Exemplar Problems

Question:

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

Answer:

Meiotic prophase -I is divided into following stages:

a. Leptotene: The compaction of chromosomes continues throughout leptotene and the chromosomes become gradually visible under the light microscope.

b. Zygotene: Chromosomes start pairing together and this process of association is called synapsis. Such paired chromosomes are called homologous chromosomes. Electron micrographs of this stage indicate that chromosome synapsis is accompanied by the formation of complex structure called synaptonemal complex.

c. Pachytene: clearly appears as tetrads. This stage is characterised by the appearance of recombination nodules, the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes. Recombination between homologous chromosomes is completed by the end of pachytene, leaving the chromosomes linked at the sites of crossing over

d. Diplotene: The recombined homologous chromosomes of the bivalent tend to separate from each other except at the site of crossovers. The dissolution of synaptonemal complex is seen. These sites (X-shaped structures), are called chiasmata.

e. Diakinesis: This is marked by terminalisation of chiasmata. During this phase the chromosomes are fully condensed and the meiotic spindle is assembled to prepare the homologous chromosomes for separation.