DNA replication II

Question 1

How does DNA replicate? (1)

Answer 1

Semi-conservatively (each new strand has one parental strand and one new strand)

Question 2

Is E.coli’s chromosome circular or linear and what direction does it replicate in?

Answer 2

Circular and bidirectional

Question 3

Why is there an issue replicating the ends of linear eukaryotic chromosomes? (3)

Answer 3

At the end of the chromosomes are telomeres
There is no problem at the 3’ end as leading strand can fall off
Problem at 5’ end as RNA primer on lagging strand is removed but cannot be replaced - the short region hasn’t been copied

Question 4

How is the telomere problem solved in replication?

Answer 4

Telomerase adds an extra DNA sequence to the 3’ end of the template DNA strand that is complimentary to the lagging strand. RNA is still last but the telomere doesn’t get shorter

Question 5

Where is telomerase active?

Answer 5

Germ cells but NOT in most somatic cells (body cells)

Question 6

Why is the telomere important in some cases?

Answer 6

Stops cells growing out of control and becoming cancerous

Question 7

How many polymerases does E.coli have?

Answer 7

5

Question 8

What is the role of DNA polymerase 1?

Answer 8

Helps remove RNA primer and replaces with DNAB in chromosome replication - repairs damaged DNA

Question 9

What is the role of DNA polymerase 2?

Answer 9

Restarting replication when blocked by damaged DNA, also role in DNA repair

Question 10

What is the role of DNA polymerase 3?

Answer 10

Chromosome replication

Question 11

What is the role of DNA polymerase 4 & 5?

Answer 11

Allow replication to bypass some types of DNA damage, also involved in DNA repair

Question 12

what are the main enzyme activities DNA polymerase 1? (4)

Answer 12

Gene: polA
Exonuclease in both directions
Polymerase in 5’-3’ direction
Slow polymerisation rate (16-20)

Question 13

What are the main enzyme properties of DNA polymerase 3? (4)

Answer 13

Gene: polC
Exonuclease in only 3’-5’
Polymerase in 5’-3’
Fast polymerisation rate (250-1000)

Question 14

What is exonuclease?

Answer 14

Proofreading - can degrade to remove errors

Question 15

What is the structure of DNA polymerase 3? (5)

Answer 15

symmetrical enzyme:
2 alpha subunits - core of enzyme DNA synthesis
2 Tau units - holds the 2 halves together
Epsilon and Theta units - involved in proofreading activity of the enzyme
2 B subunits - ring clamp for DNA to go through
Clamp loader complex at the top

Question 16

What is helicase’s role?

Answer 16

Unwind the DNA duplex

Question 17

What is the role of single-stranded DNA binding protein?

Answer 17

Keeps strands apart and stops them folding up

Question 18

What is the role of primosome?

Answer 18

Synthesizes RNA primers to initiate DNA synthesis, moves along with lagging strand

Question 19

Describe DNA polymerase 3 in action and the steps it goes through: (5)

Answer 19

1. DnaB unwinds the complex
2. a subunits synthesise DNA
3. Tau subunits ensures dimerisation of polymerase
4. B clamp encircles and clamps onto DNMA
5. Nucleotides are added

Question 20

As the DNA polymerase 3 is symmetrical, what does this mean the lagging strand template forms?

Answer 20

A loop round the primer for synthesis

Question 21

What are the 5 polymerase’s in DNA replication in eukaryotes? What are their functions?

Answer 21

alpha - initial synthesis at RNA primer
delta - DNA replication (lagging), 3’-5’ exonuclease
Epsilon - DNA replication (leading), 3’-5’ exonuclease
Beta - repair
Gamma - DNA synthesis, 3’-5’ exonuclease (mitochondria)