Crossing-over

Crossing-over is the process by which the homologous chromosomes exchange their parts with each other. We can find it especially during the first meiotic division. At this time the chromosome is formed by two sisters chromatids. The crossing-over takes place on the non-sisters chromatids of two chromosomes. The exchange of genetic material also occurs between the sister chromatids, however such exchange has no significance for genetic recombination.

The Mechanism of the Crossing-over
The DNA strands have to be broken to exchange their segments. It is a process of the homologous chromosomes – it means that the exchanging parts code the same genes. Crossing-over leads to the recombination of the chromosomes. The point of connection is called chiasma.

The exchange is usually reciprocal – the exchanging segments of the two chromosomes have the similar size. Sometimes it can be also unequivalent. The crossing over can occur on several places. Then we recognize few types of the crossing-over:
 * 1) single crossing-over;
 * 2) double crossing-over;
 * 3) multiple crossing-over.

There are several influences that can induce the crossing-over:
 * age;
 * the external influences – chemical, physical, radiations;
 * sex.

The Importance of the Crossing-over
The importance of the crossing-over is in the genetics diversity. The new recombined sex cells are created. That leads to the differences between the offsprings. Sometimes it brings the advantages, sometimes disadvantages. Anyway it ensures that at least somebody survives.

The frequency of the crossing-over depends on the distance between linked genes. Linked genes means that they are transfered together everytime and it is impossible to divide them. The crossing-over occurs just on the places between these linked groups. It is used to construct the gene maps.

Related articles

 * Meiosis
 * Genetic Mapping
 * Eukaryotic Chromosomes