L13: Gene Mutations Flashcards
What is a mutation?
Change in a DNA sequence
3 causes of mutations
1) DNA replication errors
2) Spontaneous mutations
3) Chemicals & radiations
Define somatic mutations
Occur in body cells (not gametes), not inherited
Define germ-line mutations
Occur in gametes (sperm/egg), can be inherited
3 types of base-pair substitutions
1) Silent mutation
2) Missense mutation
3) Nonsense mutation
Define silent mutation
Base-pair change but DOES NOT alter amino acid
Define missense mutation
Changes a codon, resulting in a different amino acid in a protein
Define nonsense mutation
Changes a codon into a stop codon, leading to premature termination of translation
2 types of missense mutation substitution
1) Conservative substitution
2) Non-conservative substitution
Define conservative substitution
Substitutes chemically similar amino acid
Define non-conservative substitution
New amino acid chemically different, alter protein function
When does a frameshift mutation occur?
When 1 or more nucleotides are inserted/deleted, altering reading frame of gene
- Changes all downstream codons
What do regulatory mutations affect?
Promotors/regulatory protein binding sites
What is induced mutation?
Caused by mutagens (environmental agents e.g chemicals)
What is spontaneous mutations?
Occurs without known mutagens, due to DNA replication errors
2 effects of regulatory mutations and their effects
1) Reduced Transcription: Mutations in promotors can reduce/abolish transcription
2) Enhanced Transcription: Some mutations increase transcription levels
What causes trinucleotide repeat disorders?
DNA pol slipping during replication, increasing no. of repeats in genes
What are tautomeric shifts?
Alternative structures of nucleotide bases
e.g amino -> imino
keto -> enol
What can tautomeric shifts lead to?
Base-pair mismatches during DNA replication, causing mutations
What are transposable elemtns?
DNA sequences that can move within the genome of all prokaryotes/eukaryotes
2 types of transposable elements
1) DNA transposons
2) Retrotransposons
How do DNA transposons and Retrotransposons move?
DNA: Via transposase protein
Retro: Via an RNA intermediate & reverse transcriptase
How can transposable elements cause mutation?
By inserting into a gene & causing an insertion mutation
Describe the structure of DNA transposons?
Contain a transposase gene, which is bordered by inverted repeats
Function of transposase
Cuts DNA & inserts the IS element into a new location
Types of induced mutagens
1)** Base analogs: **Mimic nucleotides
2) Alkylating agents: Adds methyl/ethyl groups to bases
3) Intercalating agents: Insert between base pairs, causing frameshift
3 types of DNA repair mechanisms
1) Photoreactivation Repair
2) Base Excision Repair
3) Nucleotide Excision Repair
Describe photoreactivation repair
Repairs thymine dimers caused by UV light
- Enzyme photolyase cleaves dimer bond
Describe base excision repair
Removes damaged bases & replaces with correct ones
Describe nucleotide excision repair
Repairs bulky DNA lesions by excising a segment of DNA & synthesising a new strand
