Mutations in DNA and repair

Question 1

Silent Mutation

Answer 1

Nucleotide substitution but codes for the same amino acid; often a base change in the 3rd position of the codon (tRNA wobble)

Question 2

Missense

Answer 2

substitution leading to change in amino acid (is conservative if amino acid is similar in structure)

Sickle cell anaemia is a Glutamic acid –> Valine substitution

Question 3

Nonsense

Answer 3

Substitution leads to an early stop codon leading to shortened reading frame.

Question 4

Frameshift

Answer 4

Deletion of insertion of nucleotide leading to alteration of reading frame. causes misreading of all nucleotides downstream of mutation and causes a truncated or non functional protein.

Question 5

Transition and transversion

Answer 5

Transition is when a A–> G or C–>T

Transversion is A/G –> C/T

Question 6

DNA Repair: Singled stranded DNA needs repaired sometimes by what methods?

Answer 6

Nucleotide excision repair: Endonucleases cut out mutations, Polymerase and Ligase fill and reseal the gap.
This repairs large helix distorting lesion and occurs in G1 cell cycle.
This mechanism is defective in xeroderma pigmentosum - pyrimidine dimers not repaired after UV exposure.

Question 7

Base excision repair

Answer 7

Base specific glycosylase removes altered base and creates empty site.
nucleotides are removed by AP-endonucleases which cleaves the 5’end.
Lyase cleaves 3’ end and DNA polymerase beta fills the gap and DNA ligase seals it.

Question 8

Mismatch repair

Answer 8

Newly synthesis strand is recognised, mismatched nucleotides are removed and the gap is filled and resealed. Occurs in G2 phase.

This is defective in Hereditary non polyposis colorectal cancer.

Question 9

Double stranded repair: Non homologous end joining

Answer 9

2 ends of DNA together to repair double stranded breaks.

Ataxia telangectasia and Fanconi anaemia has a defect in this repair.

Question 10

DNA/RNA/protein synthesis direction

Answer 10

DNA and RNA are both synthesis 5’ –> 3’ (5’ has triphosphate energy source, 3’ hydroxyl attack)
Drugs that block DNA replication often have a modified 3’OH in order to block this (chain termination)

mRNA is read 5’–>3’

Protein synthesis occurs N-terminus to C-terminus.

Question 11

Stop and Start codons are:

Answer 11

mRNA - AUG (methionine sometimes removed before translation is completed)

in prokaryotes fMet - this stimulates neutrophil chemotaxis.

Stop codons are UGA, UAA, UAG.