At the beginning of amitosis, cells take in food and grow in size. Its nucleus is also enlarged. The nucleus enlarges and takes the shape of a dumbbell. The cytoplasm of the cell folds inwards. The nucleus then divides into two fragments. The cytoplasm folds further inward. Later the cytoplasm changes into two cells with nucleus

Amitosis is formed from the Greek words a meaning not, mito meaning thread and osis meaning state. The process in which a cell directly produces two daughter cells without any complex medium is called amitosis. Walter Flemming first used the term amitosis in 1882. In 1955 scientist Remak first observed amitosis cell division in red blood cells of chick embryos. Amitosis occurs in cells such as bacteria, yeast, amoeba, nostoc, cartilage of vertebrates, embryo membrane, endosperm of seeds etc.

In animal cells, the small organelles near the nucleus that contain microtubules and form spindle fibers during cell division are called centrioles. The junction of two centrioles located side by side is called a centrosome. In 1887 scientist Von Benden discovered it. Named after scientist Theodor Bovery in 1888.

Centrioles are present in animal cells. Most plant cells do not have it. Algae, fungi, bryophyta, pteridophyta, gymnosperms etc. plants have centrioles. Each cell has only one centriole. It is absent in protozoa, diatoms, yeasts, plant and mammalian red blood cells.

Centrioles are cylindrical and small organelles. It looks like a cylindrical barrel with two openings. It is 0.3-0.5 µm in length and about 0.15-0.25 µm in diameter. Each centriole consists of three main parts. These are-

1. Cylinder wall: At the center of the centriole there is a structure like a bullock cart wheel. It is called cart wheel model. The center of the cart wheel model is called the hub. The central rod is surrounded by walls or cylinder walls. Its wall is made up of 9 bundles or microtubules. Surrounding the centriole are 9 pericentriolar satellites above and below the microtubule.
2. Triplets: The centriole consists of 9 clusters of triplet molecules or triplets surrounding a cavity. Each tertiary microtubule has three equidistant subtubules. That is, each centriole (3×9) has 27 subtubules. Microtubules are attached to a central axis like the spokes of a wheel. Scientist Threadgold (1968) identified three subducts internally as A, B and C. The diameter of each subchannel is 250Å.
3. Linker: Linker is a type of protein fiber. The dense protein fibers that connect the tertiary tubules are called linkers. Channel A is connected to channel B and channel B is connected to channel C by connectors.

Centrioles are arranged in pairs. Each pair of centrioles is called diplosomes. The dark fluid surrounding the centriole is called the centrosphere.

Chemical structure of centriole

The main chemical component of centriole is protein. Also contains lipids and ATP.

Function of Cetriole

1. Cell Division: Centriole helps in cell division. It plays a special role in cell division.
2. Mucous fibers production: During cell division, it produces mucous fibers. In the prometaphase state, the nucleus is formed. It acts as a carrier of chromosomes.
3. Aster-rays: It produces aster-rays during cell division. Aster rays are formed during prometaphase in animal cells.
4. Polar movement of chromosomes: It helps in the marginal movement of chromosomes. During anaphase, the cytoplasm contracts and the chromosomes move towards the poles.
5. Formation of microtubules: It produces microtubules. Microtubules form the cytoskeleton.
6. Sperm formation: It forms the tail of the sperm. As a result spermatozoa become motile.
7. Formation of flagella: Cilia and flagella form the basal body.

1. Mitogens: Substances that stimulate cell division are called mitogens. Cytokinin stimulates plant cell division. Steroids, lymphokines, EGF (Epidermal growth factor), PDGF (Platelet derived growth factor) etc. stimulate animal cell division. Cell division occurs due to these chemical elements.
2. Gene regulation: Cell division is a gene regulation process. Cell division begins when the amount of DNA in the nucleus doubles.
3. Increase in cell number: Cells divide to increase the number of cells. Cells do not increase in number unless they divide.
4. Increase in cell volume: Cell division occurs to increase cell volume. Cells divide repeatedly to increase cell volume.
5. Cell Metabolism: Different types of metabolism take place in cells. Cell division is necessary for carrying out metabolism.
6. Nucleo-Cytoplasmic Ratio: Nucleo-cytoplasmic ratio is maintained through cell division.

The process by which a cell divides to form two or more new cells is called cell division. Rudolf Virchow (1858) said, ‘As trees come from trees, so animals come from animals, right

Only cells are created from such cells. Life of unicellular or multicellular organisms starts from a single cell. In 1882, Walter Flemming first observed cell division in the marine Triturus maculosa. A human cell replicates and divides only 4-60 times. During human life, body cells divide about 10 quadrillion or 10¹⁵ times.

Long, cylindrical, hollow and unbranched organelles present in the cytoplasm of cells are called microtubules. In 1953, Robert and Franchi discovered it from animal nerve cells. In 1953, scientists Ledbetter and Porter first observed their location in plant cells.

Microtubules are the sub-structural components of flagella and cilia. It is attached to the centromere of the chromosome. Spindle fibers contain microtubules. It is present in centriole and basal body.

Microtubules are long, cylindrical and hollow organelles. Its diameter is 10-20 mm and length is few microns. One end of them is marked as ‘+’ and the other end as ‘-‘. Each microtubule contains 13 prototubules. Each prototubule is composed of dimeric proteins. Each dimeric protein molecule consists of α and β ubulin.

Functions/importance of microtubules

1. Microtubules provide cell shape and mechanical strength.
2. It plays the role of cytoskeleton.
3. It forms the circulatory system or transport system of the cytoplasm.
4. Assists in cellular movement.
5. It acts as spindle fibers during cell division.
6. Helps in opposite pole movement of chromosomes.
7. Helps in cell wall formation.
8. Helps in the formation and movement of cilia and flagella.
9. Centrioles form in animal cells.
10. It binds to the cell membrane and nuclear membrane to maintain biological connections.
11. It helps in the secretion of essential substances.
12. It helps in the transmission of nerve impulses between nerve cells.
13. It indicates the arrangement of microfibrils.
14. It helps in communication and transportation.

The specific point in the cell cycle at which cell division stops is called a checkpoint or decision point. Cell cycle check points are:
1. G1/S Check Point: The cell cycle stops at the G1/S checkpoint when the cell is not suitable for division and the cell’s DNA is damaged. The cell cannot enter the S phase from the G1 phase. Here cell shape, cell hormones and DNA damage are checked.
2. G2/M Check Point: Cell cycle stops at G2/M checkpoint if damaged DNA of the cell is not repaired and DNA replication is not completed. The cell cannot enter the M phase from the G2 phase. Here the cell and nucleus size and DNA damage are checked.
3. M Check Point: Cell cycle stops at M Check Point when spindle fiber formation of cell is not complete and beam fiber is not properly attached to kinetochore. As a result, the process of mitosis does not occur.

Intermediate filaments are intermediate filaments between microtubules and microfilaments in cells. Their diameter is 10 nanometers. It is made of protein. Cells have four types of intermediate filaments. Keratin, vimentin, lamin and neurofilaments.

Function/importance of intermediate filament

Gives shape to cells. Provides cell strength. Helps keep other cell fibers in place.

Microfilaments are fine contractile fibers located in the cytoplasm of cells that aid in movement. They are also called actin filaments. It was first observed by scientist Paleviz in 1974.

Microfilaments are narrow, long, contractile and twisted biaxial. It is located under the cell membrane like a ribbon. Its diameter is 30-60Å. It is composed of actin protein and myosin protein.

Function/importance of microfilaments

1. Microfilaments provide cell shape and mechanical strength.
2. Microfilaments cause cellular movement.
3. Changes the position of the cell organelle.
4. Connects between cell membrane proteins and cytoplasmic proteins.
5. It causes cytokinesis (cell division).
6. Helps in opposite pole movement of chromosomes.
7. It regulates phagocytosis and pinocytosis.

1. Cyclin-Cdk complex: Inside the cell, the cyclin-cdk complex regulates the changes in various stages of the cell cycle.
2. Cyclin-Cdk-MPF complex: MPF binds to the cyclin-cdk complex of the dividing cell to initiate the cell cycle.
3. P⁵³ protein: When DNA is damaged for some reason, P⁵³ protein stops the cell cycle. When DNA is extensively damaged, the P⁵³ protein induces cell death by apoptosis.
4. P²⁷ protein: P²⁷ protein binds to the cyclin-cdk complex and prevents cells from entering the confluent state. High levels of P²⁷ protein in cells cause breast cancer in women.
5. Cyclin: The regulator of cell cycle is cyclin protein. Timothy Hunt (1982) discovered cyclin proteins. For this, Timothy Hunt, Lee Hartwell and Paul Nurse were awarded the Nobel Prize in 2001. There are four types of cyclins in human cells.

(i) Cyclin-D: Cyclin-D transports cells from G1 to S phase and from S phase to G₂ phase.

(ii) Cyclin-E: Cyclin-E prepares the cell for DNA replication in S phase.

(iii) Cyclin-A: Cyclin-A activates and accelerates DNA replication in the S phase of the cell.

(iv) Cyclin-B: Cyclin-B performs spindle fibers and other essential functions for mitosis.

The tiny, granular, self-reproducing and enzyme-rich organelles surrounded by a membrane are called peroxisomes. They are also called microsomes. Its diameter is 0.2-17 millimicrons. It contains enzymes in crystal and granule form. At its center, fine-grained material accumulates to form an opaque core. It is called nucleoid. Its main enzymes are catalase, D-amino acid oxidase and uric acid oxidase. It breaks down fatty acids into acetyl Co by beta-oxidation process. Organelles that carry out beta-oxidation are called glyoxysomes. Belgian psychologist Christian de Duve discovered it in 1967.

Peroxisomes are abundant in photosynthetic cells of plants, such as those in the embryo membrane, liver and kidney cells. Peroxisomes arise from rough endoplasmic reticulum.

 

Function/importance of peroxisomes

1. It breaks down fatty acids into acetyl Co by beta-oxidation process.
2. Enzymes present in it react H2 and O2 to produce toxic H2O2.
3. Catalase enzyme breaks down toxic H2O2 to produce water and oxygen.
4. Catalase enzyme uses H2O2 to oxidize alcohol, phenol, formic acid, formaldehyde etc.
5. It destroys blood toxins in human liver and kidney by causing peroxidation reaction.
6. Plant cells contain a large number of antioxidative enzymes (superoxide dismutase, NADP dehydrogenase) that catalyze important reactions.
7. It regulates the concentration of oxygen in the cells.
8. It helps in the production of NAD, DNA and RNA.
9. It turns love into water.
10. It helps in making glycine and serine in plant cells.
11. It helps carry out photosynthesis in green plants.