Cell Cycle and Cell Division

Meiosis is also known as reduction division. It maintains the constant number of chromosomes generation after generation. It occurs in reproductive cells only and somatic body increases in size through mitotic division.

S- phase or Synthesis phase is the phase of the cell cycle in which DNA is replicated. It occurs between G₁ and G₂ phase.

During this phase, DNA is replicated and histones are sythesized so that each chromosomes becomes two chromatids.

Lampbrush chromosomes are a special form of chromoosme found in the growing oocytes of most animals, except mammals. They grow profusely during prophase of meiosis- I.

Prophase I consists of 5 stages:

(i) Leptotene- Chromosomes become gradually visible.

(ii) Zygotene- Synapsis, pairing of homlogous chromosomes takes place. It results in the formation of bivalents. Also, formation of special proteinaceous structure called synaptonemal complex occur.

(iii) Pachytene- Cells that eneterd this phase results in spiralling, shortening and thickening of the bivalents.

(iv) Diplotene- Dissolution of synaptonemal complex takes place. Also, recombined homologous chromosomes of the bivalents seperate.

(v) Diakinesis- Terminalisation of chiasmata occur in this phase.

Colchicine, an alkaloid obtained from Colchincum autumnale (Liliaceae), prevents mitosis by inhibiting spindle formation through disruption of microtubules. It is a mitotic poison.

Cell cycle is a series of cyclic changes through which a cell passes during its growth and division. In the cell cycle, the resting stage or interphase can be divided into three periods.

G₁ phase: RNA and Protein synthesised

S phase: DNA is formed from purines and pyrimidines. The DNA content of nucleus would be doubled.

G₂ phase: Synthesis of RNA and protein continues, but DNA synthesis stops.

Farmer and Moore (1905) proposed the term meiosis. Meiosis is a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cells, as in production of gametes and plant spores.

Chiasmata is observed in meiosis during diplotene, sub stage of Prophase-I. It is the result of crossing over. Each chiasmata is the site of an exchange between chromatids. It is produced by breakage and reunion between any two of the four strands present at each site.

Cell cycle is divided into 4 phases, namely, G₁, S, G₂ and M- phase.

DNA polymerase is synthesized in G₁- phase. DNA and histones are synthsized during S- phase. Mitotic spindle begins to form during G₂- phase. Karyokinesis and cytokinesis occur during M phase.

The term 'Meiosis' was given by Farmer and Moore (1905). To study meiosis in plants, anther is the best part. 

The process of meiosis involves two division of the genetic material. The first division is called reduction division or Meiosis I as it reduces the number of chromosomes from 46 chromosomes or 2n to 23 chromosomes or n.

Meiosis II is called equational division because it does not reduce chromosome numbers. A nuclear division that maintains the same ploidy level of the cell.