Sister chromatid exchange

Sister chromatid exchange ( SCE abbreviated according to Engl. Sister chromatid exchange) is a term used in cytogenetics, which deals with chromosomes. An SCE is an exchange of equal parts of the two sister chromatids within a chromosome. Such an exchange can take place only in the phases of the cell cycle, which already replicate the DNA of the chromosome ( doubled) exists because the two already doubled, identical DNA double strands severed and ' wrong ' about to be reconnected. Some research suggests that the process of exchanging not only after, but already during replication takes place, caused by a collapse of the replication fork. Since both chromatids have identical DNA sequence, is a SCE for the continued existence of the cell or the daughter cells without significance, as long as the circuit in both sister chromatids at the exact same location takes place.

SCEs are to be distinguished from crossing-over during meiosis in which an exchange of chromatids ( or parts thereof) between the homologous chromosomes, ie the respective chromosomal copies of both mother and father, takes place and which thus allows recombination of genetic material. In contrast, SCE's just no exchange of information takes place, since the two chromatids involved belong to the same chromosome and are identical because they are formed by doubling apart.

SCEs also occur in healthy cells, but increased in cells to radiation or to certain mutagenic substances were suspended. This also applies to interchromosomal translocations. An increased rate of SCE occur, for example in Bloom 's syndrome, wherein the patient lacks a particular DNA helicase also in hereditary diseases.

Detection of SCEs is possible by SCE staining. For this, the cells undergo DNA replication in the presence of bromodeoxyuridine (BrdU ) and another replication without BrdU. BrdU is incorporated instead of thymidine in the DNA. Due to the semi- conservative replication of each chromatid contains for this mounting a double-stranded DNA, the single-stranded labeled with BrdU. This semi- labeled DNA double strand is separated at the next round of replication (in the absence of BrdU ) due to the semi- conservative replication, so that of the two new DNA double strands of a is also semi- marked, the other is, however, no longer highlighted. In the subsequent metaphase does not lie in the normal case, a chromosome before, one sister BrdU labeled, but the other. In the case of a SCE, however, a part of one and a portion of the other is labeled with BrdU chromatid.

For the detection of the built BrdUs are several methods available. For example, it can be detected with antibodies, and then visualized by immunofluorescence in the fluorescence microscope after denaturation of the DNA by heat, acid or base. Other methods exploit the fact that the DNA fluorescent dye Hoechst 33258 less strongly fluorescent in the presence of BrdU. Also, chemical reactions can be triggered in the presence of BrdU and Hoechst 33258 by bright light, which allows a differential staining by Giemsa. Chromosomes with SCEs are also referred to as a harlequin chromosomes.