CH 8: Mutations and Repair

Mutation mistakes get made during _____

DNA replication

Mutation

A heritable change in the DNA sequence

HOw do bacteria reduce the frequency of the erros they make?

1) DNA polymerases can proofread

2) Mismatch repair

When does proofreading and mismatch repair occur

During or right after DNA replication

DNA polymerase

• Can “proofread their work”

• 3’-5’ exonuclease activity

  • removes most recent base if incorrect

  • Backspace/delete

If proofreading does not fix mistake….

Will do mismatch repair

mismatch repair

• Occurs after replication

• Distinguishes “New” strand from “Old” strand

• Removes and replaces stretch of DNA, including the mistake

Mechanism of mutation: Spontaneous

• Small mutations occur at a basal rate

• Due to normal cell processes (independent of external factors)

• They are random and infrequent

• Occur at characteristic rates

Mutation rate

Probability a gene will mutate when a cell divides

• ex/ Random Mistakes made by DNA polymerase

Mechanisms of mutation: Induced

• External factor (mutagen) increases the Mutation Rate

• or the likelihood DNA polymerase will make a mistake during replication

• ex/ chemicals or radiation

Base excision Repair

Damage base is recognized and removed

DNA glycosylases (base excision repair enzyme)

• Phosphodiester backbone is removed

• DNA bases replaced (DNA polymerase)

• Ligase seals nicks

SOS “Save our ship” repair

• DNA damage extensive

• Drastic measure taken

• Can introduce mutations

Base substitutions

Wrong nucleotide(s) used during DNA replication

base substitution: Point mutation

A single base change

Wild type

Typical phenotype of strains isolated in nature

Outcomes of base substition:

1) Silent mutation

2) Missense mutation

3) Nonsense mutation

Silent mutation

Codes for the same amino acid

Missense Mutation

Codes for a different amino acid

Nonsense mutation

Codes for a STOP codon

Insertions and deletions

One or more bases added or removed

Frameshift mutation

• Changes the reading frame

• Encoded peptide sequence is different from insertion or deletion and downstream

Null mutation

Gene product is inactivated (missense and nonsense mutations can also be Null)

• aka knock-out mutation

Inversions

DNA fragment is flipped in orientation

• outcome of inversions to coding genes: depend

How are some mutations pass on to a daughter cell

Vertical transfer

Vertical evolution

Passing gene from one generation to the next

Are mutations beneficial or no?

Most of not…. but

• environemnt determine which cells in the popoulation rae “fit” or are more stable to survive and reproduce

• Mutations are one mechanism of change and vertical evolution