Question

What is crossing over? Give its significance.

Answer 1

Hint: Crossing over which results in recombination, is a process happening in the genetic material of the cell during meiosis.

Complete answer:
Crossing over is the exchange of chromosomes between the non-sister chromatids of homologous chromosomes during meiosis.
Genetic material of the plant and animal cell are arranged in a compact thread-like structure known as chromosome, inside the nucleus. Humans have 23 pairs of chromosomes. Among the total chromosomes, half comes from the mother which are called the maternal chromosomes, and the other half comes from the father, which are paternal chromosomes.
Meiosis is the production of haploid gamete cells by the parent cells. The two haploid gametes fuse together to form a diploid zygote, which develops to a new individual.
During prophase I of meiosis, the chromosome exists in an ‘X’ shaped structure with four chromatids. The chromatids of one chromosome are known as sister chromatids. During this stage, the homologous chromosome, which are the above mentioned maternal and paternal copies of a chromosome, comes together and align side by side. Now, the non sister chromatid strands cross over and exchange genetic material from one chromosome to another. After the crossing over, the two chromosomes will have a different genetic make-up from paternal and maternal copies. They will be a new combination of parts of maternal and paternal chromosomes. These are then passed down to different gametes. This makes gametes different from the parents carrying them. The process is also known as recombination.
Crossing over ensures the variation of offsprings and generates a genetic difference within the population. Recombination changes the genetic pool of organisms by changing the gene frequency which is an important step of evolution. New combinations of traits and new phenotypes are resulted through recombination. This is very important in the field of breeding to produce improved varieties of plants and animals. Also, the crossing over frequency helps in construction of genetic maps.

Note: Crossing over results in the separation of linked genes in a chromosome.
The frequency of crossing over varies from 0 to 50 percent.
Crossing over does not occur during mitosis.