A mutation can be defined as an alteration in

DNA sequence.

Types of mutation:

Point mutations

single base-pair substitution

Frameshift mutations

deletion or insertion of one or

more base pairs

Chomosomal aberrations

a major alteration in the structure

of a chromosome

Mutations can also be classified as

spontaneous or induced.

Point mutations – Base pair substitutions

Point mutations – Base pair substitutions

Effects at the polypeptide level

Point mutations – Base pair substitutions

Effects at the polypeptide level

Frameshift mutations – Insertions and

deletions

Effects at the polypeptide level

Processes that lead to spontaneous

mutations:

• Tautomeric shifts

• Changes in base structure

–deamination

–depurination

• Replication slippage – insertions and

deletions

Figure 16.2

Tautomeric shifts

Spontaneous mutation

Tautomeric shift

Deamination

Replication slippage - addition and deletion mutants

DNA looping

Induced mutations:

Base analogs as

mutagenic agents

Induced mutations – base analogs

Mechanism of mutation

5-bromouracil (5BU)

Induced mutations – base modifying compounds

Deamination by nitrous acid

Induced mutations – base modifying compounds

•Hydroxylamine is a hydroxylating agent that reacts only with cytosine

adding an OH group so that it pairs with adenine instead of guanine. Only

CG to TA transitions.

Induced mutations – intercalating mutagenic agents

(acridine dyes)

Induced mutations – intercalating mutagenic agents

Induced mutations – UV light

DNA repair

Repair of damaged bases or thymine dimers

1. Direct repair

2. Base excision repair (BER) and

nucleotide excision repair (NER)

Mismatch repair

Direct repair

Photoreactivation

enzyme

Ultraviolet repair genes