BIOLOGY 172 Lecture Notes - Lecture 19: Dna Mismatch Repair, Proofreading, Base Pair
Document Summary
Dna pol(cid:455)(cid:373)erases (cid:862)proofread(cid:863) a(cid:374)d fi(cid:454) (cid:373)istakes duri(cid:374)g repli(cid:272)atio(cid:374): really great at its job! Base pair mismatches not caught by dna pol can be recognized and fixed by other enzymes: mismatch repair enzymes can fix any errors missed by dna pol. Must also have mechanisms to help protect dna from external factors. Spontaneous (mistakes made by dna pol during replication; most common) Mutagens (damage dna molecules) x-rays, uv light, chemicals: single or double stranded break. Breaks in the phosphate backbone chunks of dna are missing. Caused by x-rays: cross-linked base pairs. Uv light can cause adjacent pyrimidines to cross link, forming thymine dimers. Inappropriate covalent bond forms, causing them to be too close together. Two things happen: dna pol cannot replicate over the dimer, rna pol cannot replicate either. The chromosome cannot be replicated or transcribed. Base pairing to the opposite dna strand is lost: chemicals can add or modify bases. Bulky side-groups result in improper base pairing.
