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Meiosis - I

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Meiosis - I - Lesson Summary

Meiosis one, which includes prophase one to telophase one, results in the formation of two cells with half the number of chromosomes compared to the original cell. In sexually reproducing organisms, meiosis results in the formation of sperm cells and egg cells. Later, during the process of fertilisation, these haploid cells fuse to form a diploid offspring. Meiosis is preceded by an interphase stage, during which DNA replicates and each chromosome doubles.
 
Meiosis consists of two cell divisions, namely meiosis one and two. Meiosis one is divided into four stages – prophase one, metaphase one, anaphase one and telophase one. Similarly, meiosis two is divided into four phases – prophase two, metaphase two, anaphase two and telophase two. Meiosis one begins with prophase one, which is divided into five phases, namely, leptotene, zygotene, pachytene, diplotene and diakinesis. During leptotene, chromatin condenses into chromosomes that consist of two chromatids. During zygotene, homologous chromosomes, one from the paternal set and one from the maternal set, get attracted to each other and form pairs, called synapsis. During this process, synaptonemal complex is formed, which consists of a pair of synapsed homologous chromosomes called a tetrad or a bivalent.

During pachytene, the exchange of genetic material occurs between the non-sister chromatids of homologous chromosomes, at the recombination nodules on the synaptonemal complex. This process is called crossing over and the recombination of genetic material between the two chromosomes is initiated by the recombinase enzyme, which leads to variation in the offspring. During diplotene, the recombined homologous chromosomes of the tetrad separate except at the crossover sites. The x-shaped part on the homologous chromosomes that holds the two chromatids together is called chiasmata. During diakinesis, the chiasmata terminates.

By the end of prophase, the meiotic spindle assembles and nuclear envelope and nucleolus disappear. During metaphase one, the chromosomes line up along the equatorial plate of the cell and the spindle fibres get attached to the kinetochores of the homologous chromosomes. During anaphase one, the spindle fibres pull the chromosomes towards the opposite ends of the cell. During telophase one, the chromosomes are at the poles, while the spindle fibres disintegrate and the nuclear envelope and the nucleolus reappear. Telophase one is followed by cytokinesis and results in two cells referred to as a diad.

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