MoBio Repetitive DNA Sequences Chapter 3

A stretch of DNA sequence often repeats several times in the total DNA of a cell. For example, the following DNA sequence is just a small part of telomere located at the ends of each human chromosome:

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An entire telomere, about 15 kb, is constituted by thousands of the repeated sequence "GGGTTA".

DNA reassociation kinetics is used to classify the repetitive DNA sequences. The total DNA is first randomly cleaved into fragments with an average size of about 1000 bp. Then, they are heated to separate the complementary strands of each fragment. Subsequently, temperature is reduced to allow strand reassociation. If a fragment contains a sequence which is repeated many times in the total DNA, it will have greater chance to find a complementary strand and reassociate more quickly than other fragments with less repetitive sequences.

Based on the reassociation rate, DNA sequences are divided into three classes:

Highly repetitive: About 10-15% of mammalian DNA fragments reassociate very rapidly. This class includes tandem repeats.

Moderately repetitive: Roughly 25-40% of mammalian DNA fragments reassociate at an intermediate rate. This class includes interspersed repeats (also known as mobile elements or transposable elements).

Single copy (or very low copy number): This class accounts for 50-60% of mammalian DNA.