Mutations Flashcards

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1
Q

Explain why most mutations on the third position of a codon results in a silent mutation

A
  • 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
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2
Q

Name the 4 types of mutations that may affect the codons coding for amino acids

A
  • Missense
  • Nonsense
  • Silent
  • Frameshift
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3
Q

What is meant by a MISSENSE mutation?

A

Single base change that results in a substitution of amino acids e.g. GGC -> TGC substitutes Gly -> Cys

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4
Q

What is meant by a NONSENSE mutation?

A

Single base mutation that results in a codon coding for an amino acid becoming a stop codon e.g. GGA -> TGA substitutes Gly -> STOP

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5
Q

What is meant by a SILENT mutation?

A

Single base mutation that does not result in an amino acid change e.g. GUG -> GUA still codes for VALINE

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6
Q

Define a FRAMESHIFT mutation and state the consequences

A
  • 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
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7
Q

What are the consequences of a splice site mutation?

A
  • 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)
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8
Q

Explain how a missense mutation may NOT have a great effect on the overall protein structure and function

A
  • 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
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9
Q

What is the difference between a transition and a transversion?

A
  • 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
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10
Q

Describe how a single base mutation can be detected using various genetic tests

A
  • PCR amplification of DNA fragment
  • Hybridisation with ALLELE SPECIFIC OLIGONUCLEOTIDE
  • DNA sequencing
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11
Q

Explain when you would use an allele-specific PCR method to investigate a mutation

A
  • 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
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12
Q

When would you use Southern blotting over allele specific PCR when investigating a mutation?

A
  • 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
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13
Q

What is the most common base pair transition in the human genome?

A

C to T

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14
Q

Explain why the genetic code can be described as both degenerate and unambiguous

A

Each codon codes for one specific amino acid (ambiguous) but each amino acid can be coded for by multiple codons (degenerate)

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15
Q

How would a deletion of 3bp affect the reading frame of the mRNA strand?

A
  • Shorter mRNA strand produced
  • Reading frame would remain intact
  • Would result in loss of amino acid therefore a shorter protein
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16
Q

Name 3 occurrences which can result in base changes of DNA

A
  • Sequence changes during DNA replication
  • Chemical induced mutations
  • Radiation exposure
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17
Q

What 2 spontaneous occurrences can cause base changes during DNA replication?

A
  • Tautomeric shift

- DNA strand slippage

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18
Q

Describe how tautomeric shift can result in a base change of DNA

A
  • 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
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19
Q

Describe how DNA slippage can result in an altered reading frame of the new strand

A
  • 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
20
Q

Name 2 chemicals that can cause base mutations in DNA and state their methods

A
  • 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)
21
Q

What 2 external factors may induce a mutation?

A
  • Chemicals

- Radiation

22
Q

What is the difference between an oncogene and a proto-oncogene?

A

Oncogene is a MUTATED form of a proto-oncogene that causes the cells to proliferate (CANCER)

22
Q

Why do single base mutations in the 3rd position of a codon not usually result in an amino acid change?

A
  • 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
23
Q

What is meant by NONSENSE MEDIATED DECAY?

A
  • mRNAs which contain premature termination codons are destroyed before being translated
  • Results in less protein being produced
24
Q

What is the main action of IQ?

A

Disrupts the packing of DNA bases and causes mostly SINGLE BASE DELETIONS

25
Q

Name 3 environmental causes of ionising radiation

A
  • SOLAR (uv light)
  • Radon gas (granite)
  • X Rays
  • NUCLEAR POWER PLANTS
26
Q

What is the action of UVA, UVB and UVC forms of UV light?

A
  • 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
27
Q

What is the effect of UV light on DNA?

A
  • THYMINE DIMER FORMATION
  • Can cause adjacent thymines to base pair together
  • Usually resolved spontaneously
28
Q

How does DNA polymerase repair DNA?

A
  • PROOF READING

- Detects mispaired 3’ bases in the new strand and corrects them 99% of the time

29
Q

What two methods other than proof reading can be used to repair DNA?

A
  • Nucleotide mismatch repair

- Excision repair

30
Q

How can chemically damaged DNA be repaired?

A
  • EXCISION REPAIR

- Removal of a damaged patch of DNA and replacement by polymerase enzyme

31
Q

Describe how nucleotide mismatch repair occurs

A

Enzymes detect mismatched bases in newly synthesised strand and replace them, forming a new PATCH of DNA

32
Q

What is the consequence of not repairing mutations?

A

CANCER

33
Q

Explain the characteristics formed by tumour cells as a result of natural selection and evolution

A
  • 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
34
Q

What is a retrovirus?

A

Viruses that contain genes that are able to transform host cells into a CANCEROUS PHENOTYPE

35
Q

How can proto-oncogenes be activated?

A

Specific AMINO ACID SUBSTITUTIONS

36
Q

How is the cancer gene inherited?

A
  • HOMOZYGOSITY

- Both mutant alleles must be present in order for cancer to occur

37
Q

Give 4 examples of how HOMOZYGOSITY can occur

A
  • Loss of wild type of allele
  • MITOTIC RECOMBINATION
  • Deletion of normal allele
  • Point mutation of normal allele
38
Q

Why are you more likely to develop cancer if it is inherited rather than sporadic?

A
  • 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
39
Q

What is single strand conformational polymorphism (SSCP)?

A
  • Denaturing of dsDNA into ssDNA
  • ssDNA fold and form single stranded complexes (SNAP COOL)
  • Allows sequences to be distinguished via gel electrophoresis
40
Q

Give 3 ways in which foetal DNA can be obtained to test for pre-natal diagnosis of some diseases

A
  • Amniocentesis
  • Chorionic villus biopsy
  • Foetal DNA from maternal blood
41
Q

Describe the method of amniocentesis

A
  • Needle inserted into amniotic sac and amniotic fluid is extracted
  • Performed at 15-20 weeks of gestation
42
Q

Describe the method of chorionic villus biopsy

A
  • Sample of tissue taken from the chorionic villus of the placenta
  • Performed at 10-13 weeks
43
Q

Explain why chorionic villus biopsy method for extracting foetal DNA has a higher risk of miscarriage than amniocentesis

A

Performed earlier on in the pregnancy, so fetus is less developed, therefore more vulnerable to damage

44
Q

What is MLPA used for?

A
  • Duplication of exons

- Counts the copy number of many exons in parallel

45
Q

What is SSCP used to test for?

A

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