Question

Crossing over is the exchange of genetic material between?
A. Non-sister chromatids
B. Sister chromatids
C. Non-homologous chromosomes
D. Two different bivalents

Answer 1

By the breakage and rejoining of the chromatids of a homologous chromosome crossing over occurs which leads to genetic recombination and exchange of genetic material during meiosis.

Complete answer:
Crossing overs occur in the pachytene phase of prophase I of meiosis I during sexual reproduction of organisms. It leads to the exchange of genetic material between the non-sister chromatids of homologous chromosomes. The process of crossing over is known as synapsis and leads to the new set of genes generated on each chromosome due to the exchange of fragments between the non-sister chromatids. Non – sister chromatids are a pair of homologous chromosomes having different alleles.
When crossing over occurs chiasma is formed which is the point of contact between the non-sister chromatids which results in the exchange of genetic material.

Hence, the correct answer is option (A).

Additional information:
-Thomas Hunt Morgan described crossing over for the first time after studying the theory of Frans Alfons Jenssens who described crossing over as a chiasma type in 1909.
-Crossing over is considered to be a process by which the damaged DNA segments are exchanged with the healthy ones.
-Crossing over occurs with the help of an enzyme called recombinase.
-It is responsible for the normal segregation of chromosomes so that they can be transferred to the next generation.
-Crossing over always involves exchange of equal strand between the chromosomes.
-Frequency of crossing over is always between 0 to 50% and never more than this.

Note: Crossing over is very important as it works in the repair of DNA and removal of those genes which might code for some proteins that might lead to some harmful diseases. Along with that it helps to bring new genetic variations in the genetic makeup of the organism that are good for evolution of the species as a whole.