Crossing over

Crossing over is one of the key processes in genetics that occurs during meiosis. It plays an important role in the transmission of genetic information from parents to offspring and is the basis for the formation of genetic diversity in a population.

Crossing over occurs during cell division when chromosomes split into two halves. Each half of a chromosome consists of two strands of DNA, called homologous. Homologous DNA strands have a common origin and contain identical gene sequences.

In the process of crossing over, homologous strands are broken and then joined in a random order. This causes genes that would normally be on different chromosomes to move to adjacent chromosomes. Thus, crossing over allows the creation of new combinations of genes and provides genetic diversity in the population.

Although crossing over plays an important role in generating genetic diversity, it can also lead to genetic diseases and mutations. Therefore, understanding the mechanisms of crossing over and its consequences is an important aspect in the field of genetics and medicine.

So what is crossing over?

In short, this is a mutual exchange of sections between neighboring chromosomes. Each chromosome has so-called “end sections” at the ends - they are small structural elements called telomeres. In order for a cell to divide normally, periodic removal of the end sections is necessary. As a result of the natural process of DNA strand termination (in mammals, several thousand replicated double-stranded fragments of 20-30 base pairs in length occur per day). It is these terminal sequences that are used to determine the exact number of cell divisions. In addition, telomeres are protective structures that protect the terminal double-stranded part of the chromosome from degradation. In prokaryotes, telomerase activity is weakly expressed. Therefore, when such cells divide, each of the daughter cells can be the same length as the mother cell. If there is more than one maternal chromosome, the telomeric ends of all chromosomes are destroyed, except for the telomeric (“mother”) chromosome. The time has come for the next division (interkinesis), and the chromosomes are lengthened by a section of the cell center (kindly provided by the surrounding cells), with the help of the enzyme telomerase. In eukaryotes, with each cell division, unfortunately, the length of telomeric sequences, which determine the size of the cell as a whole, decreases. During meiosis—the process of determining the sex chromosomes of offspring—the final processing of telomeres occurs at the meiotic pole of the cell (which is located near the center of the nucleus). Spermatogonial cells completely lose their telomerin-dependent regulatory functions and undergo molecular mutation. Thus, in humans, spermatogenesis