Mutations Flashcards
What are the most common SNPs
C to T (two thirds)
What are the most common mutations?
SNPs - single nucleotide polymorphisms
Are transitions or transversions more common?
Transitions
Why do mutations in the third position often not cause disease?
Triplet code is degenerate, often mutations in third position are silent substitution
What is a missense mutation?
One amino acid is substituted by another
Why might a silent mutation cause disease?
Disrupt splicing
What is a nonsense mutation?
Amino acid codon is replaced by a stop codon
What is a conservative missense mutation?
Some amino acids which have similar codons have similar properties, so substitution might be tolerated in non critical regions of the protein
What are PTCs?
Premature termination codons - due to frameshift
What is NMD?
Nonsense mediated decay - mRNA that has PTC is broken down, prevents faulty protein being produced.
What is the result of splice site mutations?
Skipping of exon, may lead to frameshift.
What base pairings are observed when a base undergoes tautomeric shift?
Rare C with common A (2 hydrogen bonds)
Rare G with common T (3 hydrogen bonds)
What is tautomeric shift?
Proton briefly changes position
What is slippage during replication?
Either newly synthesised strand loops out resulting in the insertion of a base, or the template strand loops out resulting in a base being skipped and a deletion
What is the affect of nitrous acid on nucleotides?
Replaces amino groups with keto groups
C > U
A > H (pairs with C - hypoxanthine)
G > X (pairs with C - xanthine)
What is the effect of EMS on DNA?
Ethyl methane sulphonate.
Removes purine rings - apurinic sites can be paired with any base
What is the effect of IQ on DNA?
2-amino-3-methylimidazo[4,5-f]quinoline
Found in cooked meat and cigarettes
Causes single base deletions at GC base pairs, intercalated with DNA and forces bases further apart - misreading by DNA polymerase.
What is ethidium bromide?
Intercalates with DNA - mutagenic
Can also be used to stain DNA
How can you reduce exposure to radiation?
Avoid x Rays and flying
What are the effects of UV on the body?
UVB induces vitamin B production in the skin
Over production causes sunburn and some types of skin cancer
UVA and UVB destroy vitamin A
UVA, UVB, and UVC damage collagen, causes skin to age.
What is the effect of UV on DNA?
Causes thymine dimer formation - often spontaneously resolves by photo - reactivation
What is the error rate of DNA polymerase?
1 in 100,000
120,000 mistakes every time a cell replicates.
How are most errors corrected?
Proof reading - polymerase detects incorrectly paired 3’ base and corrects it 99% of the time.
What is nucleotide mismatch repair?
Enzymes recognise mismatched DNA and remove (excise) it (it is unknown how they know which strand is parent and which daughter)
Replace the patch of DNA
What damaged bases may accumulate in DNA?
Uracil
Deaminated bases
Oxidised bases
Alkylated bases
How are damaged bases repaired?
Excision repair
Short patch - polymerase, non displacing synthesis, ligase
Long patch- polymerase, displacing synthesis, flap endonuclease, ligase
What genes are commonly mutated in hereditary non polyposis colorectal cancer?
MLH1, MSH2, MSH6 - encode mismatch repair enzymes
What new capabilities must a cell acquire in order to become cancerous?
Divide independently of external growth signals
Ignore external anti growth signals
Avoid apoptosis
Divide indefinitely avoiding senescence
Stimulate angiogenesis
Invade tissues and establish secondary tumours
One mutation increases likelihood of further mutations.
What genes are involved in inherited breast cancer?
BRCA1 and BRCA2
Involved in detecting DNA damage and signalling cell cycle checkpoints
What are oncogenes?
Mutated versions of proto oncogenes (genes that control cell cycle)
Cancer causing
Why is inherited cancer apparently dominant, when the genes are recessive?
Two hit theory - probability of a second mutation over a lifetime is high
How can homozygosity be acquired?
Loss of wild type chromosome
Deletion of part of a wild type chromosome
Point mutation
Mitotic recombination
What is the most common mutation in CF?
p.F508del - 3bp deletion, phenylalalanine, at amino acid 508
How many mutations account for 92% of cases of cf?
13
What proportion of CF cases have unknown mutations?
1.5%
What is SSCP?
Single stranded conformation polymorphism, denature then snap cool - strands form sequence specific partly double stranded.
Electrophoresis in polyacrylamide gel, then stain with silver.
How can DNA be obtained from a foetus?
Chorionic villus sampling
Amniotic fluid
Fetal DNA in mothers blood
How is amniocentesis carried out?
15-20 weeks gestation
Ultrasound guidance
Cells cultured for 2 weeks
0.5-2% chance of miscarriage
How is chorionic villus sampling performed?
10-13 weeks gestation Ultrasound guidance Trans cervical or trans abdominal 2% risk of miscarriage Must separate from maternal tissue
What is MLPA?
Multiplex ligation - dependent probe
2 labelled probes - different stuffer for each template DNA - can be separated by electrophoresis
Only ligated when they both bind to template DNA.
What conditions have whole exon deletions or duplications?
Include osteogenesis imperfecta and duchenne muscular dystrophy
