DNA repair

Mutations

The Ames Test

The repair systems

• Mainly, the undamaged complementary strand acts as a template for the DNA polymerases involved in repair– Mismatch repair

– Base excision repair

– Nucleotide excision repair

– Direct repair

– Recombinational repair

– Error-prone repair

Mismatch repair

Mismatch mechanism

Mutations in mut proteins?

Base excision repair

Repairing apurinic and apyrimidinic

sites

Nucleotide excision

repair

Mutations in excision

repair systems

Direct repair

Methyltransferase

When both strands are damaged

SOS system

Recombination

Recombination• Three forms

– Homologous

– Site specific recombination

– Transposition

Recombinational repair

DNA replication stalled because of lesion

Replication continues

Repair

Recombination

Blockade

Replication following a break

Blockade

Recombination

Reseal

Meiotic recombination

Recombination between homologous a chromosomes

Recombination permits genetic

mappingIf each recombinant occurs at 2.5%, then the total map distance is 5% or 5 m.u.

1

2

3

Generation

The simple minded

mechanism (incorrect) of

meiotic recombination

Gene conversion indicates that another mechanism must be operable

Gene conversion on a molecular level

The mechanism of homologous recombination

The topology

of the Holliday structure

Resolution of the Holliday structure

Non-identical sequences

The enzymes of recombination

Rec A

Site specific recombination

Site specific recombination within one molecule

Use of recombinase to

resolve a dimeric genome

Transposition

Direct transposition

Replicative transposition

Retroviruses