Question

Arrange the following events in the correct sequence of their occurrence during meiosis.
a) Formation of a recombination nodule
b) Reduction in the number of chromosomes
c) Alignment of univalents at the equator
d) Terminalisation of chaismata

• A. a , b , d and c
• B. a, d, b, and c
• C. a, c, b and d
• D. c, d, b and a

Answer 1

Answer: (B)

a, d, b, and c

The various events of meiosis given in the question occur in the following sequence:

• Crossing over occurs between non sister chromatids of homologous chromosomes during pachytene stage of prophase I in which segments are exchanged and a recombination nodule is formed.
• The crossing over results in the formation of a X shaped structure known as chiasma, during diplotene stage of prophase I.
• The terminalisation of chiasma occur during diplotene stage of Prophase. The homologous chromosomes separate from each other except in the place of cross over.
• The chromosomes align at the equatorial plate during metaphase which the get pulled apart during anaphase.
• The cell undergo cytokinesis and divides into two cells each having half the number of chromosomes as the parent cell.
• Each cell then undergoes meiosis II which is similar to mitosis and in that case, individual chromosomes get aligned at the equatorial plate which are then pulled in opposite directions by the spindle fibres. This results in the splitting of chromosomes into sister chromatids.

Hence, correct sequence of the occurrence of events is a, d, b and c.

So, the correct answer is 'a, d, b and c'.