Human physiology

Lesson 06 : Meiosis


Like mitosis, meiosis is also divided into following chronological phases;

Prophase I: It is a long process during which homologous chromosomes pair closely and interchange hereditary material. The first meiotic prophase is divided into five sub-stages: Leptotene, Zygotene, Pachytene, Diplotene, and Dikinesis.

Leptotene stage begins when each chromosome is first seemed to have its condensed form its interphase conformation to produce a long thread. Each chromosome is attached at both of its end to the nuclear envelop via specialised structure called attachment plate. Although each chromosome has replicated and consists of two sister chromatids, these chromatids are very close to each other and therefore chromosomes appear to be single stranded.

Zygotene stage begins as soon as intimate pairing between the two members of each homologous chromosome pair is initiated by the process called synapsis or zygotene pairing. The paired homologous chromosomes comprise one chromosome derived from each parent. Synapsis often starts when the homologous ends of the two chromosomes are brought together on the nuclear envelope. It continues inwards in a zipper-like manner from both ends aligning the two chromosomes side by side.

Pachytene stage commences as soon as the synapsis is complete all along the chromosome, the cells are said to have entered the pachytene stage of prophase, where they may remain for days. At this stage, large recombination nodules appear at intervals on the synaptonemal complex. During Pachytene stage crossing over of genes occur between sister chromatids with the help of recombinase enzyme.

Diplotene stage is characterized by dissolution of synaptonemal complex, recombination of chromosomal bivalents to form X-shaped chiasmata. This stage in oocyte (in women) lasts for 10-12 or more years i.e. from birth to puberty of a girl and it is actually completed only few hours before ovulation. Therefore, diplotene stage of prophase-1 is the longest phase of cell division in human.

Dikinesis is last stage of meiotic prophase –I, characterized by termination of chiasmata, condensation of chromosomes, disappearance of nucleolus and nuclear membrane.

Metaphase-1: is represented by lining of bivalent chromosomes at equatorial plate, appearance of spindle and microtubules.

Anaphase-1: is characterized by separation of homologous chromosomes towards respective centromeres.

Telophase-1: is characterized by reappearance of nucleolus and nuclear membrane with some dispersion of chromosomes. After a brief period of interphase, the cell then proceeds into meiosis II.

MEIOSIS-II: Meiosis II is much faster and simpler division and resembles with normal mitosis, prophase II is simpler and shorter phase followed by a quick metaphase II, anaphase II and telophase II. At the end of telophase II, four daughter haploid cells are produced and meiosis division is completed.

Last modified: Tuesday, 10 April 2012, 5:37 AM