Lecture.20.Fall.2013

However most mutations are recessive and require

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: he acute effects were suffered increased incidence of many types of cancer • These excess cancers are still being detected in this population. • Why cancer? • Animal studies have detected genetic effects from sublethal doses of radiation. Radiation Exposure in Humans: The Heavy • No such effects have been detected in offspring of Japanese survivors. • However, most mutations are recessive and require several generations to detect - why? 2 copies needed to show phenotype A number of pathways repair changes in DNA • More than 999/1000 DNA lesions are repaired before being passed on. • General characteristics of DNA repair • Most DNA repair mechanisms require two strands of DNA • A template strand is needed to specify the correct nucleotide sequence. • Many types of DNA damage can be repaired by more than one pathway. • Why? A number of pathways repair changes in DNA • Four mechanisms of DNA repair: • Mismatch repair • Direct repair • Base-excision repair • Nucleotide-excision repair Mismatch repair corrects errors missed by proofreading • Mismatched bases are incorporated into a new DNA strand during synthesis at a frequency of about 1 in 10,000 or 100,000. • However, most errors are corrected and never become permanent mutations. • Proofreading by the DNA polymerase. • Mismatch repair. Mismatch repair corrects errors missed by proofreading • Errors distort the 3D structure of DNA • Mismatch enzymes detect these distortions • Also detects small unpaired loops in DNA • e.g. those caused by strand slippage in replication. • If trinucleotide repeats form secondary structures. they may escape detection. Mismatch repair corrects errors missed by proofreading • Mismatch-repair enzymes cut out the distorted part of the new strand • Fill in using the original strand as a template. • Differentiate between new and old strands • In E. coli, after synthesis, adenine nucleotides in the sequence GATC are methylated. • Immediately after synthesis, only the old strand is methylated. Mismatch repair • Mismatch repair in eukaryotic cells is similar to that in E. coli • It is not known how old and new strands are recognized. For Wednesday: Finish Chapter 18: Mutation and DNA Repair Begin Chapter 19: Recombinant DNA technology...
View Full Document

This document was uploaded on 02/25/2014.

Ask a homework question - tutors are online