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21. Cell Cycle eLearn.Punjab

Zygotene: First essential phenomenon of meiosis i.e., pairing of homologous

chromosomes called synapsis starts. This pairing is highly specific and exactly pointed,

but with no definite starting point(s). Each paired but not fused, complex structure is

called bivalent or tetrad.

Pachytene: The pairing of homologous chromosomes is completed. Chromosomes

become more and more thick. Each bivalent has four chromatids, which wrap around

each other. Non-sister chromatids of homologous chromosomes exchange their

segments due to chiasmata formation, during the process called crossing over. In this

way reshuffling of genetic material occurs which produces recombinations. Pachytene

may lasts for days, weeks or even years, whereas leptotene and zygotene can last only

for few hours.

Diplotene: The paired chromosomes repel each other and begin to separate. Separation

however, is not complete, because homologous chromosomes remain united by their

point of interchange (chiasmata). Each bivalent has at least one such point, the

chromatids otherwise are separated (Fig. 21.5).

Fig. 21.5 Chiasmata formation

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