Question

How would you explain why crossing-over is an important source of genetic variation?

Answer 1

Hint: Cell division can be classified into two types- mitosis and meiosis. Both types of cell division lead to the formation of new cells. Mitotic division results in cells that are genetically identical to the parent cell while meiotic cell division results in cells that are genetically different from the parent cell.

Complete answer:
Crossing over, an event that occurs in the prophase I stage of meiosis is responsible for this genetic variation. The gametes (sperms or eggs) formed in the end are not identical to the parent cell or even to the other gametes produced during the process.
Before the cell begins mitotic division, it will duplicate its DNA. Humans have forty-six chromosomes i.e. twenty-three pairs of chromosomes. Each parent contributes a single chromosome to that pair. These twenty-three pairs are homologous chromosomes (have similar genes, not identical genes).
When the DNA is duplicated before meiosis, identical sister chromatids are formed. These sister chromatids are still attached to the centromere of the initial chromosome, hence the number of chromosomes is still the same.
Now, that the cell is prepared the division begins, prophase I is the first stage. The homologous chromosomes along with their identical sister chromatids take part in crossing over. The non-sister chromatids of the homologous pair of chromosomes overlap. This is called synapsis.
The point where the non-sister chromatids of the homologous pair are physically joined is called the chiasma. At the chiasma, genetic information is exchanged between the non-sister chromatids of the homologous pair. This is called crossing over.
Now, both non-sister chromatids of the homologous pair have some genetic variation. Crossing over results in genetic recombination.

Note: While crossing over occurs at random regions, there are regions in the chromosomes where crossing over occurs more frequently, i.e. more recombinants are formed. These regions are called recombination hotspots.
Sometimes crossing over can happen twice between the non-sister chromatids of the homologous pair. This is called double-crossing over. However, the frequency of this event occurring is very low.