Satellite DNA

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In genetics, satellite DNA is used to describe highly repetitive sequences, i.e. multiple repetitive base sequences, in the genome of higher organisms ( eukaryotes ).

Surname

The term satellite DNA goes back to the differentiation of genome fragments by their GC content and thus by their density by means of density gradient centrifugation or sedimentation equilibrium centrifugation. If one plots the DNA concentration against its density in a diagram, one obtains, in addition to the main band (peak), several secondary bands , which are also known as satellite peaks. The main band represents the average density of the DNA.

properties

Mostly they are repeated sequences of five to ten base pairs , but they can also be much longer and extend over areas of up to 100,000 base pairs. In the average mammalian genome, around ten percent of the DNA consists of these simply structured DNA sequences . These sections have a particularly high rate of renaturation.

In mammals, most of these areas in heterochromatin are located near the centromeres , and in Drosophila melanogaster they are also located on the telomeres . In mitosis and meiosis, the microtubules of the spindle apparatus attach to the centromeres .

Demarcation

In general, there are three classes of satellite DNA.

Classic satellite DNA is between 100 and 5000 kb long. It consists of up to a million repetitions of sequences between 5 and 300 bp in length and is usually not transcribed. With the alpha sequence repetitions at the centromeres of the chromosomes , the classic satellite DNA fulfills a role as a protein binding site.

Mini satellites are smaller than classic satellite DNA. As a rule, they are between 100 bp and 20 kb long, and their repeat units consist of a maximum of 15 bases. On the telomeres they act as binding sites for proteins , which serve to protect against degradation by nucleases .

Microsatellites ( short tandem repeats ) are the shortest satellite DNAs with a few hundred repetitions of 1 to 6 bp long sequences. They themselves are repetitive units that occur up to 100,000 times over the entire genome. Microsatellite probably arise during replication , when free DNA ends and present it to a slipping ( slippage ) of the DNA polymerase occurs. The relatively random emergence of microsatellites also results in a large length polymorphism , which is used for genetics and forensics. Like the fingerprint, the microsatellite length polymorphism is also a distinguishing feature for the correct identification of an individual.

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