Week 12 - DNA Mutagenesis and Repair Flashcards
What is a mutation?
A direct altercation of a gene resulting in a new allele of that gene
Germline and Somatic Mutations
Germline - occur in cells that produce gametes - affects offspring of individual Somatic - occur in somatic cells - only affect individual - degree of effect is linked to stage of development mutation occurs
Classes of Mutations and the type of mutations that can occur
Point Mutation - substitution - insertion - deletion Chromosomal Mutation - inversion - deletion - duplication - translocation
Types of substitution point mutations
Silent - doesn’t change an amino acid but can still have phenotypic effect
Missense - change amino acid to another amino acid
Nonsense - change amino acid to stop codon
What is a Frameshift mutation?
Insertion point mutation
Deletion or insertion of a number of bases that isn’t multiple of 3
Introduces premature stop codons
Forward and Reverse Mutations
Forward - loss of wild-type allele
Reverse - restores wild-type allele
Phenotypic Effects of Mutations
Loss of function mutation - partial or total loss of functional gene product - recessive, a diploid organism has to be homozygous for this change before the loss of protein is seen phentoypically - e.g. cystic fibrosis Gain of function mutation - creation of a new trait - hyperactivation of the protein - often dominant - e.g. achondroplasia
Suppressor Mutation
Mutation that hides or suppresses the effect of another mutation
Intragenic
- within same gene as original mutation
- can restore a reading frame
Intergenic
- in a different gene to original mutation
- can restore ability of gene to interact
Spontaneous Mutations - Incorrect base pairing
When a mismatched base is incorporated it will produce an error (Wobble)
When strand replicates this error is passed on creating permanent mutation
Spontaneous Mutation - Strand Slipping
One strand forms loop
Base that is looped out isn’t read
Causes different strand to be produced
Strand replicated and mutation passed on
Spontaneous Chemical Changes
Depurination
- loss of a purine base from a nucleotide in a DNA strand
- covalent bond breaks between purine and 1’ C on deoxyribose sugar
- forms apurinic site = no template base during replication
- base randomly added
Deamination
- loss of an amino group from a nitrogenous base
- e.g. cytosine to uracil
- in next round of replication, uracil pairs with adenine
- next round adenine pair with thymine
- gone from C-G to U-A to A-T
Types of Chemically Induced Mutations
Base analogues Deamination Addition of OH groups Alkylating agents Oxidation Intercalating agents
Chemically Induced Mutations - Base Analogues
Similar to real bases e.g. 5 bromo-uracil is uracil with 5’ methyl group swapped for Br
Can from non-standard base pairs
E.g. initially binds to adenine, mispairs with guanine next replication guanine not meant to be there binds with cytosine forming mutation
U-A to C-G
Chemically Induced Mutations - Addition of Hydroxyl groups
Hydroxylamine adds OH group to cytosine
Forms hydroxylaminocytosine which binds to adenine
C-G to A-T
Chemically Induced Mutations - Alkylating agents
Add methyl or ethyl to nucleotides
Chemically Induced Mutations - Oxidation
Oxidative radicals damage DNA
E.g. 8-oxyguanine mispairs with A causing GC to TA transversion
Chemically Induced Mutations - Intercalating agents
Chemicals that insert themselves between the bases of the DNA
Distorts 3D structure
Causes single nucleotide insertions or deletions
Ionising and Non-ionising Radiation Mutations
Ionising
- causes ds breaks
- e.g. x-rays, cosmic rays, gamma rays
- dislodge electrons and change stable molecules
- alter bases and break phosphodiester bonds
Non-ionising
- UV
- bases absorb UV light
- formation of chemical bonds b/w adjacent pyrimidines e.g. thymine
- thymine dimers distort DNA
- prevents normal replication
DNA Repair Mechanisms
Mismatch repair - replication errors, mismatched bases, strand slippage
Direct repair - pyrimidine dimers
Base-excision repair - abnormal or modified bases, pyrimidine dimers
Nucleotide excision repair - DNA damage that distorts helix
Repair of ds breaks
Base Excision Repair
Removal of damaged base followed by removal of rest of nucleotide
Steps:
1. DNA glycosylase enzymes detect presence of modified base
2. AP endonucleases cuts phosphodiester bond
3. Other enzymes remove sugar
4. DNA pol adds nucleotide to exposed 3’ OH
5. DNA ligase repairs nick
Nucleotide Excision Repair
Steps:
- Recognise problem
- Damaged region is made single stranded
- Sugar-phosphate backbone of damaged strand cleaved both sides of damage
- Excised section displaced, DNA pol replaces nucleotides and ligase seals backbone
Direct Repair
Repairs the base instead of removing it
How are ds breaks repaired?
Homologous recombination
Uses identical or nearly identical genetic information contained in another DNA molecules
Usually a sister chromatid
