Mutations Flashcards
Explain why most mutations on the third position of a codon results in a silent mutation
- Multiple codons can code for the same amino acid
- Change in the third position on codon would in most cases still code for the same amino acid e.g. GUG -> GUA would be a silent mutation as they both code for Valine
Name the 4 types of mutations that may affect the codons coding for amino acids
- Missense
- Nonsense
- Silent
- Frameshift
What is meant by a MISSENSE mutation?
Single base change that results in a substitution of amino acids e.g. GGC -> TGC substitutes Gly -> Cys
What is meant by a NONSENSE mutation?
Single base mutation that results in a codon coding for an amino acid becoming a stop codon e.g. GGA -> TGA substitutes Gly -> STOP
What is meant by a SILENT mutation?
Single base mutation that does not result in an amino acid change e.g. GUG -> GUA still codes for VALINE
Define a FRAMESHIFT mutation and state the consequences
- Base insertion or deletion that results in the alteration of the mRNA reading frame
- mRNA may be destroyed or code for a different protein
- Insertions, deletions or splice site mutations
- Creation of a PREMATURE TERMINATION CODON
What are the consequences of a splice site mutation?
- Results in the corresponding EXON being cut out of the reading frame
- EXON would not be coded for
- Mature mRNA reading frame would be different (mRNA may be degraded)
Explain how a missense mutation may NOT have a great effect on the overall protein structure and function
- Amino acid substitution results in an amino acid change to one of a similar chemical nature
- For example GTN -> GCN would result in an amino acid change of Val -> Ala which are both non polar and hydrophobic
- Resulting protein will be very similar
What is the difference between a transition and a transversion?
- TRANSITION is a base change to a base of the same type e.g. purine —> purine
- TRANSVERSION is a base change to a base of a different kind e.g. purine —> pyrimidine
Describe how a single base mutation can be detected using various genetic tests
- PCR amplification of DNA fragment
- Hybridisation with ALLELE SPECIFIC OLIGONUCLEOTIDE
- DNA sequencing
Explain when you would use an allele-specific PCR method to investigate a mutation
- For known-disease causing mutations
- Two different allele specific probes have one common primer
- Primer is perfectly complementary to ONE of the probes (one will bind less tightly as it is the same apart from a single base)
- This can be either the probe complementary to the normal gene or the probe complementary to the mutated gene
- PCR primer will only amplify if the allele specific probe matches perfectly to the primer
When would you use Southern blotting over allele specific PCR when investigating a mutation?
- With partial gene inversions the gross organisation needs to be investigated
- Southern blotting allows investigation of an individual gene in a background of all other genes
- Analysis of larger segments with or around the gene
- Can be used to analyse triplet repeat/deletion disorders and resulting fragments may be of different sizes
What is the most common base pair transition in the human genome?
C to T
Explain why the genetic code can be described as both degenerate and unambiguous
Each codon codes for one specific amino acid (ambiguous) but each amino acid can be coded for by multiple codons (degenerate)
How would a deletion of 3bp affect the reading frame of the mRNA strand?
- Shorter mRNA strand produced
- Reading frame would remain intact
- Would result in loss of amino acid therefore a shorter protein
Name 3 occurrences which can result in base changes of DNA
- Sequence changes during DNA replication
- Chemical induced mutations
- Radiation exposure
What 2 spontaneous occurrences can cause base changes during DNA replication?
- Tautomeric shift
- DNA strand slippage
Describe how tautomeric shift can result in a base change of DNA
- Proton briefly changes position on base forming a rare form which has different H bonding properties
- Rare tautomeric forms cause C to bond with A and T to bond with G
- DNA polymerase reads base in tautomeric form and bonds an incorrect base into the new strand
Describe how DNA slippage can result in an altered reading frame of the new strand
- Slippage (looping out) of newly synthesised strand results in ADDITION of a single base to the new strand
- Slippage (looping out) of template strand results in OMISSION of a single base to the new strand
Name 2 chemicals that can cause base mutations in DNA and state their methods
- NITROUS ACID replaces amino acid group with a keto group (C->U, A->H and G->X)
- ETHYL METHANE SULPHONATE (EMS) causes removal of purine rings (apurinic sites can pair with ANY BASE)
What 2 external factors may induce a mutation?
- Chemicals
- Radiation
What is the difference between an oncogene and a proto-oncogene?
Oncogene is a MUTATED form of a proto-oncogene that causes the cells to proliferate (CANCER)
Why do single base mutations in the 3rd position of a codon not usually result in an amino acid change?
- SILENT MUTATIONS
- One amino acid can be coded for by multiple codons, which normally differ in the 3rd position e.g. GU-X can code for Valine where X can be any base
What is meant by NONSENSE MEDIATED DECAY?
- mRNAs which contain premature termination codons are destroyed before being translated
- Results in less protein being produced
What is the main action of IQ?
Disrupts the packing of DNA bases and causes mostly SINGLE BASE DELETIONS
Name 3 environmental causes of ionising radiation
- SOLAR (uv light)
- Radon gas (granite)
- X Rays
- NUCLEAR POWER PLANTS
What is the action of UVA, UVB and UVC forms of UV light?
- UVA and UVB both destroy vitamin A in skin
- UVB in small amounts leads to production of vitamin D in skin, however overexposure can lead to sunburn and SKIN CANCER
- ALL FORMS OF UV LIGHT DAMAGE COLLAGEN IN SKIN - leads to ageing
What is the effect of UV light on DNA?
- THYMINE DIMER FORMATION
- Can cause adjacent thymines to base pair together
- Usually resolved spontaneously
How does DNA polymerase repair DNA?
- PROOF READING
- Detects mispaired 3’ bases in the new strand and corrects them 99% of the time
What two methods other than proof reading can be used to repair DNA?
- Nucleotide mismatch repair
- Excision repair
How can chemically damaged DNA be repaired?
- EXCISION REPAIR
- Removal of a damaged patch of DNA and replacement by polymerase enzyme
Describe how nucleotide mismatch repair occurs
Enzymes detect mismatched bases in newly synthesised strand and replace them, forming a new PATCH of DNA
What is the consequence of not repairing mutations?
CANCER
Explain the characteristics formed by tumour cells as a result of natural selection and evolution
- Divide independently of external growth
- Ignore external anti-growth signals
- Avoid APOPTOSIS
- Divide indefinitely without senescence
- STIMULATE SUSTAINED ANGIOGENESIS
- Invade tissues and establish secondary tumours
What is a retrovirus?
Viruses that contain genes that are able to transform host cells into a CANCEROUS PHENOTYPE
How can proto-oncogenes be activated?
Specific AMINO ACID SUBSTITUTIONS
How is the cancer gene inherited?
- HOMOZYGOSITY
- Both mutant alleles must be present in order for cancer to occur
Give 4 examples of how HOMOZYGOSITY can occur
- Loss of wild type of allele
- MITOTIC RECOMBINATION
- Deletion of normal allele
- Point mutation of normal allele
Why are you more likely to develop cancer if it is inherited rather than sporadic?
- If inherited you already have 1 mutant allele (heterozygous) so only 1 other mutation has to occur in the homozygous allele for it to be cancerous
- If sporadic 1 allele becomes mutated in the normal cell, producing a heterozygous. It is unlikely that the same mutation would happen on the other allele, but not impossible, therefore is less likely to be cancerous
What is single strand conformational polymorphism (SSCP)?
- Denaturing of dsDNA into ssDNA
- ssDNA fold and form single stranded complexes (SNAP COOL)
- Allows sequences to be distinguished via gel electrophoresis
Give 3 ways in which foetal DNA can be obtained to test for pre-natal diagnosis of some diseases
- Amniocentesis
- Chorionic villus biopsy
- Foetal DNA from maternal blood
Describe the method of amniocentesis
- Needle inserted into amniotic sac and amniotic fluid is extracted
- Performed at 15-20 weeks of gestation
Describe the method of chorionic villus biopsy
- Sample of tissue taken from the chorionic villus of the placenta
- Performed at 10-13 weeks
Explain why chorionic villus biopsy method for extracting foetal DNA has a higher risk of miscarriage than amniocentesis
Performed earlier on in the pregnancy, so fetus is less developed, therefore more vulnerable to damage
What is MLPA used for?
- Duplication of exons
- Counts the copy number of many exons in parallel
What is SSCP used to test for?
Can IDENTIFY THE MUTATED REGION OF A GENE as different complexes of DNA are formed during SNAP COOL which move at different speeds in gel electrophoresis
