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5. Beskrive vigitigheden af repeterede sekvenser i DNA (højt og moderat repeterede sekvenser)
 Devlin, s.75-76

 

While repetition of particular DNA-sequences in prokaryotes is very limited, the DNA of eukaryotes contains nucleotide sequences that are repeated anywhere from few times, for certain coding genes, to millions of times for certain simple, relatively short, sequences.

In the human genome, 25-35% of the DNA is repetitive.

There are 2 categories of repetitive sequences:

 

Their length varies between 5-300 bp, but typically they are shorter then 20 bp. They can be repeated up to millions of times. They are most often found near the centromere, where they probably have a structural role under chromosome pairing.

 

Can be found as lines and sines.

Lines are long interspersed sequences consisting of several thousand nucleotides; there can be up to thousand copies per genome. No function established.

Sines are short interspersed sequences. Typical example is the ALU-family, which by itself constitutes 5% of the human genome.  Alu sequences are 300 bp long and repeated over 500.000 times. Their average homology with a consensus sequence is 87%.

Their function is not yet clear, but they are seen often at origins of replication, oriC.

These repeated sequences can be result of transposons incorporated in the human genome.

 

Only a small fraction of the single-copy is coding, 2-4% of the human genome. Some single-copy DNA exists as pseudogenes, sequences of DNA that have significant nucleotide homology to a functional gene but contain mutations that prevent gene expression.

Additional single-copy DNA can be found in introns and as control regions that flank the genes.

Normally, single-copy DNA and moderately repeated sequences occur in a chromosome in an orderly pattern – interspersion pattern – which consists of alternating blocks of single-copy DNA and moderately repeated DNA.    

 

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