Meiosis | Stages of Meiosis | Cell cycle | Biology lecture Video 4

MEIOSIS
The production of offspring by sexual reproduction includes the fusion of two gametes, each with a complete haploid set of chromosomes. Gametes are formed from specialized diploid cells. This specialized kind of cell division that reduces the chromosome number by half results in the production of haploid daughter cells. This kind of division is called meiosis. Meiosis ensures the production of haploid phase in the life cycle of sexually reproducing organisms whereas fertilization restores the diploid phase. We come across meiosis during gametogenesis in plants and animals. This leads to the formation of haploid gametes. The key features of meiosis are as follows:
• Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II but only a single cycle of DNA replication.
• Meiosis I is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase.
• Meiosis involves pairing of homologous chromosomes and recombination between them.
• Four haploid cells are formed at the end of meiosis II.
Meiosis I
Prophase I: Prophase of the first meiotic division is typically longer and more complex when compared to prophase of mitosis. It has been further subdivided into the following five phases based on chromosomal behavior, i.e., Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.

Some important terminologies
Chiasmata: X-shaped observable regions in diplotene in which nonsister chromatids of homologous chromosomes cross-over each other are called chiasmata.
Crossing over: Crossing over is a process in which homologous chromosomes exchange genetic material through the breakage and reunion of two chromatids with the help of enzyme recombinase. This process can result in an exchange of alleles between chromosomes.

Cytokinesis: The division of the cytoplasm of a cell following division of the nucleus that occurs in mitosis and meiosis, when a parent cell divides to produce two daughter cells.

Diakinesis: This is the final stage of meiotic prophase I in which the chromatids break at the chiasmata and exchange their parts. During this phase the chromosomes are fully condensed and the meiotic spindle is assembled to prepare the homologous chromosomes for separation.
Diplotene: This is the stage of the first meiotic prophase, following the pachytene, in which the two chromosomes in each bivalent begin to repel each other and a split occurs between the chromosomes, which are then held together by regions where exchanges have taken place (chiasmata) during crossing over.
Leptotene: This is the stage of meiosis in which the chromosomes are slender, like threads.
Pachytene: In meiosis, the stage following synapsis (zygotene) in which the homologous chromosome threads (synaptonemal complex) shorten, thicken, and continue to intertwine, and each of the conjoined (bivalent) chromosomes separate into two sister chromatids, which are held together by a centromere, to form a tetrad. During this phase the chromatids break up and corresponding regions of the non-sister chromatids of the paired chromosomes are exchanged in a process known as crossing over.
Synapsis: The pairing of homologous chromosomes along their length; synapsis usually occurs during prophase I of meiosis, but it can also occur in somatic cells of some organisms.
Zygotene: This is the synaptic stage of the first meiotic prophase in which the two leptotene chromosomes undergo pairing by the formation of synaptonemal complexes to form a bivalent structure.
Significance of meiosis
Meiosis is the mechanism by which conservation of specific chromosome number of each species occurs
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