In: Biology
What are Zygotene, Leptotene, Pachytene, Diplotene, Diakinesis?
Meiosis is a type of cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells. This process is required to produce egg and sperm cells for sexual reproduction. Meiosis can be divided into nine stages. These are divided between the first time the cell divides (meiosis I) and the second time it divides (meiosis II). Meiotic prophase is classically subdivided into five stages: leptotene, zygotene, pachytene, diplotene, and diakinesis.
Leptotene A cell destined to become a gamete enters the leptotene stage after its chromosomes are duplicated during interphase. During leptotene stage those duplicated chromosomes each consisting of two sister chromatids condense from diffuse chromatin into long, thin strands that are more visible within the nucleoplasm.
zygotene is the stage of prophase I that follows after leptotene . 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 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.
Pachytene The
paired homologous chromosomes that enter the pachytene stage are
now referred to as tetrads (for having four chromatids) or
bivalents (for two homologous chromosomes that have associated
together).
During pachytene, each tetrad shortens, thickens, and separates
into four distinct chromatids joined at the centromere. This is
also the stage of homologous recombination, e.g. chromosomal
crossover between nonsister chromatids. In sites where genetic
exchanges occurred, chiasmata form. However, chiasmata are not yet
visible under the microscope at this stage.
Diplotene The homologous chromosome pairs begin to separate and move away from one another except at chiasmata. The chiasmata become visible in this stage.
Diakinesis This is the final stage in meotic 1 prophase where the chiasmata dissappears characterised by shortening and thickening of the paired chromosomes, formation of the spindle fibers, disappearance of the nucleolus, and degeneration of the nuclear membrane.