Question

What are the stages of meiosis?

Answer 1

Meiosis:

1. It is a type of cell division in which the chromosome number is reduced by half in the daughter cells.
2. This ensures the presence of the haploid phase in the life cycle of an organism.

Cycles:

1. This division involves two sequential cycles of nuclear and cytoplasmic division but DNA replication occurs only once.
2. They are known as Meiosis I and meiosis II.

Stages:

Prophase I:

1. It is for a longer duration.
2. It is divided into five sub-stages that are leptotene, zygotene, pachytene, diplotene, and diakinesis.
3. During leptotene, the chromosomes start to condense and achieve a compact shape.
4. The pairing of homologous chromosomes in zygotene initiates a process called chromosomal synapsis, which is followed by the development of a complex structure known as the synaptonemal complex.
5. A bivalent or tetrad complex is made up of two synaptically linked homologous chromosomes.
6. Non-sister chromatids of homologous chromosomes cross over at the recombination nodules during the pachytene stage.
7. At the locations of crossing over, the chromosomes are still connected.
8. Except at the sites of cross-over, diplotene indicates the breakdown of the synaptonemal complex and the separation of the bivalents.
9. Chiasmata are the X-shaped structures that occur during separation.
10. Chiasmata are terminated during diakinesis, and the meiotic spindle is assembled to segregate homologous chromosomes.
11. The nuclear membrane disintegrates, and the nucleolus vanishes.

Metaphase I:

1. The bivalents are aligned at the equatorial plate.
2. The microtubules from the opposite poles connect to the homologous chromosomal pairs.

Anaphase I:

1. The homologous chromosomes are separated in this phase.
2. Sister chromatids are however associated with each other at the centromere.

Telophase I:

1. The chromosomes are at opposite poles.
2. The nucleolus and the nuclear membrane reappear.
3. Cytokinesis occurs and this stage is known as a dyad of cells.

Prophase II:

The nuclear membrane dissolves, the Golgi apparatus and endoplasmic reticulum complexes vanish, and the chromosomes start to condense simultaneously.

Metaphase II:

1. The microtubules at the kinetochores of sister chromatids bind the chromosomes to the centriole poles.
2. Additionally, they line together to produce the metaphase plate at the equator.

Anaphase II:

1. The centromeres of each chromosome split simultaneously during the anaphase II stage of meiosis II, and the sister chromatids are drawn apart in opposite directions.
2. The kinetochore is at the leading edge and the chromosomal arms are lagging as the chromatids travel towards the poles.

Telophase II:

1. A nuclear membrane forms around the undifferentiated mass of chromosomes as it dissolves once more.
2. Telophase II is immediately after cytokinesis.

Therefore, meiosis occurs in 2 sequential cycles consisting of 4 phases of nuclear division followed by cytokinesis.