Mutations

Question 0

What are the most common SNPs

Answer 0

C to T (two thirds)

Question 1

What are the most common mutations?

Answer 1

SNPs - single nucleotide polymorphisms

Question 2

Are transitions or transversions more common?

Answer 2

Transitions

Question 3

Why do mutations in the third position often not cause disease?

Answer 3

Triplet code is degenerate, often mutations in third position are silent substitution

Question 4

What is a missense mutation?

Answer 4

One amino acid is substituted by another

Question 5

Why might a silent mutation cause disease?

Answer 5

Disrupt splicing

Question 6

What is a nonsense mutation?

Answer 6

Amino acid codon is replaced by a stop codon

Question 7

What is a conservative missense mutation?

Answer 7

Some amino acids which have similar codons have similar properties, so substitution might be tolerated in non critical regions of the protein

Question 8

What are PTCs?

Answer 8

Premature termination codons - due to frameshift

Question 9

What is NMD?

Answer 9

Nonsense mediated decay - mRNA that has PTC is broken down, prevents faulty protein being produced.

Question 10

What is the result of splice site mutations?

Answer 10

Skipping of exon, may lead to frameshift.

Question 11

What base pairings are observed when a base undergoes tautomeric shift?

Answer 11

Rare C with common A (2 hydrogen bonds)

Rare G with common T (3 hydrogen bonds)

Question 12

What is tautomeric shift?

Answer 12

Proton briefly changes position

Question 13

What is slippage during replication?

Answer 13

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

Question 14

What is the affect of nitrous acid on nucleotides?

Answer 14

Replaces amino groups with keto groups
C > U
A > H (pairs with C - hypoxanthine)
G > X (pairs with C - xanthine)

Question 15

What is the effect of EMS on DNA?

Answer 15

Ethyl methane sulphonate.

Removes purine rings - apurinic sites can be paired with any base

Question 16

What is the effect of IQ on DNA?

Answer 16

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.

Question 17

What is ethidium bromide?

Answer 17

Intercalates with DNA - mutagenic

Can also be used to stain DNA

Question 18

How can you reduce exposure to radiation?

Answer 18

Avoid x Rays and flying

Question 19

What are the effects of UV on the body?

Answer 19

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.

Question 20

What is the effect of UV on DNA?

Answer 20

Causes thymine dimer formation - often spontaneously resolves by photo - reactivation

Question 21

What is the error rate of DNA polymerase?

Answer 21

1 in 100,000

120,000 mistakes every time a cell replicates.

Question 22

How are most errors corrected?

Answer 22

Proof reading - polymerase detects incorrectly paired 3’ base and corrects it 99% of the time.

Question 23

What is nucleotide mismatch repair?

Answer 23

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

Question 24

What damaged bases may accumulate in DNA?

Answer 24

Uracil Deaminated bases Oxidised bases Alkylated bases

Question 25

How are damaged bases repaired?

Answer 25

Excision repair Short patch - polymerase, non displacing synthesis, ligase Long patch- polymerase, displacing synthesis, flap endonuclease, ligase

Question 26

What genes are commonly mutated in hereditary non polyposis colorectal cancer?

Answer 26

MLH1, MSH2, MSH6 - encode mismatch repair enzymes

Question 27

What new capabilities must a cell acquire in order to become cancerous?

Answer 27

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.

Question 28

What genes are involved in inherited breast cancer?

Answer 28

BRCA1 and BRCA2 | Involved in detecting DNA damage and signalling cell cycle checkpoints

Question 29

What are oncogenes?

Answer 29

Mutated versions of proto oncogenes (genes that control cell cycle) Cancer causing

Question 30

Why is inherited cancer apparently dominant, when the genes are recessive?

Answer 30

Two hit theory - probability of a second mutation over a lifetime is high

Question 31

How can homozygosity be acquired?

Answer 31

Loss of wild type chromosome Deletion of part of a wild type chromosome Point mutation Mitotic recombination

Question 32

What is the most common mutation in CF?

Answer 32

p.F508del - 3bp deletion, phenylalalanine, at amino acid 508

Question 33

How many mutations account for 92% of cases of cf?

Answer 33

13

Question 34

What proportion of CF cases have unknown mutations?

Answer 34

1.5%

Question 35

What is SSCP?

Answer 35

Single stranded conformation polymorphism, denature then snap cool - strands form sequence specific partly double stranded. Electrophoresis in polyacrylamide gel, then stain with silver.

Question 36

How can DNA be obtained from a foetus?

Answer 36

Chorionic villus sampling Amniotic fluid Fetal DNA in mothers blood

Question 37

How is amniocentesis carried out?

Answer 37

15-20 weeks gestation Ultrasound guidance Cells cultured for 2 weeks 0.5-2% chance of miscarriage

Question 38

How is chorionic villus sampling performed?

Answer 38

``` 10-13 weeks gestation Ultrasound guidance Trans cervical or trans abdominal 2% risk of miscarriage Must separate from maternal tissue ```

Question 39

What is MLPA?

Answer 39

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.

Question 40

What conditions have whole exon deletions or duplications?

Answer 40

Include osteogenesis imperfecta and duchenne muscular dystrophy