Cell cycle and cell division - Notes | Class 11

Cell Cycle and Cell Division - Mitosis

Mitosis

It is the cell division occurring in somatic cells.
It is also called equational division as the number of chromosomes in the parent and progeny cells is the same.
Mitosis is generally seen in diploid cells, but it also occurs in haploid cells of some lower plants and social insects.
It involves major reorganization of all cell components.
The karyokinesis of mitosis has four stages: Prophase, Metaphase, Anaphase, and Telophase.

It is the longest phase in mitosis, following the S and G2 phases of interphase, where DNA molecules are intertwined.
Characteristic events:

Chromosomal materials (chromatin fibers) are untangled and condensed to form mitotic chromosomes, composed of two chromatids attached at the centromere.
Centrosomes move toward opposite poles, radiating microtubules called asters. The two asters, with spindle fibers, form the mitotic apparatus.

At the end of prophase, Golgi complexes, endoplasmic reticulum, nucleolus, and nuclear envelope disappear.

The nuclear envelope completely disintegrates, allowing chromosomes to spread through the cytoplasm.
Chromosome condensation is completed, making them easily observable under a microscope, each with two sister chromatids.
Chromosomes align at the equator on the metaphase plate.
Spindle fibers from both poles connect to chromatids via their kinetochores at the centromere.

It is the shortest phase in mitosis.
The centromere of each chromosome divides longitudinally, forming two daughter chromatids (future daughter nuclei chromosomes).
As spindle fibers contract, chromatids move from the equator to opposite poles.

Chromosomes cluster at opposite poles and uncoil into chromatin fibers.
A nuclear envelope develops around each chromosome cluster, forming two daughter nuclei.
Nucleolus, Golgi complex, and ER reappear.
Spindle fibers disappear.

It is the division of cytoplasm to form two daughter cells, starting during telophase.

Cytokinesis in animal cells:

A cleavage furrow appears in the plasma membrane, gradually deepening to divide the cytoplasm into two.

Cytokinesis in plant cells:

Due to the cell wall, vesicles from Golgi bodies accumulate at the equator, growing outward to form a cell plate, which separates the daughter cells. The cell plate becomes the middle lamella.

During cytokinesis, organelles like mitochondria and plastids are distributed between daughter cells.

In some organisms, karyokinesis without cytokinesis results in a multinucleate syncytium (e.g., liquid endosperm in coconut).

Produces diploid daughter cells with identical genomes.
Maintains the same chromosome number in somatic cells.
Supports body growth in multicellular organisms, with mitosis in meristematic tissues enabling continuous plant growth.
Restores the nucleo-cytoplasmic ratio disrupted by cell growth.
Facilitates cell repair and replacement (e.g., epidermis, gut lining, blood cells).

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