.

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This includes mitosis and cytokinesis
a
i t h Mitosis


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Mitosis is referred to the nuclear division which gives rise to two genetically identical daughter nuclei from a
mother nucleus.
 This may get divided into five stages; in order to ease the learning of activities of cell cycle.

1. ……………………………………………..
2. ……………………………………………..
3. ……………………………………………..
4. ……………………………………………..
5. ……………………………………………..
,
1. Prophase

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Chromatin fibers get condensed by shortening and thickening and transformed into chromosomes.


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As a result chromosomes will be visible through light microscope.
Nucleoli get disappeared and chromosomes appear with two sister chromatids attached at the centromere.

 un
Chromosomal arms of sister chromatids attached by special proteins called cohesine.
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
H a
The formation of mitotic spindles begins.
Spindle is formed by accumulated microtubule complex which includes the centrosomes , the spindle
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microtubules and the aster.

 ti ha
Centrosomes move toward opposite poles of the cell due to the lengthening of microtubules between them.
centrosomes or centrioles are not available in plant cells . however spindle is formed during cell division from
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accumulated microtubule complex.

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2. Prometaphase

 The nuclear envelope fragments.

 Chromosomes get even more condensed.
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 A special protein called kinetochore attaches the sister chromatids of each chromosome at their centromere.
 Some of the microtubules that attach to the kinetochore of the chromosomes move the chromosomes back and
forth.
r un
a
 Microtubules which are not attached to the kinetochore interact with those from the opposite poles.
H
3. Metaphase
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 R
Centrosomes reach the opposite poles.
The chromosomes have arrived to a place called metaphase plate which is located in equal distance from each
pole. The centromeres of all chromosomes are located in the metaphase plate.
 At the end of this phase, each chromosome of the cell get attached to the kinetochore microtubule at their
centromere and aligned at the metaphase plate.

4. Anaphase

 Sister chromatids are separated at the centromere.

 Microtubules attached to kinetochore get shorten and pull sister chromatids towards the opposite poles.
 Cell elongates as the non kinetochore microtubules are lengthened.

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By the end of anaphase equal and complete set of chromosomes found at each pole of the cell.

5. Telophase
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 r un
 Nucleoli reappears.
H a
Nuclear envelope reforms around each set of chromosomes at opposite poles.


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Spindle microtubules get deplolymerized.
a

 i t h
Chromosomes unwind and become less condense to form chromatin.
Two genetically identical daughter nuclei are formed.

Cytokinesis
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 The division of the cytoplasm starts at the end of the telophase.
 Therefore at the end of the mitosis two genetically identical daughter cells are produced.
 In animal cells- a cleavage furrow forms. This produces two genetically identical daughter cells.
 In plant cells- cell plate forms as a result of vesicle produced by Golgi apparatus.
 This divides the cytoplasm in to two and generates two genetically identical daughter cells to the parent cell.

Significances of mitosis

1. Maintains the genetic stability

2. Growth and development
3. Cell repair, replacement and regeneration
4. Asexual reproduction
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1
Meiosis
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Sexually reproducing organisms undergo different type of cell division called meiosis.
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H
from a diploid mother nucleus. a
Meiosis is a type of nuclear division which gives rise to four haploid, genetically non identical daughter nuclei,


j
Meiosis involves two consecutive nuclear divisions, Meiosis I and Meiosis II.
a
 i t h
Meiosis I is a reduction division and Meiosis II is similar to mitosis,


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each stage consists of four sub phases: prophase, metaphase, anaphase, and telophase.
Before meiosis one cell is in interphase, during S phase of the interphase DNA replication occur. (refer interphase
of mitosis)

Meiosis I

1. Prophase I

 Cell enters to the prophase I from interphase.

 Chromosomes begin to condense.
 Nucleolus begins to disappear.
 Next the formation of zipper like structure called the synaptonemal complex by a specific proteins holds two
homolgs tightly together.
 The pairing and physical connection of homologous chromosomes is called synapsis.

1
During synapsis part of the DNA molecule of non-sister chromatids paired homologous chromosomes break,
exchange and rejoin at corresponding point.
3
 This process is called crossing over.
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

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These points of crossing over become visible as chiasmata after the synaptonemal complex
dissembles and the homologous chromosomes slightly apart from each other..


Nuclear envelop breaks.
H a
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Centrosomes move towards opposite poles forming spindle in animal cells.

ti ha
 The kinetochore of each homologue attach to microtubule from one pole or the other.
 The homologous pair then moves toward the metaphase plate.

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2. Metaphase I


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The pair of homologous chromosomes get arranged on the metaphase plate with one chromosome of each pair


faces each pole.
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
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Both chromatids of a homologue are attached to kinetochore microtubules
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from one pole and those of the other homolog are attached to kinetochore microtubules from the opposite pole.
a
Homologous chromosome arrange randomly at metaphase plate.
H
3. Anaphase I
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 i t h


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Kinetochore microtubules of the spindle get shorten.
Homologous pair separates and one chromosome of each pair moves towards the opposite pole.
Sister chromatids of each chromosome remain attached at the centromere and move as a single unit towards the
same pole.

3. Telophase I

 One complete haploid set of chromosomes accumulate at each pole.

 Nuclear envelope reforms aroundeach set of chromosomes.
 Nucleoli reappear.
 Spindle disintegrates.
 Chromosomes decondensed into chromatin.

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Genetically non identical, haploid, two daughter nuclei are formed within one cell.

Cytokinesis I
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 r un

H a
Usually occurs simultaneously with telophase I.
Genetically non identical, haploid, two daughter cells

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are formed. In animal cells, cleavage furrow is formed.
a

 i t h
In plant cells a cell plate is formed.
No DNA replication occurs between meiosis I and meiosis II
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Meiosis II
1. Prophase II


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Centrosomes start producing spindle apparatus (spindle fibers, aster centrosome).
Chromatin fibers condense and produce chromosomes with two sister chromatids.

 -4
Nuclear envelope breaks down into fragments. 7

Nucleolus disappears.

r un
During the late prophase II centromere of the chromosomes are moved to the metaphase II plate.

H a
2. Metaphase II
a j

i t h
All Chromosomes get attached to the microtubules at their centromere and aligned on the metaphase plate.



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Kinetochores of sister chromatids are attached to microtubules extending from both poles.
Due to the crossing over in meiosis I, the two sister chromatids of each chromosome are not genetically identical.
Meiosis II usually takes place in the perpendicular direction of Meiosis I.
 Therefore, metaphase plate of meiosis II is perpendicular to the metaphase plate of meiosis I.

3. Anaphase II
 Due to the breakdown of proteins attaching sister chromatids, they are separated at centromere.
 As a result of shortening of microtubules , sister chromatids of each chromosome move towards opposite
poles.

4. Telophase II
 Nuclear envelope and nucleolus reform.
 Chromosomes decondense into chromatin.
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Spindle disassembles.
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Genetically non identical, haploid, four daughter nuclei are formed from one parent cell.

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Cytokinesis

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 Cytokinesis occurs as in mitosis. a
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 Genetically non identical, haploid, four daughter cells are formed.
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 These four daughter cells are not even identical to their parent cell.

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Significance of meiosis

1. Maintain the constant number of chromosomes through generations in sexually reproducing species.
2. Produce new genetic variations leading to evolution.
3. Genetic variation occur due to crossing over ,recombination and independent assortment.

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Tumor, cancer and galls

Tumors and cancers in animals
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Cell division is driven by external and internal factors.

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They may be chemical or physical factors
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Cancer cells do not respond to normally to the body’s control mechanism
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They divide excessively and invade other tissues. If unchecked they can kill the
organism.
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Cancer cells do not consider the normal signals that regulate the cell cycle.
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They do not need growth factors. They may make required growth factors

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themselves or giving signals to continue for the cell cycle without growth factors.
they possess abnormal cell cycle control system.
The problem begins when a single cell in a tissue undergoes transformation, the process converts a normal cell to
abnormal cell.
 If the body immune system cannot recognize and destroy it this leads proliferate cells and form a tumor.
 If the abnormal cells remain at the original site, the lump is called benign tumor.
 Most benign tumors do not cause serious problems and can be completely removed by a surgery.

 A malignant tumor becomes invasive and attack one or more organs.

 An individual with a malignant tumor is said to have a cancer.
 A few tumor cells may separate from the original tumor, enter blood vessels or lymph vessels and travel to other
parts of the body.
 They may proliferate and form a new tumor.
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This spread of cancer cells to locations distant from their original site is called metastasis.
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Galls in plants

 This occurs due to uncontrolled mitotic division of plant cell.

 The plant cell division is controlled by maintaining a proper balance between plant growth regulators such as

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auxins and cytokinins.
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When this balance is lost plant cells produce undifferentiated mass of cells.
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Galls are the bumps and growths that develop on different parts of plants

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after being invaded by some very unique organisms.
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Galls have range of causes, including viruses, fungi, bacteria, insects and

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mites.
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Usually the gall causers in some way attack or penetrate the plants growing

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tissues and causes the host to reorganize its cells and to develop an
abnormal growth

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Dr.Charitha Munasinghe | Biozone 36