Biology: CELL CYCLE AND CELL DIVISION

CELL CYCLE AND CELL DIVISION

Cell cycle

It is a series of events that takes place in a cell, leading to the formation of two daughter cells from a single mother cell.

Cell cycle is divided into two basic phases: Interphase and M phase Phases of cell cycle.

·         Interphase

·         M  phase (mitosis phase) karyokinesis and cytokinesis

Interphase

o   G1 phase

o   S phase

o   G2 phase

o   Go phase-quiescent stage

Mitotic phase

·         Karyokinesis (nuclear division): – Prophase, Metaphase, Anaphase and Telophase.

·         Cytokinesis (division of cytoplasm)

Interphase

Interphase involves a series of changes that prepares the cell for division. It involves the period of cell growth and cell division in an orderly manner.

It is divided into three phases:

• G1 phase – It involves growth of cell and preparation of DNA for replication.
• S phase – It involves DNA synthesis. The amount of DNA doubles, but the chromosome number remains the same.
• G2 phase – It involves protein synthesis and further growth of cell, which prepares it for division.
• G0 phase or Quiescent phase – It is the stage when metabolically active cell remains quiescent for long period of time.

I Mitosis

• It is a process of cell division where chromosomes replicate and get equally distributed into two daughter cells. Hence, it is also called equational division.
• The process of mitosis keeps the chromosome number equal in daughter as well as parental cell.
• Mitosis usually takes place in somatic cells.

Mitosis involves four stages:

Prophase

• It involves initiation and condensation of chromosomes.
• Nucleolus and nuclear membrane disappears.

Metaphase

• Chromosomal material condenses to form compact chromosomes that get aligned in the middle of nucleus at equatorial plate.

Anaphase

• Centromere splits and chromosomes move apart towards two opposite poles due to shortening of spindle fibres.

Telophase

• Chromosomes finally reach their respective poles.
• Nuclear envelope assembles around each chromosome clusters.
• Nucleolus and other organelles reform.

Karyokinesis and Cytokinesis

• Karyokinesis is the division of nucleus during mitosis or meiosis which is followed by cytokinesis.
• Cytokinesis involves the division of cytoplasm of a cell.
• Cytokinesis is achieved in animal cell by cleavage, which deepens and divides the cell into two.
• It is achieved in plant cell by cell plate formation.
• When karyokinesis is not followed by cytokinesis, a multinucleated condition arises. This is called Syncytium.

Significance of mitosis

• Results in formation of diploid genetically identical daughter cells
• Growth of the body takes place by mitosis.
• Cell repair and replacement of worn out tissues
• Maintenance of nucleo-cytoplasmic ratio
• Vegetative reproduction in plants takes place by mitosis.

II Meiosis

• It is the process which involves the reduction in the amount of genetic material.
• It mainly occurs in germ cells.
• At the end of meiosis II, four haploid cells are formed.
• It is comprised of two successive nuclear and cell division with a single cycle of DNA replication.
• The phases of meiosis are as shown below-

Meiosis I

1. Prophase I – It comprises of 5 stages:

i. Leptotene

• Chromosomes start condensing.

ii. Zygotene

• Pairing of chromosomes called synapsis occurs.
• A pair of synapsed homologous chromosomes is called bivalent or tetrad.

iii. Pachytene

• Exchange of genetic material (crossing over) between non-sister chromatids occurs.
• Chiasmata formation

iv. Diplotene

• Bivalents formed during pachytene separate from each other (except at chiasmata) due to dissolution of synaptonemal complex.

v. Diakinesis

• Terminalisation of chiasmata can be observed.
• By the end of this stage, the nucleolus disappears and the nuclear envelope breaks.

2. Metaphase I

• Bivalents (tetrad) get aligned along metaphase plate through spindle fibres.

3. Anaphase I

• Homologous chromosomes separate while chromatids remain attached at their centromere.

4. Telophase 

• Nucleolus and nuclear membrane reappear around chromosome clusters at each pole.
• Inter-kinesis – It is the stage between two meiotic divisions.

Meiosis II

1. Prophase II

• Chromosomes become compact.
• Nuclear membrane disappears.

2. Metaphase II

• Chromosomes align at the equator.
• Kinetochores of sister chromatids attach to spindle fibres at each pole.

3. Anaphase II

• Chromatids separate by splitting of centromere.
• As a result, chromatids move towards their respective poles in the cell.

4. Telophase II

• Nuclear envelope and nucleolus reform around the chromosome clusters.

Cytokinesis:

• After meiosis II, the process of cytokinesis results in the formation of four haploid cells.

Significance of meiosis:

• It results in reduction of chromosome number by half in gametes, which again doubles during fertilization. Therefore, it helps to conserve the chromosome number of species from generation to generation.
• Crossing-over, occurring in pachytene stage of meiosis I, is a source of genetic variation in sexually reproducing organisms.
• The variation thus formed helps in evolution.

The Difference Between Meiosis and Mitosis