Sister chromatid exchange

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Scheme of a sister chromatid exchange. The ends of the chromatids are reversed in the lower area.
Metaphase preparation of chromosomes of a cell line with a ring chromosome (R) and several sister chromatids, some of which are marked by arrows.

Sister chromatid exchange (abbreviated to SCE by Engl. Sister chromatid exchange) is a term used in cytogenetics , which with chromosomes employed. An SCE is an exchange of equal parts of the two sister chromatids within one chromosome. Such an exchange can only take place in the phases of the cell cycle in which the DNA of the chromosome is already replicated (doubled), since the two identical DNA double strands that have already been doubled have to be severed and reconnected 'upside down'. Some research suggests that the process of swapping does not take place after, but rather during replication, caused by a collapse of the replication fork . Since both chromatids have an identical DNA sequence, an SCE is irrelevant for the continued existence of the cell or daughter cells, as long as the break in both sister chromatids takes place at exactly the same place.

SCEs are to be distinguished from crossing-over , in which an exchange of chromatids (or parts of these) between the homologous chromosomes, i.e. the respective chromosome copies of mother and father, takes place during meiosis and which thus enables the genetic make-up to be recombined. In the case of SCEs, on the other hand, there is currently no exchange of information, since the two chromatids involved belong to the same chromosome and are identical, since they emerged from one another through duplication.

SCEs also occur in healthy cells, but increasingly in cells that have been exposed to ionizing radiation or certain mutagenic substances. This also applies to interchromosomal translocations . An increased SCE rate can also occur in hereditary diseases , for example in Bloom syndrome , in which the patient lacks a certain DNA helicase .

SCE staining can be used to detect SCEs . For this purpose, the cells undergo a DNA replication in the presence of bromodeoxyuridine (BrdU) and a further replication without BrdU. BrdU is built into DNA instead of thymidine . Due to the semiconservative replication , after this incorporation, each chromatid contains a DNA double strand, one of which is marked with BrdU. This half-marked DNA double strand is separated in the next round of replication (in the absence of BrdU) due to the semiconservative replication, so that one of the two new DNA double strands is also half-marked, but the other is no longer marked. In the following metaphase, there is usually a chromosome, one of which is marked with a sister chromatid BrdU, but the other is not. In the case of an SCE, however, part of one and part of the other chromatid is labeled with BrdU.

Several methods are available to verify the built-in BrdU. For example, after the DNA has been denatured by heat, acid or base, it can be detected with antibodies and then made visible in a fluorescence microscope using immunofluorescence . Other methods take advantage of the fact that the DNA fluorescent dye Hoechst 33258 fluoresces less strongly in the presence of BrdU. In the presence of BrdU and Hoechst 33258, chemical reactions can be triggered by strong light irradiation, which enables a differential coloration by Giemsa . Chromosomes with SCEs are also known as harlequin chromosomes .

See also

source

  • Jan Murken, Timo Grimm, Elke Holinski-Feder: Human Genetics ; Thieme, Stuttgart; Edition: 7th, completely revised. A. (August 2006)

Individual evidence

  1. ^ R. Rodríguez-Reyes and P. Morales-Ramírez: Sister chromatid exchange induction and the course of DNA duplication, two mechanisms of sister chromatid exchange induction by ENU and the role of BrdU. In: Mutagenesis , 18 (1): 65-72, January 2003.
  2. ↑ David M. Wilson III and Larry H. Thompson: Molecular mechanisms of sister-chromatid exchange. Mutat Res 616 (1-2): 11-232007. PMID 17157333 doi: 10.1016 / j.mrfmmm.2006.11.017
  3. ↑ Norma F. Neff, Nathan A. Ellis, Tian Zhang Ye, James Noonan, Kelly Huang, Maureen Sanz and Maria Proytcheva: The DNA Helicase Activity of BLM Is Necessary for the Correction of the Genomic Instability of Bloom Syndrome Cells. Mol Biol Cell. 1999 March; 10 (3): 665-676. PMID 10069810 , PMC 25194 (free full text)
  4. ↑ SA Latt and RR Schreck: Sister chromatid exchange analysis. At the J Hum Genet. 1980 May; 32 (3): 297-313. PMID 6992563 . PMC 1686078 (free full text)
  5. ↑ Keiko Goto, T. Akematsu1, H. Shimazu and T. Sugiyama: Simple differential Giemsa staining of sister chromatids after treatment with photosensitive dyes and exposure to light and the mechanism of staining. Chromosoma, 53 (3): 223-230, 1975. doi: 10.1007 / BF00329173 .