Question

What happens during Meiosis I

Answer 1

Meiosis is reductional cell division takes place in Gamets. In this division Diploid chromosome number reduced to haploid and at the end of cell division four daughter cells formed from single mother cell. It takes place in two phases I.e Meiosis I and meiosis II

Meiosis I divided into two parts one is karyokinesis and second is cytokinesis.

A) Karyokinesis have 4 phases as follows

1)Prophase 1

It is the most complicated and longest phase . It again divided into 5 substages viz leptotene, zygotene, pachatane, diplotene and diakinesis.

a. Leptotene -

During this phase the chromatin network condenses and resolves onto long and thin thread like chromosome. 3acj chromosome consist of two chromatids but appear as a single thread.

b. Zygotene-

It is the phase in which homologous chromosome begin to pair lengthwise ( The morphologically and structurally similar chromosome - one paternal and the other maternal - Present in the Diploid cell are called homologous chromosome). Such pairing of chromosome is called synapsis. The pairs at this stage are called bivalent.

c. Pachytene

During this phase condensation of chromosome progress and they become short and thick. Each of the chromosome now show two chromatids so that the bivalent now appear to be composed of four chromatid known as tetrad. The twisting of homologous chromosome becomes compact resulting in breakage and rejoining of the chromatid. During this process crossing over I.e exchange of genetic material between non sister chromatid of homologous chromosome takes place. The points where crossing over takes place are called chiasmata.

d. Diplotene

The homologous now start repelling each other and begin to separate . However at the points of crossing over they remain attached and thus chiasmata can be seen. Nucleolus and nuclear membrane start disappearing.

e. Diakinesis

During this phase chromosome continue to condense and shorten. The separation of homologous chromosome proceeds and the chiasmata get shifted to the ends of chromatids. This process is called terminalization. The nucleolus and nuclear membrane completely disappear and kinetic spindles become apperarnt.

2) Metaphase I

The spindle formation is completed and the bivalent move and arrange themselves at the equatorial plane. Chromosome fiber extend from the poles to the centromere of corresponding homologous.

3. ANAPHASE I

The chromosomal fibers contract and pull the homologous and inter chromosomal fibers appear and extend so that homologous chromosome start moving towards opposite poles. The centromere do not divide . During this phase homologous chromosome which are still attached at chaismata , finally get separate. This is called disjunction. Each chromosome is with two chromatids and one centromere . At the end of this phase half the number of chromosome gather at respective poles

.4 ) TELOPHASE I

The chromosome uncoil to form chromatin. The spindle fiber disappear. The nuclear membrane and nucleolus reappear forming two daughter nucleoli

B​​​) Cytokinesis I

Telophase I is often followed by division of cytoplasm. The plasma membrane constricts in the middle in animal cell while a cell plate is formed in plant cell to form two daughter cells. Each daughter cell is with single nucleus consisting single ( haploid) set of chromosome.