5. DNA replication

Question 1

Where does DNA replication initiate?

Answer 1

Origin of replication - multiple origins along each chromosome in S-phase

Question 2

What enzymes are involved in the initiation of DNA replication, and what are their roles?

Answer 2

1. Topoisomerases - create nick in DNA strand
2. Helicases - unwind double helix so that Y-shaped replication forks can open
3. Primases - attach small complementary RNA sequences as primer at forks. Provide 3’-OH group needed as substrate for DNA polymerase to start synthesis
4. DNA polymerase - synthesises new strand by adding deoxynucleoside monophosphate (dNMP) to 3’-OH of growing strand

Question 3

What type of DNA polymerase replicates nuclear DNA? What advantages does it have?

Answer 3

Family B DNA polymerase

High fidelity, 3’-5’ proof-reading function to remove errors

Question 4

What happens in the elongation phase of DNA replication?

Answer 4

Primers removed and replaced with new nucleotides, backbone sealed by DNA ligase

2 replication forks allow DNA polymerase to move in opposite directions

Question 5

How is the leading strand synthesisesd?

Answer 5

Continuously, movement of replication fork in same direction as 5’ to 3’ synthesis of the strand

Question 6

How is the lagging strand synthesised?

Answer 6

Discontinuously, movement of replication fork in opposite direction to the 5’ to 3’ synthesis of the strand

DNA polymerase elongates short fragments (Okazaki fragments) before moving to new primer

Okazaki fragments joined by DNA ligase

Question 7

How are telomeres replicated?

Answer 7

No template ahead of replication fork for primase, telomerase (specialised reverse transcriptase) binds to overhanging 3’ end of lagging strand

TERC sub-unit of telomerase is tandem repeat sequences complementary to telomere repeat - provides template to extend telomere of lagging strand by one repeat –> creates space for primer

Question 8

Disorder associated with origin of replication defect?

Answer 8

Meier-Gorlin syndrome (ORC1, ORC4, CDT1, CDC6)

Primordial dwarfism

Inability to initiate replication origins

Question 9

Disorder associated with helicase defect?

Answer 9

Bloom syndrome

Growth deficiency, predisposition to malignancy

Defective DNA replication –> genomic instability, increased sister chromatid exchange

Question 10

Disorder associated with polymerase defect?

Answer 10

Hutchinson-Gilford progeria syndrome (LMNA)

Premature ageing