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NBME Answers

nbme15/Block 0/Question#0 (5.5 difficulty score)
An investigator is studying Helicobacter ...
Slipped-strand mispairing🔍

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submitted by hungrybox(800),

Excess pattern repeats lead to strand slippage/errors due to an unstable region (in this case, excess Cytidine bases).

It could be a repeated pattern as well (ie the trinucleotide repeat CAG in Huntington's).

here's a more in depth explanation (from wikipedia article on Slipped-strand mispairing):

A slippage event normally occurs when a sequence of repetitive nucleotides (tandem repeats) are found at the site of replication. Tandem repeats are unstable regions of the genome where frequent insertions and deletions of nucleotides can take place, resulting in genome rearrangements.

hungrybox  Anyone know why it's not Transposon insertion? I was thinking maybe because transposons have to be longer than one nucleotide, but I'm not sure. +1  
bingcentipede  @hungrybox I think it's because transposons are usually gene segments rather than a single nucleotide insertion - plus w/ what you said about the repeated pattern, I think slipped-strand mispairing (which is a concept the NBME loves) more likely. +12