Question

Explain prophase 1 of meiosis.

Answer 1

Concept 4: Meiosis I: Prophase I. Chromosomes become visible, crossing-over occurs, the nucleolus disappears, the meiotic spindle forms, and the nuclear envelope disappears. ... The duplicated homologous chromosomes pair, and crossing-over (the physical exchange of chromosome parts) occurs.

Answer 2

Answer:

Meiosis is a cell division which occurs in diploid cells and give rise to four haploid cells.

The daughter cells have half number of chromosomes as compared to the parent cell.

Meiosis cell division produces gametes.

This cell division is also called as reductional division.

There are two divisions in meiosis cell division that is meiosis 1 and meiosis 2.

The Meiosis 1 is initiated after the parental chromosome have replicated to produce identical sister chromatids at the S phase of interphase.

The Meiosis 1 is also called as heterotypic or reduction division. It contains prophase 1, metaphase 1, anaphase 1 and telophase 1.

Prophase 1 :

It consists of five stages called lepotene, zygotene, pachytene, diplotene and diakinesis.

Lepotene :

In this stage, the nucleus enlarges and the chromosomes become clear. They poses a string of swollen area called the chromomeres. In short, the the compaction and condensation of chromosomes takes place in this stage.

Zygotene :

The Homologous chromosomes pair up in this stage. The paired up Homologous chromosomes are seen as bivalents or tetrads. The pairing process is called synapsis and the process is accompanied by a a complex structure called the synaptonemal complex.

Pachytene :

The chromosomes of the bivalent gets spirally twisted around each other. The process of crossing over takes place, which is a process where transfer of genetic material between both the chromosomes in the bivalent takes place. It is an enzyme mediated process where recombinase is the enzyme.

Diplotene :

The bivalents start repelling. The chromosomes that were joined at several points is called a chiasmata, which appears in this stage. These are the sites of the sites of crossing over.

Diakinesis :

Chiasmata terminates, and the homologous pairs stars moving towards the equator. The centrioles moves towards the pole and the nuclear membrane dissapears. Due to the repulsion between the chromosomes in the bivalent, they form a particular shape. Spindle fibres starts forming. Then, the nucleolus dissapears and the nuvlear membrane breaks.