6. (6 points) Very briefly describe two pathways of DNA damage bypass, one error-free and one error-prone.
In class we learned about two types of error-free damage bypass pathways. The first type is based on template switching during DNA synthesis. When the replication fork encounters damage in one of the template strands, thus blocking the DNA polymerase on that strand, synthesis can continue past that point on the other template strand. The blocked polymerase can then switch to utilize the newly synthesized sister nascent strand (which was extended past the damage and has the correct polarity) as template. After passing the damaged region, the polymerase can then switch back to its original template strand.
The second type of error-free damage bypass is mediated by polymerases that can incorporate correct nucleotides when they synthesize past a damaged region in the template. The example we studied is that of polymerase eta, which incorporates two A residues when it encounters a TT dimer in the template strand. Since TT codes for AA, the new strand is synthesized correctly (i.e. error-free).
Error-prone damage bypass is mediated by polymerases that synthesize past damaged regions but insert random rather than template-encoded nucleotides. The example we studied is that of pol zeta.
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