What are the various stages of meiotic prophase-I? Enumerate the chromosomal events during each stage?


Prophase-I occurs over a long duration and involves several complicated changes in meiotic cell division. It is important because genetic recombination and variation in sexually reproducing organism occurs due to the events of this phase.
(i) The chromatin network opens out and threads become clear.
(ii) The chromosomes are thin, slender and long.
(iii) Chromosome number is diploid.
(i) Corresponding chromosomes become intimately associated.
(ii) The process of pairing is known as synapse. It is so exact that pairing is not merely between corresponding chromosomes but between corresponding individual! units.
(iii) The chromosomes become shorter and thicker.
Pachytene or Pachynema
(i) The synaptic chromosomes become very intimately associated.
(ii) The pair of chromosomes becomes short and thick.
(iii) Crossing over occurs at this stage. Chiasmata are clearly seen.
(i) Homologous chromosomes start separating from one another.
(i) Chiasmata tend to slip out of the chromosomes. This is known as terminalisation of chiasmata.
(ii) Chromosomes start separating out but the separation is not complete.
(iv) Nuclear membrane and nucleolus start disappearing.
(i) The bivalents condense further and get randomly distributed.
(ii) The separation of paired chromosomes is almost complete.
(iii) Terminatisation of chiasmata is almost complete.
(iv) Nuclear membrane and nucleolus disappear.