noun, plural: zygotenes
The sub-stage of prophase I of meiosis I in which the homologous chromosomes pair or come together in synapse
Meiosis is form of cell division that gives rise to genetically diverse sex cells or gametes. It is comprised to two successive nuclear divisions namely meiosis I and meiosis II. Meiosis I is comprised of four stages: prophase I, metaphase I, anaphase I, and telophase I. Prophase I is the first stage and consists of the following sub-stages: leptotene, zygotene, pachytene, diplotene and diakinesis.
The zygotene is the stage of prophase I that follows after leptotene and precedes pachytene. Prior to zygotene, the chromosomes start to condense into long strands inside the nucleus and the chromosomes appear as threadlike. Zygotene is that phase wherein the homologous chromosomes pair or come together in synapse. The pairing or coming together of homologous chromosomes is called synapsis. It may be facilitated by the synaptonemal complex.
The term zygotene is derived from Greek words that mean paired threads. The pairing is zipper-like in fashion. The pairing may start at any part of the chromosomes, e.g. near the end or near the centromere. The pairing is also highly specific, i.e. homologous chromosomes that come in pairs must be of equal length and have the same centromere position. The paired chromosomes are called bivalent or tetrad chromosomes.
The zygotene stage is also described as bouquet stage since the telomeres cluster at one end of the nucleus.
Word origin: from Ancient Greek words: zygon (junction, yoke) + –tene (filament)