CYTOLOGY

Cytology or cell biology
The branch of biology which deals with the study of all aspects of the cell is known as cell biology. Previously the term cytology was used for the study of cell structures that can be observed under compound microscope but now it is used as synonym of cell biology.
Discovery of cell
An English scientist, Robert Hooke, discovered the cells in 1665 A.D. He examined thin slices of cork under his microscope and observed that the cork was composed of box like compartments which were termed as the 'cells' (Latin: cellula = hollow space) by him. He published his work in a book Micrographia which drew immediate attention of the scientists all over the world.

Some other discoveries in cell biology
Antony von Leeuwenhoek (1672) a Dutch scientist, described some protozoan, bacteria, spermatozoa, red blood cells etc. for the first time.

Corti (1772) observed the presence of some sort of substance within the cell.


Lamarck (1809) wrote, "Every living organism is essentially a mass of cellular tissue in which more or less complex fluid move."

H.J. Dutrochet (1824), a French scientist, builds some tissues in an acid and separated the cells. He believed that tissues are made up of cells.


Felix Dujardin (1835) showed that a cell contains a living substance ‘sarcode’ in it.

J.E. Purkinje (1840) used the term "Protoplasm" for the cells contents.


Vol Mohl (1846) described the protoplasm as the fundamental substance of living beings and used this name for the embryonic cells of the plants.

M. Schuttze (1861) proposed protoplasm theory which states that "a cell is accumulation of living substance or protoplasm limited by an outer membrane and having nucleus". The portion of the protoplasm between membrane and nucleus is cytoplasm and within the nucleus is nucleoplasm.


J.S. Huxley (1868) and O. Hertwig (1892) described protoplasm as “physical basis of life”.

In 1831, Robert Brown discovered nucleus in the cells of the orchid roots which were later in 1880 described by Strasburger in detail.

Cell as a basic unit of life
Living things are composed of material units called cells. But there are many organisms like Amoeba, Bacteria, Chlamydomonas, Paramecium etc. whose bodies are made up of single cell only i.e. unicellular and almost all life processes occur within the cell i.e. intracellular. Contrary to this, there are many organisms which are made up of number of cells i.e. multicelular and there is division of labours in the cells constituting the body. Due to this division of labours, the cell in the body of multi-cellular organisms undergoes a wide range of differentiation and specialization and coordinates their activities which maintain their life. There is a definite hierarchy in the body organization of most organisms which can be shown as:

Cell Tissues Organ Systems Organization.
In animal:
Cells Tissues Organ System an individual

RBCS Nerves Stomach. Digestive

WBCS Muscles Pancreas Circulatory
Bones Liver
In Plant:
Cells Tissues Organ System an entire Plant
Meristematic Parenchyma Leaf Root system Plant
Parenchyma Cellenchyma Flower Shoot system
Cellenchyma Xylem
Companion Cell Phloem

Various organ systems further work in a coordinated manner to form an individual of the living world. In this way cell is a basic unit of structure and function in the bodies of all living-beings.

Concept of cell –theory
After the discovery of the nucleus two biologists, Matthias J. Schleiden, a German botanist, and Theodor Schwann, a German zoologist, postulated a theory known as the cells theory or cell doctrine which were based on the works of their predecessors. With the works of many cytologists like Virchow, Nageli and Pasteur the cell theory was modified. The modern versions of the cell theory are as follows:
All living organisms are composed of cells and their products.
New cells arise from pre-existing cells only.
All cells are basically similar in their chemical compositions and metabolic activities.
The function of an organism as a whole is the outcome of the activities and interactions of the cells constituting the body of that organism.
The cells are structural and functional units of all living organisms.
Growth of an organism occurs by cellular growth in unicellular, or by cellular multiplication in multicellular.
All the living cells are totipotent.

Exceptions to cell theory
1. Viruses are living organisms but they lack the internal organization which is found
in the cells i.e. they are acellular.
2. Certain algae such as Vaucheria and certain fungi, such as Rhizopus are unexplainable according to cell theory because their bodies are made up of the undivided mass of protoplasm in which many nuclei remain scattered.
3. The bacteria and blue green algae also have no true cell and in them the nuclear material doesn't remain bounded by the nuclear membranes and have direct contact with that of cytoplasm.

The cell as a self contained unit
In unicellular organisms, all the life activities occur within the cell i.e. intracellular. A cell is also able to regulate its own life span. In other word, a cell also has total independent existence. That is why a cell is regarded as a self contained unit. But in multicellular organisms the cells interact and cooperate with one another to give rise to a cumulative effect exhibiting life process. In this case, the cells develop specialized structures and functions leading to the division of labours e.g. some cells are responsible for energy release processes, some for reproduction etc.
If cells in them are isolated, they show the capacity for independent existence and multiplication. This can be demonstrated by cell culture techniques.

Cellular totipotency concept
Every vegetative cell of a plant body has all the potentialities to give rise to a new plant because each cell is derived from zygote and has inherent capacity to produce the whole organism. This is called cellular totipotency. The first evidence of cellular totipotency was given by F.C. Steward and his coworkers in 1950. They grew small pieces of mature carrot roots in a liquid medium supplemented with coconut milk. By gently shaking the medium, the cell and cell clusters became free from one another. Some of them multiplied and formed rooting clumps. When these were transferred to the tubes containing another medium, they gave rise to whole plants that flowered and set seeds.

Differences between unicellular and multicellular organisms
Unicellular Organism
Multicellualr Organism
Body is made up of single cell.
Body is made up of many cells.
All the life activities are carried out by single cell.
The specialized cells carry out different life processes.
Mitosis cell division helps in reproduction in these organisms.
Mitosis helps in growth and development.

Flow of energy through the cell
The law of conservation of energy states that energy can neither be created nor destroyed but it can be transformed from one from to another. Similarly, according to second law of thermodynamics, i.e., Physical law of entropy all systems (both living and non-living objects) need a constant supply of energy for their existence and maintenance. If they are left to themselves (no free energy provided) they tend to disorganize and disintegrate finally leading to death, the highest state of entropy. Therefore all living organisms must get a constant supply of energy to prevent entropy.

The source of energy to the living world is sun i.e., solar energy. This energy is trapped by green plants only which convert solar energy into chemical energy by a process photosynthesis. This chemical energy is passed on to the other non green organisms and there it is oxidized to provide usable form of energy. This is done in the process of respiration. The energy which a living organisms use is a chemical form of energy i.e. adenosine triphosphate (ATP). It is in this form only that energy can be easily stored, transferred and transformed.

Flow of information within the cell
There are mainly two ways in which information is carried out within the cell.
Flow of genetic (intrinsic) information
DNA contains genetic information as the sequence of nitrogen bases for the synthesis of proteins or enzymes which control the physical as well as physiological characteristics of living beings. DNA transcribes the stored information into RNA molecules which translate the genetic information into specific proteins.

DNA Transcription RNA Translation Specific proteins.
Flow of extrinsic informations
Various hormones act as information molecules that bring the information into the cell from outside. Some of them can directly enter into cell surface and act as 'membrane receptors'. Even such membrane bindings can change the cellular activities significantly.
Common features of the cells
Though cells show great variation in morphological and organizational aspects, they all exhibit fundamental similarities very common to them which are as follows:
All the cells possess nucleic acids DNA and RNA
Structure of biological membranes and their properties are common.
Mechanism of aerobic respiration is uniform
Protein synthesis and nucleic acid synthesis follow the same pattern in all the cells.
Cytologists have recognized two basic types of cells on the basis of structural complexity which are prokaryotic and eukaryotic. These terms were suggested by Hans Ris in the year 1960.
Prokaryotic cell
The prokaryotic (Gk., pro. = primitive or before; karyon=nucleus) cells are the most primitive cells from the morphological point of view. They occur in bacteria, mycoplasma, and cynobacteria or blue green algae. They have primitive nucleus without membrane and also lacking in organelles like chloroplasts, mitochondria, golgibodies, lysosomes etc. The genetic information here is located mostly in the single chromosome consists of double strand of DNA and that also without histone protein. Mitotic apparatus and nucleolus are also absent. The prokaryotic cells are surrounded by cell wall made up of carbohydrates and aminoacids. The plasma membrane lying directly beneath the cell wall often forms intrusions into cytoplasm i.e. the mesosomes.
Eukaryotic cell
The eukaryotic cells (Gk; eu=true; karyon=nucleus) are found from algae to angiosperms in plants and protozoa to mammals in animals. They show high degree of organization. They possess extensive membrane system e.g. endoplasmic reticulum, golgibodies, well formed nucleus, plastids, mitochondria, lysosomes etc. Nuclei and more than one chromosome with DNA and histones are present.

A Comparison between prokaryotic and Eukaryotic cells
Features
Prokaryotic Cells
Eukaryotic Cells
1. Size
Mostly 1-10mm
Mostly 10-100mm
2. Cell wall
Amino acids and polysaccharides
Cellulose in green plant, chitin in fungi, absent from animal cells.



3. Cell-capsule
When present made up of mucopolysaccharides
Absent
4. Cell organelles


a. Mitochondria
b. ER
c. Vacuole
d. Lysosomes
e. Chloroplasts
f. Centrioles
g. Ribosomes
h. Microtubules
i. Nucleus

Absent
“
“
“
“
“
Present (70 S)
Absent
Represented by single circular DNA also called incipient nucleus or nucleoid.
Present
“
“
“
“ (only in plants)
“ (absent in higher plant)
“ 80 S and 70 S
“
With nuclear membrane, nucleolus, nucleoplasm and chromatin reticulum.
j. Flagellum
Simple structure composed of the protein flagellin
Complex 9+2 structure of tubulin and other proteins.
5. Respiration
many strict anaerobes (oxygen fatal)
All aerobes, but some facultative anaerobes by secondary modification.
6. Photosynthetic enzymes
Bound to plasma membrane as composite chromatophores
Enzymes packed in plastids bound by membrane.
7. Sexual system
Rare: if present one way (and usually partial). Transfer of DNA from donor to recipient cell occurs.
Both sex involved in sexual participation and entire genomes transferred; alternation of haploid and diploid generations is also evident.

Shape of the cells
The shape of various cells varies considerably according to their functional properties. Cells may be oval, round, plate like, tubular, cylindrical, polygonal, irregular etc. The cell shape is said to be controlled by several factors like function, age, cell wall, external pressure, location, tension and viscosity etc.

Size of the cells
The cells may be very small (microscopic) or very large (macroscopic). The eukaryotic cells are typically larger (mostly ranging between 10-100mm) than the prokaryotic cells (mostly ranging between 1-10mm). Size of the cells of the unicellular organisms is larger than a typical muticellular organism’s cells.
· The smallest cells so far known is of Micoplasma or PPLO i.e. Pleuropneumonia like organism which is 0.1-0.3 mm size.
· The egg of an ostrich is considered to be the largest cell (which measures 170x135 mm in size).
· The nerve cells are the longest in human body, measuring up to 90cm in length.
· Acetabularia, an alga consists of a single cell about 10 cm to 90 cm in length. Mean volumes of few cells have been given below:
Cell type
Mean volume (cubic micron)
Ostrich egg
1.1x1015
Men egg
5.0x1013
Human egg
1.4x106
Human sperm
1.7x101
Largest bacterium
7.0x100
Smallest bacterium
2.0x10-2
Smallest virus
7.0x10-7

Differences between a plant and an animal cell
Plant Cell
Animal cell
1. Cells are covered by a cell wall made up of cellulose.
1. A cell wall is absent
2. Centrioles with centrosome are absent except a few lower plants.
2. Centrioles with centrosome are present near the nucleus.
3. Plastids containing pigments are present
3. Plastids are absent
4. Golgi bodies are in the form of a number of unconnected units called dictyosomes.
4. Golgi bodies are localized and consist of connected complex.
5. Vacuoles are large in size and more in number
5. Vacuoles are either absent or a few only.




Cell


Cell wall

Protoplasm
Ø Middle lamella

Ø Primary wall

Ø Secondary wall


Cytoplasm

Nucleus


Plasma membrane

Cytosol

Cytoplasmic structure


Ectoplasm

Endoplasm



Cytoplasmic organelles (organoids)

Vacuole

Cytoplasmic inclusions
Ø Mitochondria
Ø Plastids
Ø Endoplasmic reticulum
Ø Golgi bodes
Ø Lysosomes
Ø Microbodies
Ø Ribosomes
Ø Centriole
Ø Flagella and Cilia
Ø Starch grain
Ø Glycogen granule
Ø Fat droplets
Ø Aleurone grain
Ø Mineral
An Outline of Cell


Structural organization of cell

Cell wall
It is the outermost, protective layer of a typical plant cell and taken as an outstanding point of difference between plant and animal cell. However naked protoplasm is seen in lower group of plants and even reproductive cells i.e. gametes of higher plants. Cell wall is the secretory product of the protoplasm. It is non-living and permeable layer formed at the time of cytokinesis. During cytokinesis, protoplasmic matters accumulate in the equatorial region in the form of small droplets, which cohere in course of time and form a continuous plate called cell plate.
This plate undergoes physical and chemical changes and is ultimately converted into the intercellular substance, the middle lamella. Middle lamella is optically inactive and colloidal in nature. It is composed of pectates of calcium and magnesium so hard in nature.

The protoplast goes on secreting cell wall materials on the middle lamella and ultimately a soft delicate wall is formed which is primary wall. It contains mainly of cellulose and pectic compound and may also contain other polysaccharides. This is really the first formed cell wall. The primary cell wall is optically active.

Primary wall gets more and more stretched during the growth of the cell. In course of time secondary cell wall materials are deposited on the primary wall. The secondary wall is mainly composed of cellulose, which may undergo other modifications. In many cells it is usually three layered, the layers differing in physical and chemical properties of which the middle one is the thickest. While secondary materials are deposited on the primary wall, small thin areas are left out. These areas are called primary pit fields through which fine fibres of cytoplasm pass from one cell to another and thus establish the organic continuity of protoplasm. These fibrils are called as plasmodesmata.

Due to localized thickening on primary cell wall the following peculiar design or patterns are produced:
Annular
Spiral or helical
Scalariform
Reticulate
Pitted (simple and bordered pits)
In most of the plant cells, the cell wall is made up of cellulose, hemicellulose, pectin and protein. In many fungi the cell wall is formed of chitin and in bacteria, the cell wall contains protein-lipid-polysaccharide complexes.

Cellulose: It is insoluble polysaccharide. It is the principal material in building up the framework of a plant. Normally cellulose wall is not digestible excepting in case of some fungi and bacteria it is not used as food. Cellulose wall is permeable to water and solutes.

Lignin: It occurs in the walls of the woody tissues. It can give sufficient strength and rigidity to the plant. Cellulose may be modified into lignin or may be associated with the same. With gradual lignification of the wall most cells loose protoplasm and ultimately become dead. It appears in the middle lamella and primary wall and later may even be found in the secondary wall. Like cellulose, lignin is also permeable to water and solute.

Cutin: It is waxy or fatty modification occurring on the outer wall of epidermal cells of aerial organs. It is impermeable.

Suberin: Fatty substance, an important constituent of the walls of cork cells. It is also impermeable.

Mucilage and gum: Mucilage and gum are substances related to carbohydrates of the cell wall. They can absorb water quite quickly and can retain the water. In plants inhabiting dry regions like deserts, mucilage is definitely helpful to germination. In aquatic plants mucilage protects them from insects.
Minerals: Silicon, oxalates of calcium, calcium carbonate, resin, tannins, fatty matters are also found in cell wall.




Plasma- Membrane
It is the outermost layer in animal cell while inner to cell wall in case of prokaryotes, fungi and plant cells. It is ultra thin (75-100A0 in thickness), living, elastic and selectively permeable. Chemically plasma-membrane is made up of lipid (20-40%), protein (59-75%) and carbohydrate (1-5%). Four major classes of lipids are commonly present in the plasma-membrane are phospholipids (most abundant), spingolipids, glycolipids and sterols (e.g. Cholesterol). Each phospholipid molecule consists of a polar head (hydrophilic) containing phosphate and two non polar hydrocarbon tails (hydrophobic) from the fatty acids used to make the molecule. According to the position in plasma-membrane, the proteins fall into two groups: extrinsic and intrinsic proteins. Carbohydrates are present as short, unbranched or branched chains of sugars (oligosaccharides) attached either to exterior ectoproteins forming glycoproteins or to the polar ends of phospholipids at the external surface of the plasma-membrane forming glycolipids.

Structures
Several types of models have been put forward to explain the structure of the plasma membrane. The most important are as follows:

1. Sandwich Model
This model was proposed by Danielli and Davson in 1935. It is trilaminar structure consisting of two layer i.e. bimolecular plate of phospholipids sandwiched by two surface-active layers (i.e. outer and inner layers) of protein molecules. They are arranged in such a way that the hydrophobic tails of the two layers come to lie nearby while the hydrophilic heads is directed towards the protein.

2. Fluid Mosaic Model
In 1972, Singer and Nicolson put forward the ‘fluid mosaic model’ of biological membrane, which is widely accepted one. According to this model lipids and proteins are arranged in mosaic fashion and so do not represent a rigid structure. They have described this model as ‘proteins icebergs in a sea of lipids’. This model thus explains the dynamic and functional properties of the membrane.

This model holds the concept that lipids and proteins are in a quasifluid structure and the proteins are able to move within the lipid layers. The lipid molecules show intermolecular movements or they may rotate about their own axis resulting in transfer of one side of bilayer to the other. The proteins are of two kinds:

Integral or intrinsic proteins: There are some large globular integral proteins which project beyond the lipid layers on both the sides. Other smaller globular integral proteins penetrate the phospholipids layers partially and are exposed on one surface only. The integral proteins account for 70% of the total membrane proteins. These proteins are tightly held by strong hydrophilic or hydrophobic or both types of interactions and cannot be easily separated by physical or chemical methods.

Peripheral or extrinsic proteins are superficially arranged on either side and can be easily separated. These proteins have enzymatic activities and also make the membrane selectively permeable. These proteins are commonly referred to as permeases. Some proteins and lipids have short branch carbohydrate chains like antennae, forming glycoproteins and glycolipid respectively. Membrane also contains cholesterol.

Functions
· It forms outermost protective covering of animal cell and protects from injury.
· It helps in active and passive transport of materials
· It helps in bulk transport i.e. endocytosis and exocytosis.
· Help in flow of information (extrinsic). Some of the informational molecules also bind with certain molecules present on the cell membrane. Then it becomes membrane receptor ultimately produce the cellular effect.
· Substances attached to cell membrane determine antigen specificity.
· In nerve cells, the cell membrane takes place in transmission of impulses.
· Plasma lemma provides sheaths for cilia and flagella.
· It helps in the movement of some cells by either developing undulations (e.g. fibroblasts) or pseudopodia (e.g. Amoeba).
[Figure Book no 1/ Page no 142/ fig no 5.16 B.]


Nucleus
It was first discovered by Robert Brown in 1831 A.D. It is the largest cell organelle typically about 10 mm in diameter and they were the first to be described by light microscopists. They uniformly occur in all eukaryotic cells except in mature sieve tube and RBC of mammals. Normally a cell contains only one nucleus but Paramecium contains two nuclei whereas Mucor and Rhizopus contain numerous nuclei. Their position is normally at the centre but may shift here and there within the cell. The size of the nucleus depends on amount of DNA, proteins and metabolic phase of the cell. A typical eukaryotic nucleus consists of the following components:

Nuclear envelope or nuclear membrane or karyotheca
It is 60 A0 thick, double membrane separated by 150-300 A0 which delimits the nucleus from cytoplasm. Nuclear membrane is perforated by nuclear pores. The outer membrane is continuous with endoplasmic reticulum and like ER it may be covered with ribosome. The space between outer and inner membrane is called perinuclear space.

Nucleolus
It is rounded, darkly stained structure inside the nucleus and variable in number. It is made up of DNA and RNA. During nuclear division nucleoli disappear as its DNA disperses. But it reassembles after nuclear division. The main function of nucleolus is the biogenesis of ribosome. DNA associated with nucleolus has the genetic information for the synthesis of rRNA and ribosomal proteins, which associate to form ribonucleoprotein. It has significant role in mitosis. Even if one of the two nucleoli is injured by some means, the process of mitosis stops permanently.

Nucleoplasm
It is transparent, semisolid, granular and acidophilic ground substance of nucleus also knows as nuclear sap or karyolymph. It is mainly made up of nucleic acids, proteins, enzymes, lipids and minerals. It takes part in spindle fibre formation and also the site for various enzymatic reactions.

Chromatin reticulum
It is thread like network embedded in nucleoplasm, which condenses into chromosomes during division. It is composed of DNA bound to basic proteins called histone. The darkly stained region of chromatin is called heterochromatin and lightly stained region is called euchromatin.

Differences between Heterochromatin and Euchromatin:
Heterochromatin
Euchromatin
· Dark stained in prophase and found in condensed region of chromatin
· Light stained in prophase and found in diffused region of chromatin
· Supposed to be genetically inert.
· Supposed to be genetically active
· Contains small amount of DNA and large amount of RNA
· Contain large amount of DNA
Functions
1. Chromosomes contain DNA, the molecule of inheritance.
2. DNA is organized into genes, which control all the activities of the cell.
3. Nuclear division is the basis of cell replication and hence reproduction.
4. Nucleolus manufactures ribosomes.
Plastids
They are large sized organelles found only in plant cells. They are absent in fungi and prokaryotes. Plastids are mainly of three types:
Chloroplast
Chromoplast
Leucoplast
All three types of plastids can change from are one to another.

Chloroplasts

Leucoplasts Chromoplasts


Chloroplasts
They are green coloured plastids due to abundance of green pigment chlorophyll. They are present in all green coloured eukaryotic cells. In prokaryotic cells, organized chloroplasts are absent. Its number varies from one (e.g. Chlamydomonas) to 100 (as in mesophyll palisade cells) per cell. Generally the plant cells contain 20-40 chloroplasts per cell. They are variable in shape e.g. biconvex, cup shaped, spiral, stellate etc. They are 4-8 m in diameter and 2 m in thickness.
Each chloroplast is double membrane-bounded organelle made by lipo-protein, the space between two membranes is called periplastidal space. The internal structure of the plastid shows two distinct parts:
i) Colourless ground substance called stroma and
ii) Closed flat, stack like membrane system called grana.
Stroma is watery and proteinaceous ground substance. It contains starch grains, lipid droplets, DNA and free ribosomes (70 S types). The dark reaction of photosynthesis takes place in the stroma.
Grana are densely packed stacks of membrane layers called thyllakoids. Each chloroplast contains about 40-60 grana while each granum contains 10-100 thyllakoids.
The space enclosed by a thyllakoid is called loculus or lumen. Each thyllakoid is bounded by a single membrane. Light reaction of photosynthesis takes place in the grana as photosynthetic pigments are located there. Two adjacent grana are joined with one another by lamellae called intergranal lamellae or stroma lamellae or fret channel. Chloroplast is semiautonomous organelle due to the presence of DNA.

Chromoplasts
They are coloured plastid other than green. These are present in petals and fruits imparting them different colours for attracting insects and animals. These may arise from the chloroplasts due to replacement of chlorophyll by other pigments (e.g. tomato and chilies or from leucoplasts by the development of pigments (e.g. carrot)
Leucoplasts
They are colourless plastids present in those part not exposed to sunlight e.g. seeds, underground stems, roots, tubers, rhizomes etc. These are also found in sex cells, embryonic cells, meristematic cells and parenchymatous cells. These are oval, spherical, rod like, filamentous and are of three types:
Amyloplasts: Synthesize and store starch grains as in potato tuber, wheat and rice grain.
Elaioplast: Stores lipid (oil) as in cells of endosperm in castor seeds.
c. Proteinoplast (Aleuroneplast): Stores proteins as in the cells of maize grains.

Functions of plastids:
Photosynthesis: Chloroplasts also called photosynthetic apparatus involve in photosynthesis
Proteins synthesis: DNA of the chloroplasts code for rRNA, tRNA and ribosomal protein of chloroplast. It also codes for proteins of thyllakoids. Chloroplasts are semiautonomous organelles.
Food storage: Different types of leucoplasts store starch, proteins and fat in them.
Colour: chloroplasts contain xanthophylls and carotene that provide different colours of flowers and fruits.


Golgi Bodies
This organelle was first observed by Camilo Golgi (1898) in the nerve cells of barnowl. These are present in all eukaryotic cells except RBC of mammals, sperm cells of bryophytes and peridophytes and sieve tubes of plants. These are entirely lacking from prokaryotes. It occurs in two forms:
Localized form: Near the nucleus in vertebrate cells.
Diffused form: Scattered in the cytoplasm and not easily distinguishable known as dictyosomes in plant cells.
These exhibit variation in shape and size. Each golgi complex is made by following different structures:
Cisternae: They are elongated, tubular or fattened sacs arranged parallel or concentrically like stack of coins. A stack contains 6-30 or more cisternae. Each cisterna constitutes the functional unit of golgi complex. The margins of each cisterna are gently curved. As a result entire glogibody becomes a polarized structure having a convex forming face or cis-face. At this end of the stack, new cisternae are constantly formed by fusion of vesicles derived from SER. The concave face of the stack is called maturing face or tans-face directed towards the plasmamembrane. At this maturing face, the distal cisterna is continuously lost by its breaking off into vesicles called secretory vesicles.

Tubules: They form a complicated network towards the periphery and maturing face of the apparatus. Actually tubules arise due to fonestration of the cisternae. They have a diameter of 30-50nm. The tubules interconnect the different cisternae.
Vesicles: The small droplets like structures about 60 nm in diameter. These are associated with the convex surface of cisternae i.e. on the forming face. They develop by budding off from smooth endoplasmic reticulum.

Large golgian vacuoles: Large spacious rounded sacs present on the maturing face of golgi complex.

Functions

Storage, condensation and packing of materials: Golgi bodies absorb compound of iron, cupper and gold and accumulate them in their membrane. They are also concerned with the absorption and storage of lipids.

Hormone: Golgi bodies are involved in the production of hormones by endocrine glands.

The Golgi complex processes and packages and help in the transport and release of secretory proteins as well as intracellular enzymes contained in peroxisomes and lysosomes.

Formation of plasma-membrane and other structures: The cisternae of golgi-bodies form plasma-membrane when they discharge their contents outside the cells. They also form lysosomes by blebbing.

Cell wall formation: In plant cell, they synthesize pectin, hemicellulose and other polysaccharides needed for the formation of cell wall. They also form a cell plate at the centre of the mitotic cell plate.

Glycoprotein formation: Linking of carbohydrates and proteins in the formation of glycoproteins occur in the golgi-bodies.

Acrosome formation: Golgi bodies play a prominent role in the formation of acrosome during maturation of spermatids into spermatozoa
Golgi bodies activate mitochondria to produce ATP which is utilized for various anabolic reactions of the body.

Yolk deposition: In the formation of ovum or oocyte, the yolk is synthesized and deposited by Golgi bodies in the process of vitellogenesis.

Matrix formation: Golgi bodies are involved in the formation of matrix of connective tissues.



Endoplasmic reticulum
Porter first reported endoplasmic reticulum or ergastoplasm in 1945 as a network of channels of different shapes and sizes. Since these channels are more concentrated inside the cell cytoplasm (endoplasm) they are called as endoplasmic reticulum. They are found in all eukaryotes except the RBC of mammals. They are absent in prokaryotes. It is extensively developed in those cells which are actively synthesizing proteins and hormones e.g. the cells of liver and pancreas. In muscles, they are known as sarcoplasmic reticulum.
Endoplasmic reticulum is a double unit membrane lamellated structure that has taken various morphological shapes depending upon the type of the cells. They are usually 60nm in diameter, cris-crossing the cytoplasm and continuous with the nuclear envelope and plasma-membrane. ER constitutes more than half of the total membrane in a cell. The internal space of the system is called ER-lumen occupies more than 10% of the cell volume and is filled with a fluid called endoplasmic matrix. Depending upon the various morphological forms, the following three types are recognized:

Cisternae: They are long flat and unbranched plates or lamellae arranged in parallel rows. They are more concentrated near the nuclear region and usually get associated to nuclear membrane. Cisternae are more numerous in those cells which are actively synthesizing proteins, like liver, pancreas, notochord and brain.

Vesicles: They are round or ovoid sacs found isolated in the cytoplasm measuring 25 to 500 m. They are also found in those cells which are actively synthesizing proteins.

Tubules: They are irregularly branched tube like structures surrounded by thin unit membrane and their lumen is filled with the secretory products of the cell. They are more numerous in those cells which are actively synthesizing steroids.

Types of endoplasmic reticulum:
Two types may be present in the same or different types of cells.
Agranular or smooth endoplasmic reticulum: The surface of this type of ER is smooth, ribosomes being not attached. Smooth ER is present in cells which are actively engaged in steroid synthesis, (e.g. cholesterol, progesterone, testosterone etc.) carbohydrate metabolism and pigment production.

Granular or rough endoplasmic reticulum: The rough ER has ribosomes attached throughout the surface. This type of ER is preset in cells which show active protein synthesis.

Differences between Ser and RER
SER
RER
· SER doesn't bear ribosomes.
· RER possesses ribosomes
· It is mainly formed of vesicles and tubules.
· It is mainly formed of cisternae and a few tubules.
· It involves in the synthesis of glycogen, lipids and steroids.
· It involves in the synthesis of protein and enzymes.
· It gives rise to sphaerosomes.
· It helps in the formation of lysosomes.
· Pores are absent so that materials synthesized by SER do not pass into its channels.
· RER possesses narrow pores below its ribosomes for the passage of synthesized polypeptide into ER channels.
· SER is often peripheral. It may be connected with plasmalemma.
· It is often internal and connected with nuclear envelope.


Functions
Mechanical support: ER provides an ultrastructural skeletal framework to the cell and gives mechanical support to the colloidal cytoplasmic matrix.

Transportation: ER acts as an intracellular transporting system. Various secretory products of granular endoplasmic reticulum are transported to various organelles as follows:
SER to RER to Golgibody to Lysosomes, transport vecicles or secretory granules. The exchange of molecules by the process of osmosis, diffusion and active transport occurs through the membrane of endoplasmic reticulum.

Enzyme activity: SER is the site for the metabolism of steroids, phospholipids and sex hormones (testosterone and progesterone). These membranes provide an increased inner surface for these reactions to occur and also facilitate the union of enzymes with their substrate.
Protein synthesis: RER involves in protein synthesis as they contain ribosomes on their outer surface.

Glycogen metabolism: The enzymes necessary for the conversion of glycon to glucose are localized in the smooth endoplasmic reticulum of liver.

ATP hydrolysis: SER is involved in the hydrolysis of ATP as it contains the enzyme ATPase.

Detoxification: ER chemically modifies xenobiotics (toxic materials of both endogenous and exogenous origin), making them more hydrophilic, hence, more readily excreted.

Inorganic ions transport: It is involved in the secretion of chloride ions in oxyntic cells lining the mammalian stomach and the gills of fishes.

Receptor: ER present in the epithelial cells of retina receives the light stimuli in the eyes.

Impulse conduction: The sarcoplasmic reticulum shows ionic gradients and thus an electrical potential is generated across the membrane. This helps in the transmission of impulses.

Formation of other membranes: Nuclear membrane and golgi bodies are known to differentiate from endoplasmic reticulum.


Lysosomes
C. de Duve first discovered lysosomes in 1955. These are tiny membrane-bound vesicles found commonly in eukaryotic cells. They are more common in animal cells than in plant cells. These contain about 40 types of hydrolytic enzymes including proteases, nucleases, lipases, glucosidases and phosphatates. These enzymes become active only in acidic medium and are also called acid hydrolases. The enzymes contained within the lysosomes are synthesized in endoplasmic reticulum and transported to the Golgi complex. Golgian vacuoles containing the processed enzymes later on bud off to from the lysosomes.

Types of lysosomes
Lysosomes exhibit the phenomenon of polymorphism i.e. they exist in more than one morphological form. A lysosome passes through different stages at different times in the same cell. Following four types of lysosomes have been recognized in different types of cells;

Primary lysosomes: These are newly formed lysosomes containing enzymes only. These are produced from golgi bodies or endoplasmic reticulum.

Secondary lysosomes: They are also called heterophagosomes or heterophagic vacuoles. These contain the enzymes and ingested food particles and are formed by the fusion of primary lysosomes with the vacuole containing engulfed food. The digestion of engulfed substances takes place by the action of the associated hydrolytic enzymes.

Autophagosomes: They are also called autophagic vacuoles. These are formed when primary lysosomes digest intracellular structures including mitochondria, ribosomes and glycogen granules, which occur under certain physiological (metamorphosis) and pathological conditions. The process is known as autophagy. Old organelles are replaced by new ones by the process.

Residual bodies: When the autophagosomes and heterophagosomes contain undigested and indigestible materials, they are formed as residual bodies. These are eliminated from the cell by the process of exocytosis.





Function of lysosomes

Intracellular digestion: Lysosomes digest the food contents of phagosomes or pinosomes when they fuse with the primary lysosomes and comes in contact with the hydrolytic enzymes.

Extracellular digestion: The lysosomes of certain cell such as sperms discharge their enzymes outsides the cell during fertilization to digest the limiting membrane of ovum and form penetration path for sperm.

Autolysis: Under pathological conditions, lysosomes start to digest the various organelles of the cells. This process is known as autolysis or cellular autophagy. Due to this action, they are also called suicidal bags. During starvation lysosomes digest the food stored in the cell. When a cell dies, the lysosome ruptures and the enzymes are liberated which digest the deal cell.

Metamorphosis: In the process of metamorphosis of amphibian’s gills, fins, tail are digested by the lysosomes.

Mitochondria
Mitochondria are thread like or granular organelles present in the cytoplasm of cells except RBC of mammals and all prokaryotes. Mitochondria are commonly known as “power houses of cell” as they contain enzyme for respiration. Kolliker (1880) first observed these from the muscle cells of insects. Later on Benda (1898) used the term mitochondria.
Each mitochondrion is 1.5mm to 10mm in length, 0.25-1mm in width and up to 1mm in diameter. They also vary in shape e.g. spherical, cylindrical, sausage-shaped, tubular or filamentous. Their number also vary in different cells e.g. Microsterias, an alga, has only one mitochondrion; an Amoeba, Chaos chaos may have 50,000 mitochondria, human liver cell may have about 1,000; while a kidney cell may have 300-400.
The mitochondrion is bounded by a double membrane structure. The outer membrane is smooth and continuous and shown similarities with endoplasmic reticulum. It is freely preamble to various metabolites that enter into mitochondria.
The surface area of inner membrane is much greater than the outer membrane and so it is folded into finger like projections called cristae. There is correlation between the number of cristae and the chemical activity of mitochondria. The space in between the two membranes is known as perimitochondrial space. The inner space into which these cristae project is known as matrix. Matrix has mitochondrial DNA, RNA and dense granules of about 300-500 A°. Granules contain inorganic salts and believed to be the building site for Ca++ and Mg++ ions. The enzymes involved in the Kreb cycle are present in the matrix, while those of glycolysis are present outside in the cytoplasm. The outer surface of inner membrane has number of elementary particles or F1 particles or oxysomes. These particles are evenly distributed at a distance of about 100 A°. There are about 104 to 106 elementary particles present per mitochondrion. Each particle has a base and a stalk (F0 sub unit) and a rounded head (F1 sub unit). These particles contain enzymes necessary for electron transport chain. Mitochondria are self duplicating cell organelles which are also called semi-autonomous. This is because of their own DNA. Mitochondrial DNA produces its own mRNA, tRNA and rRNA. The organelles possess its own ribosomes so can synthesize its own structural proteins. The organelles synthesize all the enzymes required for their functioning. However, mitochondria are not fully autonomous. Their functioning is partially controlled by nucleus of the cell and availability of materials from cytoplasm. Mitochondria are believed to be symbionts in the eukaryotic cells which became associated with them quite early in the evolution.


Function of Mitochondria

Respiration: The oxidation of pyruvic acids takes place in mitochondria. Large amount of energy is evolved which is used to synthesize ATP by the process of oxidative phosphorylation. ATPs are stored by mitochondria so are called power houses of cells.

Yolk formation: Mitochondria are believed to help in yolk formation in a developing ovum. They get converted to yolk storing bodies.

Maturation of sperm: During the process of spermiogenesis, mitochondria form a spiral sheath in the middle part of the maturing sperm. In insects, all the mitochondria form a compact body called ‘nebenkern.’

Fat metabolism: Mitochondria are known to be associated with the synthesis of lecithin from acids.


Microbodies
There are three types of microbodies found in a cell.

Peroxisomes
They are small sized organelles present in both plants and animals although they were first described in the kidney and liver cells of rodent. The term peroxisome was introduced by Beautaytt and Berther in 1963. They are believed to develop from endoplasmic reticulum. They measure about 1.0m in diameter and are bounded by a single unit membrane. The matrix of peroxisomes contains enzymes like catalases and oxidases. They are involved in the photorespiration of plant cells and lipid metabolism of animal cells. They also protect the cellular organelles from the toxic effect of hydrogen peroxide.

Glyoxysomes
These organelles were first discovered by Beevers from castor bean endosperm. They are usually spherical in shape, about 0.8 m in diameter and consist of fine granular stroma surrounded by a single membrane. These contain key enzymes of glyoxylate cycle (i.e. Isocitratase and malate synthetase) and therefore, are usually found in those plant tissues where fats are being actively converted into sugars such as in germinating fatty seeds.



Sphaerosomes
These are usually minute spherical structures about 0.4-3 m in size and surrounded by a membrane. Spherosomes are also called as oleosomes or oil bodies because they store fats (triglycerides). These structures are called wax bodies in JoJoba (Simmondsia chinensis) cotyledons as here they store wax esters instead of triglyceride. Sphaerosomes of some tissues (e.g. tobacco endosperm and maize root tip) contain hydrolytic enzymes. Therefore they are considered to have lysosomic activity.

Ribosomes
Ribosomes are tiny cell organelles, about 20 nm in diameter, that are found in large numbers throughout the cytoplasm of living cells, both prokaryotic and eukaryotic. They also occur inside the mitochondria and chloroplast. Chemically ribosomes are composed of rRNA and proteins, both occurring approximately in equal proportions. These are non-membrane bounded cell structures.
Each ribosome consists of two subunits one being larger and other smaller. According to the size and the sedimentation coefficient (S), two types of ribosomes have been recognized.

70S Ribosomes: They are found in all prokaryotic cells and chloroplasts and mitochondria of all eukaryotic cells. Its sedimentation rate is 70S and molecular weight is 2.7x106 Daltons. It consists of two units 50 S (larger) and 30 S (smaller).

80S Ribosomes: They are found in all eukaryotic cells. Their sedimentation rate is 80 S and molecular weigh is 40x106­­ Daltons. It consists of two units 60 S (larger) and 40 S (samller).
The chief function of ribosomes is protein synthesis. So they are also known as protein factories of cells. An aggregation of ribosomes attached to mRNA chain forms polysomes or polyribosomes at the time of protein synthesis.

CENTRIOLES
Centrioles are hollow, cylindrical structures that occur in pairs in cytoplasm near the nucleus. The pair of centrioles is called diplosome. The centrioles of a diplosome are commonly arranged at right angle to each other. Centrioles occur in animal cells and the cell of lower plants. They act as microtubules organizing centres (MTOCs) and form spindle of microtubules during mitosis and meiosis or sometimes get arranged beneath the plasma membrane to bear flagella or cilia. When centrioles bear flagella or cilia, it is called basal body. Centrioles lack limiting membrane.

Structure
A centriole consists of nine triplets of microtubules forming the wall of an imaginary cylinder. The triplets are arranged like the blades of a turbine or the vanes of a pinwheel. The triplets are equally spaced.
Chemically, the microtubules of centrioles and basal bodies are composed of a protein called tubulin along with lipid molecule. They also contain high percentage of ATPase enzyme.

Functions
At the time of cell division they form spindle fibers which help in the movement of chromosomes.
They are involved in the formation of cilia and flagella.
In spermatozoa, centrioles forms axial filament of tail or flagellum.

Cilia and Flagella
Cilia and flagella are microscopic, contractile and filamentous processes of cytoplasm which create food currents, and perform many mechanical functions of the cell. Cilia occur in the protozoan of class ciliata and ciliated epithelium of metazoan. Flagella are found in the protozoan of class flagellata, choanocyte cells of sponges, spermatozoa of metazoa and among plants in the algae and gamete cells.

Structure
Cilia and flagella have identical structures. They are the outgrowth of a cell. They arise from basal bodies or blepharoplasts, which have the same structure as that of centrioles. Each cilium or flagellum is surrounded by a membranous covering which is an extension of the plasma membrane. They contain two central microtubules surrounded by nine sets of microtubules. This characteristics arrangement of microtubules is called ‘9+2’ arrangement. Each peripheral microtubule is a doublet consisting of two sub-microtubules. Peripheral microtubules are composed of the protein tubulin and the central tubule is of dyenin.

Differences between cilia and flagella
Cilia
Flagella
· They are comparatively shorter
· They are comparatively longer
· They are numerous in a cell
· They are fewer is a cell
· They are present throughout the surface of the cell.
· They are present at one end of the cell
· They beat in a coordinated rhythm
· They beat independently
· They show sweeping movement.
· They show undulating movement

Function of cilia and flagella
They provide locomotion to the cell or organism.
Cilia create food current in lower organisms like Paramecium.
Cilia in the respiratory tract of higher animals help in the elimination of solid particles from it.
The eggs of amphibians and mammals are driven out from the oviduct with the help of cilia.

Vacuoles
A vacuole is fluid filled structure bounded by a single membrane called tonoplast or vacuolar membrane. Plant cells have more and large sized vacuoles as compared to animal cells. Animal cells contain different types of vacuoles e.g. phagocytic vacuoles, food vacuoles, autophagic vacuoles and contractile vacuoles. The vacuolar content is called cell sap. It is concentrated solution of mineral salts, sugars, organic acids, oxygen, carbon-dioxide , pigments and some waste and secondary products of metabolism.

Functions
Cell sap maintains the osmotic relations of the cells.
Colour of vast majority of flowers are chiefly due to the presence of anthocyanin pigments dissolved in the cell sap.
Plant vacuoles sometimes contain hydrolytic enzymes and act as lysosomes. After cell death the tonoplast, like all membranes, loses its partial permeability and the enzymes escape causing autolysis.
Waste products and certain secondary products of plant metabolism e.g. crystals of calcium oxalate, alkaloids, tannin, latex etc. may accumulate in vacuoles.
Some of the dissolved substances act as food reserves which can be utilized by the cytoplasm when necessary for examples sucrose and mineral salts.

Microtubules
Nearly all eukaryotic cells contain microtubules. They are non membranous, very fine, unbranched, hollow tubes. They have an external diameter of about 24nm and walls about 5nm thick made up of helically arranged subunits of a protein called tubulin. Microtubules in plants may be ground into three categories:
Microtubules constituting the nuclear spindle in mitosis.
Microtubules, which are structural components of flagella and cilia of motile cells such as gametes of, lower land plants and algae.
Microtubules found in cytoplasm which are believed to be concerned with cell wall formation by directing the alignment of the cellulose microfibrils as the latter are deposited. Probably, these microtubules also direct the golgi vesicles (which carry polysaccharides found in cellular matrix) through the cytoplasm to those regions of plasma membrane in the near vicinity of which active cell wall synthesis is taking place.

Cell Division
One of the postulates of cell theory is, “cells arise from pre-existing cells only” as suggested by Rudolf Virchow in 1855. Continuity of life depends on cell division. The growth and development of every organism depends exclusively on the multiplication and enlargement of the cells. The development of multicellular organisms from unicellular zygote is achieved by cell division, growth and differentiation. There exists a definite ratio between cytoplasmic and nuclear volumes of the cell; it is referred to as karyoplasmic index. As and when this ratio is disturbed, the cell tends to divide.

The division of nucleate cells is achieved by two integral activities such as the division of the nucleus, i.e. karyokinesis reported by Schleicher (1887), and the division of the cytoplasm, i.e., cytokinesis reported by Whiteman (1887). Usually karyokinesis is followed by cytokinesis but sometimes the cytokinesis doesnot follow the karyokinesis that results into multinucleate cells i.e. coenocyte.
Cell-Cycle
Each cell capable to undergo division passes through a cycle, referred to as the cell-cycle. It can be defined as the entire sequence of events happening from the end of one nuclear division to the beginning of the next. It is represented by DNA duplication followed by mitosis which in turn is followed by cytokinesis. W. Flemming first described mitotic cell cycle in animals in 1882.In the same year; Strasburger described mitotic cell cycle in plant.

Howard and Pelc (1953) have divided cell cycle into four phases or stages: G1, S, G2 and M passes. The G1, S, G2 phases are combinely called interphase.

Interphase
It is a state in between two successive cell divisions and when the cell doesn’t show any mitotic or meiotic events but it prepares itself for the process by synthesizing new proteins and nucleic acids. The nuclear envelop remains intact. The chromosomes occur in the form of diffused, long, coiled and indistinctly visible chromatin fibres. The DNA amount doubles. Due to accumulation of rRNA and protein, the size of nucleolus is greatly increased. In animal cells, a daughter pair of centrioles originates near the already existing centrioles and thus an Interphase cell has two pairs of centrioles. This stage is divided into the following three sub stages:

1. G1 or gap -1 or period of initial growth.
This phase starts immediately after the cell division. It is the period of growth of the cell; therefore, the cell grows in size. There occurs an active synthesis of RNA and protein needed for various metabolic activities of the cell. The cell carries out all its physiological activities. It occupies nearly 40-50% time of the cell cycle and there occurs no change in the DNA content of the cell during this stage. A non-dividing cell does not proceed beyond G1 stage.




2. S or synthetic phase:
During this stage replication of DNA and synthesis of histone protein occur. At the end of S phase each chromosome has two DNA molecules and a duplicate set of genes. Synthetic phase occupies roughly 35-45 % of cell cycle.
3. G2 or Gap -2 phase.

During this phase, RNA and new proteins are synthesized. The main proteins synthesized in this stage are those responsible for the formation of spindle fibres or astral rays. Cell organelles multiply in number.

Mitosis
The type of division in which a mother cell divides into two daughter cells, which are identical qualitatively as well as quantitatively with each other and with mother cell as well. This is also called vegetative or somatic cell division as they mainly occur in body cells. As chromosome number remains constant, it is also known as equational division. Though mitosis is remarkably similar in all animals and plants, it occurs only in growing apices of plant i.e. meristematic cells while most of the body cells undergo mitosis division in case of animal. The division of cell is initiated by the division of nucleus (i.e. karyokinesis). For the study point of view the karyokinesis process is divided into phases like prophase, metaphase, anaphase and telophase.

Prophase
It is the longest phase and further divided into early middle and late prophase. Following events occur during prophase:
· The chromatin network begins to coil and appear as long thread like structures called chromosomes.
· An important characteristic of mitotic prophase is longitudinal splitting of each chromosome into two sister chromatids, which remains attached at a point called centromere. The position of centromere varies in different chromosomes but is characteristic of a particular chromosome.
· In animal cell the centrioles move to opposite poles of the cell.
· Short microtubules may be seen radiating from the centrioles. These are called aster.
· The nucleoli disappear as their DNA passes to certain chromosomes.
· At the end of prophase the nuclear envelope is no longer visible because it breaks up into small vesicles, which disperse in cytoplasm.
· The microtubules of the cytoplasm orient themselves in between the two centrioles and form the spindle known as amphiaster. The spindle fibres and aster rays are composed of 3.5% RNA and 95-97% protein tubulin.

Metaphase:
Following events take place in mitotic metaphase:
· Chromosomes become shortening by coiling and become more distinct and visible under the compound microscope.
· Chromosomes orient themselves towards the equator in such a way that all the centromeres become arranged on the equator forming metaphase plate.
· A series of spindle fibres can be seen which attach the centromeres to the opposite poles. These fibres are made up of proteins rich in sulphur containing amino acids. In animal cells, the spindle fibres originate from centrioles whereas in plant cells, they are formed in the general cytoplasm.

Anaphase
Anaphase is very short stage with following events:
· Centromere of each chromosome divides into two so that each chromatid comes to possess its own centromere.
· The chromosome divides into two sister chromatids and moves towards the opposite poles by the shortening of spindle fibres. As the chromosomes move to their poles through cytoplasm, they assume characteristics V-, J- or I- shaped configurations. Such variable shapes are because of variable position of centromere in different chromosomes.
·
· Termination of anaphase movement occurs when the chromosomes form a densely packed group at the two poles.

Telophase
Telophase is long and complex like the prophase. In this phase nucleus is reconstructed from each group of chromosomes. It involves the following events:
· The chromatids reach the poles of the cell, uncoil and lengthen to form chromatin again, loosing the ability to be seen clearly.
· The spindle fibres disintegrate and the centriole replicates.
· The nuclear envelop reforms around the chromosomes at each pole and the nucleoli reappear.
· Telophase may lead straight into cytokinesis.

Cytokinesis
Cytokinesis is the division of cytoplasm. This stage normally follows telophase and leads into G1 of interphase. In preparation for division, the cell organelles become evenly distributed towards the two poles of telophase cell along with the chromosomes. Cytokinesis is different in animal and plant cells.

Cytokinesis in animal cell
It takes place by invagination or cleavage method. Microfilaments collect in the middle region of the cell below the cell membrane. They induce the cell membrabe to invaginate. The furrow proceeds centripetally and cleaves the cell into two daughter cells.

Cytokinesis in plant cell
It takes place by cell-plate formation. The spindle fibres at the equatorial region form barrel shaped structure known as phragmoplast. The membranes of the vesicles form two sheets which enclose a matrix film and get solidified to form cell plate or middle lamella. It grows centrifugally and comes in contact with lateral walls of parent cell. With the formation of cell plate, the spindle or phragmoplast disappears. The daughter protoplasts deposit cellulose, hemicellular and pectin on either side of the cell plate. They form the primary wall.

Significance of mitosis cell division
· Genetic stability: Mitosis produces two nuclei having same number of chromosomes with same hereditary information in their genes. This result in genetic stability within population of cells derived from the same parental cell.

· Growth: The number of cells within an organism increase by mitosis and this is the basis of growth in multicellular organisms.

· Cell replacement: Replacement of cells and tissues also involves mitosis. Cells are constantly dying and being replaced by mitosis.

· Regeneration: Some animals are able to regenerate whole part of the body such as legs in crustaceans and arms in starfish.

· Asexual reproduction: Mitosis cell division is the mode of asexual reproduction in case of unicellular organism as it helps in increase in no. of individuals.

Meiosis
It is a type of division in which a diploid cell divided into four haploid cells through the two successive nuclear division i.e. Meiosis –I and Meiosis –II. Meiosis occurs during the formation of sperms and eggs (gametogenesis) in animal and during microspores and megaspores formation (sporogenesis) in plants. Like mitosis, meiosis is a continuous process but conveniently divided into prophase, metaphase, anaphase and telophase. These stages occur in the first meiotic division and again in the second meiotic division.

Meiosis-I
1. Prophase-I
i. Leptotene
ii. Zygotene
iii. Pachytene
iv. Diplotene
v. Diakinesis
2. Metaphase-I
3. Anaphase-I
4. Telophase-I
Meiosis-II
1. Prophase-II
2. Metaphase-II
3. Anaphase-II
4. Telophase-II

Heterotypic Division or First Meiotic Division
In this division, the chromosome number is reduced to half. The new daughter cells produced are dissimilar to the parent cell. It is also called reduction division due to reduction in chromosome number.

Prophase –I
It is the longest stage further divided into following sub stages:
1. Leptotene or Leptonema:
· Nucleus increases in volume by absorbing water.
· The chromatin fibres of interphase nucleus shorten and elongated chromosomes become clear.
· Chromosomes are extremely thin, uncoiled, elongated, longitudinally single and with swollen areas called chromomeres.
· Centrioles move towards the opposite poles and a definite type of orientation and polarization of chromosomes towards the centrioles takes place. This type of peculiar arrangement is known as bouquet.
2. Zygotene or Zygonema
· Shortening of chromosomes by coiling.
· Lengthwise pairing of homologous chromosomes (i.e. one maternal and other paternal), takes place due to formation of proteinaceous fibres called synaptonemal complex. This process is called synapsis.
· The paired homologous chromosomes of the zygonema stage are called bivalents.
· The bivalents shorten and thicken, partly by coiling. Each chromosome and its centromere can be seen clearly.

3. Pachytene or Pachynema
· Synapsis of bivalents which started in zygonema gets complited.
· The chromomes appear more thick and clear and nucleoi become more evident and are seen attached to nucleolar organizer region (NOR) of certain chromosomes.
· Homologous chromosomes appear to repel each other and partially separate
· Longitudinal splitting of each chromosome of a bivalent and formation of tetrad occurs.
· The chromosomes are seen to be joined at several points along their length. These points are called chiasmata (chiama, a cross)
· Exchange of chromatid segments between the nonsister chromatid of homologous chromosomes occur by breakage and reunion. This is called crossing over. One bivalent may divide transversely by the help of an enzyme the endonuclease. The broken chromatid segments are united with the chromatids with the help of enzyme ligase.
· Nucleolus remains intact up to this stage between homologous chromosomes lapse and they seem to separate from each other.

4. Diplotene or diplonema

Force of repulsion develops and separation of homologous chromosomes of a bivalent occurs except at chiasmata called disjunction. Nucleolus is extremely diminishing in size.



5. Diakinesis
· The bivalent chromosomes become more condensed and evenly distributed in the nucleus
· The nuclelus and nuclear membrane disappear
· The chiasma moves from centromere towards the end of the chromosomes and the intermediate chiasma diminish. This is called terminalization. The chromatids still remains connected by the terminal chiasmata and these exist up to the metaphase.
· Bivalents assume particular shapes depending upon the number of chiasmata, bivalent having single chiasma appear as open crosses, two chiasmata produce a ring shape and three or more chiasmata produce loop lying at right angle to each other.
Metaphase –I
· It is characterized by the complete disappearance of the nuclear membrane and the nucleolus and formation of spindle fibres.
· The bivalents become arranged around the equator of the spindle, attached by their centromeres.

Anaphase –I
· Spindle fibres pull homologous chromosomes, centromeres first, towards the opposite poles of the spindle.
· Separation of chromosomes into two haploid sets, one set at each end of the spindle.
· However, the chromatids of these chromosomes are not genetically identical because of crossing over.

Telophase –I
· The arrival of homologous chromosomes at opposite poles marks the end of meiosis –I.
· The chromosomes become uncoil.
· Reappearance of nucleolus and nuclear membrane.
· The spindle fibres disappear.
· Each daughter nucleus thus at the end of meiosis-I has half the number of chromosomes as compared to the parent nucleus.
Interphase –II
This stage is present usually only in animal cells and varies in length. No further DNA replication occurs.

Homotypic or second Meiotic division
This is actually the mitotic division, which divides each haploid meiotic cell into two haploid cells. The second meiotic division includes following four states:

Prophase II
It is very short and is characterized by disappearance of nucleolus and nuclear membrane and coiling of chromosomes leading to their shortening. The chromosomes here show a unique feature wherein the chromatids are flaring apart because of their non identical nature in certain segments as a result of crossing over in prophase –I.

Metaphase II
Chromosomes line up separately around the equator of the spindle to form metaphase plate. Two spindle fibres join each centromere, one originating from each pole of the spindle.

Anaphase II
The centromeres divide and the spindle fibres pull the chromatids to opposite poles, centromeres first.

Telophase II
As soon as, the chromosomes reach the respective poles, they start uncoiling and become elongated. The nucleoli and nuclear membranes are organized. Thus the end result of meiosis is the production of four nuclei, each having one half of the chromosome number of that in the parent nucleus.

Cytokinesis
Division of cytoplasm can occur after each nuclear division (Meiosis-I as well as Meiosis-II), called successive or can be delayed until four nuclei are formed, called simultaneous. In animals, it occurs by furrowing whereas in plants, by cell plate formation.

Signification of Meiosis
1. It maintains a definite and constant number of the chromosomes in sexually reproducing organisms.

2. By crossing over, the meiosis provides an opportunity for the exchange of the genes and thus, causes the genetic variations among the species. Thus it helps in the process of organic evolution.


Differences between Mitosis and Meiosis
Mitosis
Meiosis
1. It occurs in all somatic or vegetative cells of haploid, diploid or polyploid bodies. It helps in gametogenesis of plant.
1. It occurs in the reproductive cells at the time of gamete or spore formation.
2. The number of chromosomes remains same in the daughter cells produced.
2. The number of chromosomes is reduced to half in the daughter cells.
3. Two daughter cells are produced as a result of mitosis.
3. Four daughter cells are produced as a result of meiosis.
4. Daughter cells produced are similar to the parent cells, qualitatively and quantitatively.
4. Daughter cells are genetically different from the parent cells.
5. It may occur in reproductive cells of gametophytes.
Prophase
5. It is absent in gametophyte.
6. Homologous chromosomes remain separated.
6. Homologous chromosomes pair up called synapsis.
7. No formation of chiasmata
7. Chiasmata form
8. No crossing over.
Metaphase
8. Crossing over may occur
9. Pairs of chromatids line up on the equator of the spindle.
Anaphase
9. Pairs of chromosomes line up on the equator
10. Centromeres divide
10. Centromeres don’t divide in anaphase- I but divide in anaphase- II.
11. Chromatids separate and separating chromatids are identical.

Telophase
11. After anaphase I, each pole receives a mixture of paternal and maternal chromosome. Separating chromosomes and their chromatids may not identical.
12. Same number of chromosomes present in daughter cells as parent cells.
12. Half the number of chromosomes present in daughter cells.
13. Both homologous chromosomes present in daughter cells if diploid.
13. Only one of each pair of homologous chromosomes present in daughter cells.