Errors in DNA replication

Question 1

What causes DNA replication stress?

Answer 1

replication machinery factor has defect, factors hinder replication fork progression, defects in response pathways

Question 2

What does slippage mean?

Answer 2

as the strands replicate they can come apart, if they come back together they may not math up correctly, this causes a loop in the DNA. This base may be missed in replication and cause expansions and contractions in DNA. Slippage on synthesised strand = addition. slippage on template strand = deletion.

Question 3

How would you write complementary DNA?

Answer 3

5-3 prime, reversing sequence, A-T, G-C

Question 4

How could a nucleotide be misincorporated into DNA?

Answer 4

defect in exonuclease DNA polymerase, proof reader

Question 5

What is an exonuclease?

Answer 5

proof reader. enzymes that work by cleaving incorrect nucleotides one at a time from the exposed end of a polynucleotide chain, a hydrolysing reaction that breaks the phosphorites term bond at either the 3 or 5 end

Question 6

What is endonuclease?

Answer 6

Ends: cleaves polynucleotides by separating nucleotides other than the two end ones

Question 7

What is the main function of DNA polymerase?

Answer 7

assemble nucleotides

Question 8

What is the function of an exonuclease domain in a DNA polymerase?

Answer 8

Many DNA polymerase contain an exonuclease domain, which acts in detecting base pair mismatches and further performs in the removal of the incorrect nucleotide to be replaced by the correct one. Proof reading

Question 9

Summarise the stages of DNA replication

Answer 9

1. separation of 2 strands by helicase = spins incoming DNA to unravel, separated strands = 3 prime + 5 prime, distinguished by direction nucleotides join up
2. 3’ = leading strand template, diverted to DNA polymerase, used as a continuous template for synthesis of leading strand
3. 5’ = lagging strand template, has opposite 5’ - 3’ orientation, DNA polymerase won’t work in this direction. As it emerges from the helicase, the lagging strand is organised into Okazaki fragments = RNA primase synthesises starting from RNA primers
4. Okazaki fragments are presented to second DNA polymerase in preferred 3’ to 5’ orientation and are then effectively synthesised backwards and added by DNA ligase, creating lagging strand
5. Finished sections are released and next loop is drawn back for replication

Question 10

Which direction does DNA polymerase run?

Answer 10

Moves 3’ to 5’, synthesises 5’-3’

Question 11

What factors hinder replication fork progression?

Answer 11

Base excision repair (BER) defect: causes DNA single-strand breaks (SSBs) to persist, replication fork arrests at SSB

Question 12

What is the most dangerous type of DNA damage?

Answer 12

Double strand breaks

Question 13

Name the defect that causes Huntingtons

Answer 13

HTT gene, normal function not clear, CAG normal gene 6-39 repeats, CAG disease gene 35-121 (polyglutamine), triplet repeat expansion leads to neurodegeneration, Mutant protein disrupts normal functions within neurons

Question 14

What are the differences between the major and minor DNA grooves?

Answer 14

major groove occurs where backbones are far apart, minor groove occurs where they are close together, easier for DNA binding proteins to interact with bases on the major groove side because backbone not in the way

Question 15

What is BRCA deficiency defect?

Answer 15

BRCA is a repair mechanism, defect = DNA double stranded breaks to persist = loss of chromosome integrity, eventual cell death = most dangerous type of DNA damage

Question 16

What is the base excision repair defect?

Answer 16

Causes DNA single stranded breaks to persist, replication fork arrests at SSB