DNA replication

Question 1

First step

Initiation

Answer 1

separation of the two DNA strands

Question 2

Second step
Elongation
3 points

Answer 2

1. Each “old” strand acts as a template that determines the order of nucleotides along “new” complementary strands.
2. Nucleotides “plug” into specific sites along the template according to the base-pairing rules.
3. The nucleotides are connected to form the sugar-phosphate backbones of
   the new strands.

Question 3

semi-conservative mechanism

Answer 3

each daughter duplex contains one strand of the progenitor DNA

Question 4

Replication bubbles

Answer 4

see desktop

bubbles join to complete replication

Question 5

4 proteins involved in DNA replication

Answer 5

1. DNA polymerase I and III
2. DNA helicase
3. RNA primase
4. Single-strand DNA binding protein (SSB)

Question 6

Initiation

Answer 6

Helicase unwinds DNA

SSB maintains single strand

Question 7

DNA synthesis problem

Answer 7

1. DNA replication catalysed mainly by DNA polymerase
   But DNA polymerase can only add onto existing nucleotide polymer
2. the solution…..
   Primase can synthesise RNA from scratch
   Then DNA polymerase can add onto the RNA primer …this is elongation

Question 8

DNA polymerases

Answer 8

Nucleotides align with template DNA strand and are added one by one to the growing end of the new strand

Question 9

Elongation 5’ -> 3’

2 points

Answer 9

1. The DNA polymerase only joins new bases onto the free 3’ end of DNA or RNA
2. Therefore replication always occurs in a 5’ -> 3’ direction

Question 10

Elongation 5’ -> 3’ problem

Answer 10

The DNA strands are antiparallel and the polymerase can not make one strand backwards.

Question 11

Replication in the Lagging strand

4 points

Answer 11

1. DNA polymerase III adds DNA onto RNA primer
   Okazaki fragments form
2. RNA is then destroyed by DNA polymerase I
3. DNA polymerase I replaces RNA with DNA
4. Helicase causes further unwinding

Question 12

RNA

3 points

Answer 12

1. RNA (ribonucleic acid) is a nucleotide polymer
2. Nucleotides are adenine, guanine, cytosine and uracil
3. Sugar in RNA is ribose

Question 13

DNA repair

Answer 13

1. Mutations can occur in DNA
   Errors during replication
   Environmental DNA damage
2. DNA proofreading
   corrects errors as DNA polymerase makes them
3. Mismatch repair
   scans DNA and corrects any base pairing mismatches

Question 14

Mismatch repair

Answer 14

1. Three proteins MutS, MutL and MutH
2. MutS/L bind to DNA in area of mutation
3. Guides MutH to the region
4. DNA polymerase III re-synthesises DNA