DNA replication

Question 1

What part of the cell cycle does DNA replication occur in

Answer 1

S phase

Question 2

Why is DNA replicated

Answer 2

So that each daughter cell has the same volume of DNA as the parent cell after cell division

Question 3

Why is DNA replication semi-conservative

Answer 3

Each new strand consists of one parental and one newly synthesised strand

Question 4

Which direction is DNA synthesised

Answer 4

5’ to 3’

Question 5

What are the 3 steps in DNA replication

Answer 5

Initiation, Elongation and Termination

Question 6

What is a replication fork

Answer 6

The 2 separated parent strands form a Y shape, where DNA replication occurs, 2 replication forks form a replication bubble

Question 7

Why is there a leading and lagging strand of DNA

Answer 7

DNA can only be synthesised from 5’ to 3’, so the lagging strand (antisense) must be synthesised in small Okazaki fragments which is a slower process

Question 8

What happens in stage 1 (initiation)

Answer 8

• Origins of replication (specific nucleotide sequences) are recognised by proteins
• DNA helicase unwinds and opens up DNA
• primase synthesises RNA primers on both strands as a starting point for synthesis of new DNA (initial priming event)

Question 9

What is the role of single stranded DNA binding proteins in replication

Answer 9

Associate with the parent strands to prevent them from reannealing after helicase breaks the H bonds

Question 10

What happens in stage 2 (elongation)

Answer 10

• New DNA strands are synthesised by DNA polymerase
• Continuous on leading strand, via Okazaki fragments on lagging stands

Question 11

What happens in stage 3 (termination)

Answer 11

-exonuclease removes all DNA primers from both strands
- Another DNA polymerase molecule fills in the gaps of DNA by adding dNTPs to preceding fragments
- DNA ligase links the DNA to form continuous daughter strands

Question 12

What are telomeres

Answer 12

Repeated sequences present at both ends of chromosome that do not code for any genes

Question 13

What is the DNA sequence that would be paired with this strand
5’- CGATCC-3’

Answer 13

5’- GCTAGG-3’

Question 14

What is the end of replication problem

Answer 14

Because the last primer on the lagging stand can be placed 70-100 nucleotides away from the chromosome end a large gap can be left in DNA which is not replicated, shortening the chromosome

Question 15

What is the role of telomerase

Answer 15

The enzyme that prevents the shortening of telomeres in DNA by extending the ends by adding more repeating telomere sequences so the chromosome does not shorten over time