Mutations Flashcards
What is a silent substitution?
Single base substitution (usually at third codon) which does not result in an amino acid change
What is a single-base substitution? What are the two different types?
Single nucleotide base change
TRANSITION = pyrimidine to pyrimidine or purine to purine
TRANSVERSION = pyrimidine to purine
What is the difference between missense and nonsense mutations?
Missense mutation = one amino acid is substituted for another
(note: conservative mutations are well-tolerated due to similar properties in the amino acids/location of mutation)
Nonsense mutation = amino acid mutated to a STOP codon
What is a frameshift mutation?
Change in reading frame caused by insertion or deletion of base pairs not in a multiple of 3bp, or by a splice site mutation.
STOP codons are often found in the alternate reading frame —> premature termination codons —> nonsense mediate decay (prevents truncated non-functional protein being produced)
What causes base changes?
TAUTOMERIC SHIFT = proton briefly changes position in the base, causing altered base-pairing (altered template during DNA replication)
SLIPPAGE =
- newly synthesised strand loops out -> addition of one nt on new strand
- template strand loops out -> deletion of one nt on new strand
CHEMICAL MUTAGENS =
- nitrous acid replaces amino groups with keto groups
- EMS causes removal of purine rings (therefore, apurinic sites can be paired with any base during replication)
BASE STACKING MUTAGENS = IQ & ethidium bromide disrupts packing of bases by intercalating, which forces the bases further apart, therefore DNA pol misreads the strand and a single base is deleted
IONISING RADIATION = high energy (short wavelength & higher frequency) e.g. X-rays, UV, gamma rays
THYMINE DIMER FORMATION = UV light photons cause adjacent thymines to base-pair (fixed by photoreactivation)
What are two types of DNA repair?
Mismatch repair = enzymes detect mismatched bases and replaces them by strand-excision and re-synthesis
Excision repair = damaged bases excised and repaired
What does a failure in DNA repair lead to? Give an example of a disease caused by this?
Cancer
e. g. genes encoding mismatch repair are mutated -> hereditary non-polyposis colorectal cancer
e. g. mutation in BRCA1/2 (which detect DNA damage & signal cell-cycle checkpoint -> halts replication of damaged cells) -> breast cancer
What qualities do cells need to have to cause tumour formation?
- divide independently of external growth signals
- ignore external anti-growth signals
- avoid apoptosis
- divide indefinitely without senescence
- stimulate sustained angiogenesis
- invade tissues and establish secondary tumours
- exhibit chromosomal instability & microsatellite instability
What are proto-oncogenes and oncogenes?
Proto-oncogenes = genes that may activate and become oncogenes
Oncogenes = genes which can transform normal cells into cancerous cells
What is the two-hit theory?
For a cell to become cancerous, both tumour-suppressor alleles must be mutated
What is single strand conformation polymorphism detection?
Looks for splice site mutations.
PCR -> Heat to denature -> Snap cool -> Sequence-specific partly double-stranded DNA forms (NO base pairing) -> Electrophoresis & silver stain
What are the different types of foetal screening?
Chorion Villus Sampling = (membrane between foetus and mother)
- 10-13 wks gestation
- ultrasound-guided
- trans-cervical or trans-abdominal
- foetal villi separated from maternal tissue
- 2% miscarriage risk
Amniocentesis = (amniotic fluid containing foetal cells)
- 15-20 wks gestation
- ultrasound-guided
- cells recovered and cultured for 2 weeks
- 1% miscarriage risk
Isolate foetal DNA from mother’s blood (still not ready for routine diagnostic use)
How do you screen for exon deletions or duplications?
Multiplex Ligation-Dependent Probe Amplification = bind forward and reverse primers to ligate and amplify probe
No amplification product = exon deletion/duplication (as primer does not bind)
50% = heterozygous for exon deletion/duplication
