Lect 20 dna mutation and repair

Mutations can be large or small, what is another name for this

Large scale (chromosomal); small-scale (point-mut.)

Large scale

• alter chromosome structure or #

• deletion, inversions, dups. translocation

Small scale

• a single bp is changed, addded, deleted

• can alter structure and func. of protein

Transition point mut.

• more common

• purine → purine

• pyri → pyri

Transversion

Purine → pyrimidine

Missense mut.

causes single amino acid change

Neutral mut.

• amino acid change does not affect protein function

Nonsense mut

Changes sense codon into nonsense codon

Silent mut.

changes codon, but amino acid stays the same

Frameshift mut.

addt./deletion of multiples of 1 or more bp’s

Somatic mut.

• not heritable

• not all cells are mutated

• sometimes increased cell division

Germline mut.

• heritable

• gametes

Spontaneous mut.

• error during DNA rep.

• normal biochem in cell

Induced mut.

result to exposure to external agent

external agents

1. base analogues

2. base modifiers

3. interculating agents

4. UV light

Depurination

loss of a purine, generally corrected

Deamination

loss of an amine group

Ames test

to identify mutagens

How to set up Ames test

1. mutant bacteria strain his-

2. mix one tube with chemical and one with liver enzymes

3. plate on his- media

Results of Ames test

growth = mutation. Bacteria reverts and can produce his and grow on plate

Base analogues

• chemical looks like normal base

• added during S phase

• change bp properties during DNA rep

BAse modifiers

add or remove chemical group to prevent pairing during DNA rep.

Interculating agents

molecules that insert themselves b/w stacked bases to change helix structure during DNA rep.

UV light

causes adjacent pyrimides to bind together during DNA rep

Intergenic suppression

mutation in one gene masks or reverses the harmful effects of a mutation in a different gene

Intragenic suppression

second mutation within the same gene as an original mutation partially or fully restores the normal (wild-type) function

DNA repair mechanisms need…

2 strands of DNA

Direct repair

UV light to activate photolyase to break thymine dimers

Excision repair

fixes damaged DNA by cutting out (excising) the faulty segment and replacing it with a new, correct sequence

Nucleotide repair

1. distorted helix is recognized as an error

2. endonuclease activity to cut it out

3. fill gap by DNApol.

4. DNA ligase to seal nick

Base repair

1. DNA glycosylase to remove base

2. DNA endonuclease to nick DNA

3. DNA pol adds nucleotide

4. DNA ligase seals nick

Mismatched repair

correct mismatches that happened during DNA rep

Mechanism of Mismatched repair

1. enzyme loks for distortions

2. section w/mut. is excised

3. DNA pol I fills gap

4. DNA ligase seals