Zygotene is the second stage of meiosis prophase-1. It is preceded by the leptotene stage, where uncoiling and condensing of chromosomes takes place.
In the zygotene stage, homologous chromosomes (one from mother and one from father) come together and pair. The process of the pairing of homologous chromosomes is known as synapsis.
The process of synapsis is very specific and exact. The process of synapsis may start at the end of the chromosome and progress towards the centromere or vice-versa. In the random type, synapsis occurs at various points.
The paired chromosomes are joined together by a protein complex known as the synaptonemal complex.
The pair homologous chromosomes at this stage are called tetrad or bivalent. The number of bivalents is half the number of chromosomes present in the cell initially. Each bivalent contains one chromosome from each parent.
Synaptonemal complex stabilises the pairing and also facilitates crossing over or recombination in the Pachytene stage.
Zygotene is followed by the Pachytene stage. Four chromatids of tetrads become clearly visible in the pachytene stage.
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