Deletion and insertion mutations can be formed in two ways: deletions or insertions of short regions can occur by strand slippage, and deletions or insertions of longer regions can occur via homologous recombination.
Strand slippage occurs by mispairing of the template strand and the newly synthesized strand during DNA replication. If the newly synthesized strand denatures from the template during DNA synthesis and if it is complementary to different stretches of the template strand, it will occasionally pair with the wrong sequence. Such mispairing occurs in runs of a single nucleotide or in short, directly repeated sequences. If the template strand loops out, then a deletion will result.
In contrast, if the newly synthesized strand loops out, then a deletion will result.
In addition to strand slippage, deletions or insertions can be formed by intra-molecular recombination between direct repeats of homologous DNA. For example, in the figure shown below recombination between the direct repeats (shown as arrows) results in deletion of the intervening Q R S genes. Only one product of the recombination event is shown because the other product (a circular DNA molecule which carries the Q R S genes) does not have an origin of replication so it will be unable to reproduce when the cell divides. (Begin at one end of the DNA and follow the recombination events with a pencil.)
Recombination can also result in an insertion mutation, duplicating the material between a pair of direct repeats. One way that this can happen is by recombination between sister chromosomes at the DNA replication fork, as shown below.
The probability of forming a deletion or insertion depends on the length of DNA homology between the direct repeats. Recombination between longer repeats will occur more often than recombination between shorter repeats. RecA dependant homologous recombination requires at least 25 bp of perfect homology. However, recombination between even between very short repeats -- less than 10 bp -- is a major source of deletions (Albertine et al. 1982. Cell 29: 319-328). Thus, direct repeats are "hot spots" for deletion formation.
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Last modified July 15, 2002