HOW CELL DIVISION OCCURS- PROCESS OF MITOSIS AND MEIOSIS

MITOSIS

Mitosis is a type of cell division by which a cell divides into two identical daughter cells. It occurs in all somatic cells. Mitosis produces diploid cells with two sets of chromosomes .

Mitosis occurs in four phases – prophase, metaphase, anaphase and telophase. It is also known as equational division as the number of chromosomes in the parent and progeny cells is the same. During the process of mitosis, division of nucleus (karyokinesis) is followed by division of cytoplasm (cytokinesis).

PROPHASE

Prophase is the first phase of mitosis. It occurs after G₂ phase of cell cycle. Following events occur during prophase :

• Condensation of chromosomal material to form chromosomes.
• The centrioles which had duplicated during the S phase of cell cycle begin to move towards opposite poles of the cell.
• Initiation of the assembly of mitotic spindle.

METAPHASE

• Complete disintegration of nuclear membrane.
• Chromosomes are completely condensed so that they are clearly visible under microscope.
• All the chromosomes align at the equator with one chromatid of each chromosome connected by its kinetochore to spindle fibers.

ANAPHASE

• The chromosomes split into chromatids.
• Each chromatid is pulled by spindle fibers towards opposite poles of cell.

TELOPHASE

• Chromosomes reach opposite poles of cell and decondense into chromatin.
• Nuclear membrane reforms around chromosomal material.
• Nucleolus, golgi complex and endoplasmic reticulum reform.

Cytokinesis occurs after nuclear division.

CYTOKINESIS

The cell is divided into two daughter cells by cytokinesis thus completing cell division.

In an animal cell, this is achieved by the appearance of a furrow in the plasma membrane which deepens gradually and finally joins at center. This divides the cell into two.

In plant cells, wall formation starts in the center of the cell and grows outward.

MEIOSIS

It is also known as reductional division as the number of chromosomes is reduced by half resulting in the production of haploid daughter cells.

Meiosis produces gametes : sperm cells in males and egg cells in females. It involves two sequential cycles of nuclear and cell division called, meiosis I and meiosis II. Four haploid cells are formed at the end of meiosis II.

MEIOSIS I

It occurs in four stages : prophase I, metaphase I, anaphase I and telophase I.

PROPHASE I

It starts with a diploid cell. It is subdivided into five phases, namely, leptotene, zygotene, pachytene, diplotene and diakinesis.

Leptotene – Compaction of chromosomes.

Zygotene – During this stage, chromosomes start pairing together. This is known as synapsis. Such paired chromosomes are called homologous chromosomes. The complex formed by homologous chromosomes is called tetrad.

Pachytene – Crossing over occurs between non-sister chromatids of homologous chromosomes. Crossing over or recombination is the exchange of genetic material between two homologous chromosomes. It is mediated by enzyme recombinase.

Diplotene – The recombined homologous chromosomes of the tetrad separate from each other except at the site of crossovers.

Diakinesis – Chromosomes get fully condensed. Spindle fibers appear. Nuclear envelope breaks down and nucleolus disappears.

METAPHASE I

• The bivalent chromosomes align at the equatorial plate.
• Spindle fibers from opposite poles attach to the pair of homologous chromosomes.

ANAPHASE I

Spindle fibers separate homologous chromosomes in each tetrad.

TELOPHASE I

• Spindle fibers disappear.
• Nuclear membrane reforms around chromosomes.
• Nucleolus reappears.
• Cytokinesis occurs.
• Two genetically different haploid cells are produced.

The period between meiosis I and meiosis II is called interkinesis. Interkinesis is followed by prophase II.

MEIOSIS II

PROPHASE II

• Nuclear membrane disappears.
• Spindle fibers fan out from centrioles.
• Chromosomes become compact.

METAPHASE II

• Chromosomes align at the equator.
• Spindle fibers get attached to the centromere of chromosomes.

ANAPHASE II

Chromosomes split at centromere and move towards opposite poles of the cell.

TELOPHASE II

• Chromosomes get enclosed by nuclear envelope.
• Spindle fibers disappear.
• Cytokinesis occurs in both the cells.
• Four genetically different haploid cells are are produced. (Each containing only one set of chromosomes).