Sexual reproduction in organisms takes place through the fusion of male and female gametes, sperm and egg respectively. Gametes are haploid in nature, they contain only half the number of chromosomes. This genetic content makes them different from other body cells. A haploid cell is attained by the cell division called meiosis. Let’s see the different stages and the process of meiosis.
Mitotic cell division is equational in nature while meiosis is a reduction division.The salient features of meiotic division that make it different from mitosis are as follows:-
- It occurs in two stages of the nuclear and cellular division as Meiosis I and Meiosis II. DNA replication occurs, however, only once.
- It involves the pairing of homologous chromosomes and recombination between them.
- Four haploid daughter cells are produced at the end, unlike two diploid daughter cells in mitosis.
Let us discuss the different phases of meiosis I in brief:
- Prophase I: Prophase I is longer than the mitotic prophase and is further subdivided into 5 substages, namely leptotene, zygotene, pachytene, diplotene, and diakinesis. The chromosomes begin to condense and attain a compact structure during leptotene. In zygotene, the pairing of homologous chromosomes starts a process known as chromosomal synapsis, accompanied by the formation of a complex structure called as synaptonemal complex. A pair of synapsed homologous chromosomes forms a complex known as bivalent or tetrad. At pachytene, crossing over of non-sister chromatids of homologous chromosomes occurs at the recombination nodules. The chromosomes remain linked at the sites of crossing over. Diplotene marks dissolution of the synaptonemal complex and separation of the homologous chromosomes of the bivalents except at the sites of cross-over. The X-shaped structures formed during separation are known as chiasmata. Diakinesis is marked by the termination of chiasmata and assembly of the meiotic spindle to separate the homologous chromosomes. The nucleolus disappears and the nuclear envelope breaks down.
- Metaphase I: The bivalent aligns at the equatorial plate and microtubules from the opposite poles attach to the pairs of homologous chromosomes.
- Anaphase I: The homologous chromosomes separate while keeping the sister chromatids associated at their centromeres.
- Telophase I: The nuclear membrane reappears and is followed by cytokinesis. This gives rise to a dyad of cells.
Stay tuned with Byju’s to learn more about different phases of meiosis I and II.