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
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
Photoreactivation
enzyme
Ultraviolet repair genes