Mutations Flashcards
Single nucleotide polymorphism (SNPs “snips”)
- single bases substitution
- 2/3 of SNPs are C to T changes
- transitions (purine to purine or pyrimidines to pyrimidines) are more common than transversions (purine to pyrimidines)
Missense mutations
- one amino acid is substituted for another (normally by a single base change)
- e.g. GGC (glycine) to TGC (cysteine)
- substitutions of e.g. a valine to alanine may be tolerated in non-critical (not the active site) regions of the protein
Silent mutations
- a single base substitution which doesn’t substitute the amino acid
- e.g. GGA changed to GGG (both glycine)
- some can disrupt RNA splicing and result in heritable diseases
Nonsense mutations
- an amino acid codon is changes to a stop codon
- e.g. GGA (glycine) to TGA (stop)
- mRNAs that contain a PTC will be degraded by “nonsense mediated decay” - a protective mechanism
Frameshift mutations
- the reading frame of mRNA is altered in some way
- insertion, deletion (of bp that are not in multiples of 3) or slice site mutations
- a stop codon is often found in alternative reading frame causing the ribosome to terminate translation prematurely
- mRNAs that contain a PTC will be degraded by “nonsense mediated decay” - a protective mechanism
Causes of base changes
- sequence changing during replication
- chemicals can induce mutations
- exposure to radiation
Tautomeric shift
- proton (H+) briefly changes position
- altered base pairing properties
- can behave as an altered template base during DNA
replication (C pairs with an A and T pairs with a G)
Chemical mutant base change
Nitrous acid replaces amino group with keto groups
C –> U pairs with A
A –> H pairs with C
G –> X pairs with C
Ethyl methane sulphate EMS causes removal of purine rings
apurinic sites can be paired with any base during replication - are unpredictable
Consequence of mutations
About 1-2% of DNA is protein coding
- mutations close or within genes are most likely to cause disease
- type and location of mutation will determine its consequences
IQ
a heterocyclic aromatic amine food mutagen
Found in cooked meats and cigarette smoke condensates
- disrupts the packaging of DNA bases & causes mostly single base deletions at GC base pairs
- intercalation of IQ forces the bases further apart leading to misreading by DNA polymerase and deletion of a single base
Ionising radiation
- radiation that produces ions during interaction with cellular molecules
e. g. UV, X-Rays, nuclear power plant accidents, radon gas - is unavoidable
UV light effects
- UVB exposure induces the production of vitamin D in the skin, over exposure causes sunburn and some types of cancer
- UVA, UVB, UVC can all cause damage to collagen fibres leading to skin ageing
- UVA and UVB destroy vitamin A in the skin,
UV light protons cause adjacent thymine bases to pair - the diners often resolve spontaneously through a process known as photo-reactivation
DNA repair processes
Most errors are corrected by “proof reading” - mis paired 3’ base in newly synthesised strand is detected and corrected by polymerase 99% of the time
Nucleotide mismatch repair
- enzymes detect mismatched bases in the newly synthesised strand (after replication) and replace them
- a “patch” of DNA sequence is replaced
IQ
a heterocyclic aromatic amine food mutagen
Found in cooked meats and cigarette smoke condensates
- disrupts the packaging of DNA bases & causes mostly single base deletions at GC base pairs
- intercalation of IQ forces the bases further apart leading to misreading by DNA polymerase and deletion of a single base
Ionising radiation
- radiation that produces ions during interaction with cellular molecules
e. g. UV, X-Rays, nuclear power plant accidents, radon gas - is unavoidable
UV light effects
- UVB exposure induces the production of vitamin D in the skin, over exposure causes sunburn and some types of cancer
- UVA, UVB, UVC can all cause damage to collagen fibres leading to skin ageing
- UVA and UVB destroy vitamin A in the skin,
UV light protons cause adjacent thymine bases to pair - the diners often resolve spontaneously through a process known as photo-reactivation
DNA repair processes
Most errors are corrected by “proof reading” - mis paired 3’ base in newly synthesised strand is detected and corrected by polymerase 99% of the time
Nucleotide mismatch repair
- enzymes detect mismatched bases in the newly synthesised strand (after replication) and replace them
- a “patch” of DNA sequence is replaced
Excision repair
Types of damaged bases - oxidised bases - alkylated bases - de-aminated bases - uracil Are repaired by base excision repair
Cancer
Failure of DNA repair mechanisms can result in cancer
- humans: MLH1, MSH2 and MSH6 encode for mismatch repair enzymes
- the genes are commonly mutated in cases of hereditary non-polyposis colorectal cancer
Tumour is result of growth advantage with 6 new characteristics
- divide independently (no need for internal growth signals)
- ignore external antigrowth signals
- avoid apoptosis (programmed cell death)
- divide indefinitely
- stimulate sustained angiogenesis (blood supply for nutrients)
- invade tissues and establish secondary tumours
Successive advantage
- mutations affect functions which raise the probability of successive mutations occurring
- all cancer cells exhibit chromosomal instability and microsatellite instability
Inherited breast cancer genes - BRCA1 or BRCA2
Oncogenes
- retro-viruses have genes that are able to transform cells into a cancerous phenotype
- several human genes have sequence similarities to vital oncogenes (Proto-oncogenes)
- key amino acid substitutions can activate the proto-oncogenes into cancer causing oncogenes
Heritable or sporadic
- inherited cancer genes tend to harbour recessive mutations
- development of cancer is a dominant pattern of inheritance
Initiation of tumour formation requires that both copies be mutated or that the functional wild-type copy be deleted
SSCP mutation scanning
- identify the mutated region of the gene - perform targeted DNA sequencing
- amplified DNA is heated to denature, then rapidly cooled - the individual strains will adopt a sequence-specific partly double stranded forms
- the DNA is electrophoresed in polyacrylamide gel and detected by staining with silver (3D protein haves will have different mobilities in the gel)
Obtaining foetal DNA
- Amniocentesis (amniotic fluid cells) - performed at 15-20 weeks gestation with ultrasound guidance - 0.5-1% risk of causing a miscarriage
- Chorion villus biopsy - performed at 10-13 weeks gestation with ultrasound guidance - tissue sample from placenta - 2% risk of causing a miscarriage
- Foetal DNA in mother’s blood - can be isolated from mothers blood along with mothers DNA - largely a research tool
