1.2a Replication of DNA

Question 1

Why must DNA be replicated exactly?

Answer 1

So that the chromosome complement is maintained and no genetic information is lost - the new cells receive an exact copy of the genes it requires to function.

Question 2

What process requires DNA replication?

Answer 2

Mitosis (cell division)

Question 3

What does DNA replication require?

Answer 3

• DNA
• Primers
• Appropriate enzymes (DNA polymerase and ligase)
• Supply of the four types of DNA nucleotide
• ATP

Question 4

What is a primer?

Answer 4

A short strand of nucleotides which binds to the 3’ end of the template DNA strand, allowing DNA polymerase to add nucleotides.

Question 5

What is the role of DNA polymerase in DNA replication?

Answer 5

DNA polymerase adds complementary nucleotides using base pairing to the deoxyribose 3’ end of an exposed DNA strand. It needs primers to start DNA replication.

Question 6

What is the role of ligase in DNA replication?

Answer 6

Ligase joins fragments of DNA together.

Question 7

What is the leading strand?

Answer 7

The strand of DNA that is being replicated continuously.

Question 8

What is the lagging strand?

Answer 8

The strand of DNA that grows in the opposite direction to the replication fork and is replicated in fragments.

Question 9

Extended response question:

Process of DNA replication

Answer 9

• Double helix unwinds
• Hydrogen bonds between bases break causing strands to separate, forming a replication fork
• At leading strand;
  A primer binds to the deoxyribose sugar 3’ end of the exposed DNA leading strand; free DNA nucleotides from the cytoplasm move to find their complementary base; DNA polymerase adds these nucleotides using base pairing from the 3’ end to the 5’ end to synthesise a new complementary strand (only in one direction)
• At lagging strand;
  Strand ‘opens up’ the wrong way to add nucleotides to 3’ end so a number of primers are added to the exposed strand; strand is replicated in fragments and ligase joins these fragments together.
• This results in two new identical DNA molecules which rewind to form double helixes

Question 10

What does DNA replication produce?

Answer 10

Two new identical DNA molecules, each of which contains one original parent strand and one new strand (semi conservative replication)