Gene Mutation and Repair

Flashcards covering DNA mutations, the Ames Test significance, and the detailed mechanisms of error-proof and error-prone DNA repair pathways.

Mutation

A heritable change in DNA.

Mutagenic agent

A substance that causes DNA mutations, such as Nitrates or 5-Bromouracil.

Ames Test

A test of the mutagenicity of a substance that uses $Salmonella$ defective in $hisG$, which cannot grow on media lacking histidine unless a reversion mutation occurs.

Rat liver homogenate

A substance mixed with a potential mutagen in the Ames Test to simulate the metabolic chemical modifications performed by the human liver.

Error-proof pathways

DNA repair mechanisms like Methyl mismatch repair and Nucleotide excision repair that prevent or repair mutations.

Methyl mismatch repair

A repair pathway that uses the methylation pattern of the parental DNA strand to recognize and discriminate from newly replicated, unmethylated DNA.

MutS

A methyl-directed mismatch repair protein that binds to the DNA mismatch site.

MutL

A protein that binds to an adjacent hemimethylated $GATC$ site and draws $MutH$ to the $MutS$/mismatch complex.

MutH

An enzyme that cleaves the unmethylated strand $5'$ to the $GATC$ site during methyl mismatch repair.

UvrD

A helicase that unwinds the nicked DNA strand during methyl mismatch repair and nucleotide excision repair (where it is also called helicase II).

Nucleotide excision repair

A repair process where an endonuclease removes a patch of single-stranded DNA containing damaged bases, such as thymine dimers, without distinguishing between parental and daughter strands.

UvrA and UvrB

Proteins that form a complex to bind to damaged DNA and cause the DNA to bend.

UvrC

An enzyme recruited by $UvrB$ that cleaves the phosphodiester backbone of the damaged DNA strand at two places.

Transcription coupled repair

A process where RNA polymerases that stall during transcription recruit $Uvr$ proteins to repair the DNA.

Error-prone repair pathways

Repair mechanisms, such as SOS repair, that operate only under severe damage and risk introducing mutations because they lack proofreading capacity.

SOS repair

A collaborative 'Save Our Ship' response induced by extensive DNA damage where polymerases replicate through damaged DNA to ensure survival.

RecA

A protein that monitors the level of single-stranded DNA ($ssDNA$) and, upon binding to it, triggers the autodigestion of the $LexA$ repressor.

LexA

A repressor protein that prevents the transcription of DNA repair genes in the SOS system when DNA damage is low.

Pol IV and Pol V

'Sloppy' polymerases synthesized during the SOS response that lack proofreading but allow the cell to tolerate mutations to survive.