DNA damage, mutations, repair

Sources of damage

• polymerase errors

• radiation

• reactive chemicals from environment and cellular metabolism (covalently modify part of DNA molecule)

repair mechanisms

• direct

• using information in complementary to homologous strands

Consequence if not repaired

• mutations

• block DNA replication often

• cell death

High fidelity DNA repair pathways and mechanisms

• No mutation

• mutations often cause cancer susceptibility and tumors often require addition mutations in these pathways

• mismatch repair

• base excision repair

• photoreactivation

• nucleotide excision repair

High and low fidelity pathways

double-strand break repair

Low fidelity pathways

translesion synthesis

translesion synthesis

polymerase bypass damaged bases that would otherwise stall DNA replication (not actually a damage repair pathway)

error prone and causes mutations

Ionizing radiation (x-rays)

causes single and double strand breaks causes broken chromosomes lethal for cells, nick causes replication fork to collapse

Homologous end joining in DNA damage repair

repairs double strand breaks with low fidelity in absence of another copy of the chromosome

occurs primarily in euk (some bacteria)

common in non-replication cells in which homologous chromosome not located in close physical proximity

usually causes small deletions (mutagenic), better than broken chromosome

Homologous end joining in DNA damage repair step 1

break recognized by endonuclease

Homologous end joining in DNA damage repair step 2

Break widened

Homologous end joining in DNA damage repair step 3

strands annealed

Homologous end joining in DNA damage repair step 4

ligation

Repair of double-strand breaks by homologous recombination step 1

end-processing creating 3’ overhangs helicase and nuclease: 5’ to 3’ exonuclease degrade/resect one strand

Repair of double-strand breaks by homologous recombination step 2

first strand invastion recombinase: recombination sequence repuired for strand invasion

Repair of double-strand breaks by homologous recombination step 3

Strand extension, 3’ ends of invading strands act on primers

Repair of double-strand breaks by homologous recombination step 4

completion of replication and ligation

Repair of double-strand breaks by homologous recombination step 5

holiday intermediate/holiday junction formed when 2 homologous DNA molecules exchange strands leading to structure with two overlapping cross-linked DNA helices

Repair of double-strand breaks by homologous recombination step 6

crossover or noncrossover

crossover

resolvase

non crossover

resolves cuts and re-ligates strands

Homologous recombination second copy of chromosome

sister chromatid, homologous chromosome

homologous recombination

responsible for exchange between homologous chromosome leading to diversity in meiosis