DNA REPLICATION

Question 1

3 steps of prokaryotic replication

Answer 1

1. Initiation
2. Elongation
3. Termination

Question 2

Prokaryotic initation

Answer 2

DNA Pol III initiates from a single replication origin called oriC

Question 3

Structure of DNA Pol III

a) CORE enzyme
b) gamma complex
c) epsilon subunit
d) funny t sub unit
e) beta subunit

Answer 3

a) alpha sub unit (5’ - 3’ polymerisation)
sigma sub unit (proof reading exonuclease)
theta core assembly

b) 5 sub units for loading and unloading polymerase onto DNA template via a clamp
c) used as a proof reading subunit - exonuclease
d) connects two polymerase alpha sub units to create dimeric polymerase unit
e) sliding clamp to tether polymerase to the dna

Question 4

How are RNA primers synthesised?

Answer 4

DNA primase

Question 5

Function of DnaB helicase

Answer 5

in prokaryotic initiation it separates the strands

Question 6

How does prokaryotic DNA replication occur?

Answer 6

• DnaA- ATP proteins recognise oriC and DnaB helicase unwinds strands
• held apart by single stranded binding proteins
• RNA primase synthesises primers
• DNA Pol III polymerises strands

Question 7

Replication of the leading strand (5’ - 3’) is ______

Answer 7

Continuous

Question 8

Replication of the lagging strand is _____, producing ____ fragments.

Answer 8

discontinuous

Okazaki

Question 9

What slows polymerases

Answer 9

Fork traps

Question 10

Why is lagging strand looped around?

Answer 10

Both polymerases need to function in the same direction and simultaneously

Question 11

What removes RNA primers?

Answer 11

DNA pol II using its exonuclease action in a 5’ to 3’ direction

Question 12

What seals gaps?

Answer 12

DNA ligase

Question 13

What does DNA pol I do?

Answer 13

maturation of Okazaki fragments

Question 14

Why is eukaryotic replication different?

Answer 14

• Humans have 3 billion bp compared to E.coli 30 million
• therefore humans have multiple origins of replication
• chromatin disassembly
• telomeres are used to protect DNA from exonuclease action

Question 15

Why do telomeres shorten?

Answer 15

• Eukaryotes have difficulty replicating their linear chromosomes completely - ‘the end replication problem’
• telomerase extends 3’ end of chromosomal DNA by a G-rich repeat, to counteract loss of DNA by incomplete replcation

Question 16

Somatic cells have ___ telomerase levels. Meaning what?

Answer 16

reduced

• fail to compensate for depletion of DNA at each replication round, leading to telomerase shortening and cell death after 40- 80 cell divisions

Question 17

What is the Hayflick limit?

Answer 17

the point at which telomere length is so short that the cells die

Question 18

What can reactivation of telomerase activity in somatic cells lead to?

Answer 18

Cancer