Answer:
Prophase-I occurs over
a long duration and involves several complicated changes in meiotic cell
division. It is important because genetic recombineiion and variation in sexually
reproducing organism occurs due to the events of this phase.
Leptotene
(i) The
chromatin network opens out and threads become clear.
(ii) The
chromosomes are thin, slender and long.
(iii)
Chromosome number is diploid.
Zygotene
(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.
Diplotene
(i) Homologous
chromosomes start separating from one another.
(ii)
Chiasmata tend to slip out of the chromosomes. This is known as terminalisation
of chiasmata. .
(iii)
Chromosomes start separating out but the separation is not complete.
(iv) Nuclear
membrane and nucleolus start disappearing.
Diakinesis
(i) The
bivalents condense further and get randomly distributed.
(ii) The
separation of paired chromosomes is almost complete.
(iii)
Terminalisation of chiasmata is almost complete.
(iv) Nuclear
membrane and nucleolus disappear.
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