Unformatted text preview: DNA Repair Mechanisms Many agents such as alkylating agents, X-rays, oxygen radicals, etc. cause DNA damage (see
supplemental info). Repair complexes with DNA polymerase enzymes
attempt to repair the DNA. Recognize distorted regions caused by mispaired base pairs (Proofreading) ; Remove DNA section with mispaired base from the newly synthesized nucleotide chain; Resynthesize the section correctly, using original template chain as a guide.
19 BIO 1140 Unit 2-1:Replication Proofreading by DNA Polymerase If a replication error
causes a base to be mispaired, DNA polymerase reverses and removes
the most recently
added bases. The enzyme then
resumes DNA synthesis
in the forward direction 20 Fig. 12-19 p. 276 BIO 1140 Unit 2-1:Replication Mismatch Repair: general model for post-replication damage 21 Fig. 12-20 p. 277 If this occurs right after DNA synthesis, then you can
recognize a strand that is new and a strand that use
as a template. After s-phase that distinction cannot
be made. There may be a further error then, that we
are trying to correct the G attached to T, but it
replaces the G with a C. BIO 1140 Unit 2-1:Replication DNA Repair Mechanisms
In spite of proofreading and mismatch repair, mutations arise!
cell range from 104–106 per day, requiring therefore in
an adult human (1012 cells) about 1016–1018 repair
events per day … it is clear that DNA-repair systems effectively Crude estimates of the number of DNA-damage events in a counteract this threat. (from Schärer (2003) Angewandte Chemie Int. Ed.
2003, 42: 2946–2974). Do mutations still occur? Yes, they are the “stuff ‘” of evolution. Here’s a quick
calculation. In most plants the error rate per base pair is 7 per 109. How many SNPs (single-nucleotide polymorphisms) in a field of soybeans if planted at a
density of 240,000/hectare? And there are other types of mutations that are
occurring at the same time. 22 BIO 1140 Unit 2-1:Replication DNA Replication in Eucaryotes Key Points to CONSIDER (Summary)
Why do you need 3 types of DNA synthesis for a chromosome? What
Do happens when you need more or faster DNA synthesis? all DNAs have ends? What happens to them? When DNA replicates what happens to the chromosome?
(think, what is a chromosome? what needs to be synthesized?) What signals the cell to replicate the DNA? Organelles
23 and replication (more DNA polymerases) if we didnt have a repair mechanism-skin damage....
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- Winter '07
- DNA, DNA Damage, Eucaryotes