Question

Chiasmata formation takes place during
a. Prophase I
b. Metaphase I
c. Anaphase II
d. Telophase I

Answer 1

Hint: At this stage, this phenomenon is the point of contact where two (non-sister) chromatids belonging to homologous chromosomes are physically related.

Complete answer:
> Option A is correct. The chiasmata become visible during the diplotene stage of meiosis prophase I but the actual "crossing-over" of genetic material is believed to occur during the previous pachytene stage. As a tetrad, consisting of two pairs of sister chromatids, begins to break, the only contact points are in the chiasmata.

> Option B is incorrect. Step Metaphase I happen when tetrads line-up around the spindle equator. Spindle fibers bind to each homologous pair of chromosomes in the centromere region. The pairs of homologous chromosomes or bivalents are closely coiled and condensed like they are going to be in meiosis, arranged on an equidistant plane from the poles called the metaphase plate.

> Option C is incorrect. Separate the centromeres and now individual chromosomes transfer the sister chromatids towards the opposite poles of the cell. When the spindle apparatus drags the chromosomes along, their arms can be seen moving backward, so that the chromosomes form V-shapes.

> Option D is incorrect. As a consequence of the spindle action, the homologous chromosome pairs complete their migration towards the two poles. Now at each pole, there is a haploid set of chromosomes, with each chromosome only having 2 chromatids. A nuclear envelope transforms across any set of chromosomes, the spindle vanishes, and cytokinesis proceeds.

So, the correct answer is Option A.

Note: In humans, one chiasma per chromosome arm tends to be present and in mammals, the number of chromosome arms is a strong indicator of the number of crossovers. Even evidence shows in humans and probably other organisms that the number of crossovers is regulated at the level of a whole chromosome and not an arm.