Question

Crossing over takes place during which of the following stages?
(a)Zygotene
(b)Pachytene
(c)Diplotene
(d)Diakinesis

Answer 1

Hint: They are the third stage of meiosis prophase-1 (a five-stage process). It’s preceded by the zygotene stage, where synapsis, i.e. pairing of homologous chromosomes, happen. During this stage, the tetrad formed due to synapsis becomes distinct, and 4 chromatids become clearly visible. There are four kinetochores present in each tetrad.

Complete answer:
Crossing over is the interchange of genetic material during sexual reproduction between two homologous chromosomes' non-sister chromatids that leads to recombinant chromosomes. It’s one of the ultimate phases of genetic recombination, which occurs within the pachytene stage of prophase I of meiosis during a process called synapsis.

Additional Information: The linked frequency of crossover between two gene loci (markers) is the crossing-over value. For a fixed set of genetic and environmental conditions, recombination during a particular region of a linkage structure (chromosome) tends to be constant and the same is then true for the crossing-over value which is employed within the production of genetic maps.
There are two popular and overlapping theories that specify the origins of crossing-over, coming from the various theories on the origin of meiosis. The primary theory rests upon the thought that meiosis evolved as another method of DNA repair, and thus crossing-over may be a novel way to replace possibly damaged sections of DNA.
So, the correct answer is ‘Pachytene’.

Note: Crossover was described, in theory, by Thomas Hunt Morgan. He relied on the invention of Frans Alfons Janssens who described the phenomenon in 1909 and called it "chiasma type". Crossover and DNA repair are very similar processes, which utilize many of equivalent protein complexes.