Sister chromatid exchange

A sister chromatid exchange, or SCE for short, is an exchange of equal parts of the two sister chromatids within a chromosome. Such an exchange takes place in the phases of the cell cycle (a sister chromatid exchange can only take place in this phase) in which the DNA of the chromosome is already replicated (duplicated). The two identical pieces of DNA that are arranged in parallel at this stage of the cell cycle are therefore also referred to as "sister chromatids".

During this cell division phase, whole, more or less large pieces of chromosome can be exchanged. This is partly because the DNA strands are close together and partly because the DNA molecules are pulled apart by enzymes in order to be duplicated for the newly developing cell, causing tension in the molecule.

At these open sites, the replication forks, the crossing over of the chromosome pieces takes place. This leads to strand breaks - even under natural conditions - which are mended by the cell's repair system. During repair, it can happen that "wrong" pieces are joined together. This gives the DNA new properties, a normal process in evolution. Cross-linking of the DNA molecules also occurs, which can lead to inhibition or even loss of DNA function. The cross-links are again strong covalent chemical bonds that are not easy to break.

SCEs are to be distinguished from crossing-over, in which an exchange of chromatids (or parts thereof) takes place between the homologous chromosomes, i.e. the respective chromosome copies of the mother and father, during meiosis, thus enabling recombination of the genetic material. In SCEs, on the other hand, there is no exchange of information, as the two chromatids involved belong to the same chromosome and are identical, as they have diverged by duplication.

In the laboratory, chemicals such as mitomycin C (long known to trigger sister chromatid exchanges) or phytohaemagglutinin (a substance from beans that causes human blood cells to clump together - agglutinate - and stimulates lymphocytes to divide) are used to increase strand breakage rates in order to test the stability or susceptibility of chromosomes.

The sister chromatid exchange test is a common method for characterizing chromosome properties. In addition to mutagenic chemicals, ionizing radiation (X-rays and UV radiation) can also lead to strand breaks and sister chromatid exchange.

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