Question

Arrange the following events of meiosis in correct sequence
(a) Crossing over
(b) Synapsis
(c)Terminalisation of chiasmata
(d) Disappearance of nucleolus
A. b,a,d, c
B. b,a,c, d
C. a,b,c, d
D. b,c,d, a

Answer 1

Hint: The cell division occurring in the germ cells of sexually-reproducing organisms to produce gametes (sperm in males and egg in females) is called meiosis. There is a sequential occurrence of events in meiosis, each step having its own significance. Let us know a little more about meiosis to find the correct answer.

Complete answer:
 Let us first know about meiosis.
So as stated above, meiosis is the type of cell division occurring in sexually reproducing organisms wherein a single cell undergoes two rounds of division to produce four daughter cells having the half number of chromosomes as that found in the parent cell.
Meiosis completed in majorly two rounds- Meiosis I and Meiosis II.
Meiosis I- It is known as reductional division. Its stages-
> Prophase I- It can again be divided into five substages-
> Leptotene- First substage during which chromosome condensation takes place.
> Zygotene- Synapsis, in which homologous chromosomes come to lie even closer and stably paired. Installation of the central and transverse elements of the synaptonemal complex mediates this phenomenon. The paired complexes are known as bivalents or tetrads.
> Pachytene- Fully functional synaptonemal complex exists and there is exchange of genetic material between non- sister chromatids. This is called crossing-over and is responsible for the production of variations in the offsprings.
> Diplotene- There is disassembly of synaptonemal complex and homologous chromosomes begin to dissociate from each other, except at the site of crossing-over called chiasmata. Eventually, chiasmata also gets terminalis.
> Diakinesis- Disappearance of nucleoli, disintegration of nuclear membrane into vesicles and the beginning of formation of meiotic spindles are the major events of this phase.
> Metaphase I- There is a line-up of homologous pairs of chromosomes at the equator or metaphase plate.
> Anaphase I- There is separation of homologues and they move towards opposite poles. However, sister chromatids remain attached.
> Telophase I- Nuclear membrane reappears and two cells having half the genetic material are produced.
> Meiosis II- It is known as equational division and is similar to mitotic division. There is ultimately production of four daughter cells from two haploid cells produced in meiosis I .

From the above description, it is clear that the correct answer is (B)- b,a,c,d.

Note:
Variations in meiosis can lead to various syndromes. For example, nondisjunction of chromosomes in meiosis I or of sister chromatids in meiosis II are the basis for trisomy or monosomy leading to Down syndrome, Edwards syndrome,Klinefelter syndrome, etc.
Each step of the meiosis is highly regulated so as to ensure production of normal gametes.