1.5.2 DNA replication

Question 1

When does DNA replication occur in the cell cycle?

Answer 1

S phase

Question 2

Why is DNA replication semi-conservative?

Answer 2

• in each new molecule produced, one of the polynucleotide DNA strand is from the original DNA molecule
• The other strand is newly created by the cell
• Therefore, the DNA molecule has conserved HALF of the original DNA

Question 3

Describe the process of semi-conservative replication: (5)

Answer 3

• DNA helicase breaks the hydrogen bonds between complementary base pairs, separating the polynucleotide strands.
• Each original strand acts as a template for the new strand.
• Complementary base pairings mean that free floating nucleotides are attracted to their complementary exposed bases on each template strand.
• Condensation reactions join the nucleotides together, catalysed by DNA polymerase.
• Each new DNA molecule contains 1 strand of original DNA and one strand of new DNA

Question 4

What does helicase do in DNA replication?

Answer 4

Unwinds the DNA double helix by breaking the hydrogen bonds between base pairs.

Question 5

What does DNA polymerase do in DNA replication?

Answer 5

Catalyses the condensation reactions between the deoxyribose sugar, and the phosphate groups of new, adjacent nucleotides.

Question 6

In the process of semi-conservative DNA replication, the two strands within a DNA molecule are separated. Each then acts as a template for the formation of a new complementary strand. Describe how the separation of strands occurs. (2)

Answer 6

DNA helicase
- breaks hydrogen bonds between base pairs

Question 7

Describe the role of DNA polymerase in the semi-conservative replication of DNA. (2)

Answer 7

• joins adjacent nucleotides
• catalyses condensation reactions
• catalyses the formation of phosphodiester bonds.

Question 8

Name the two scientists who proposed models of the chemical structure of DNA and of DNA replication. (1)

Answer 8

Watson and Crick

Question 9

Give two features of DNA and explain how each one is important in the semi-conservative replication of DNA. (2)

Answer 9

1. weak hydrogen bonds between bases - allows the two strands to separate
   - 2 strands - both can act as templates

Question 10

Describe the role of two named enzymes in the process of semiconservative replication of DNA. (3)

Answer 10

• DNA helicase breaks hydrogen bonds between DNA strands
• DNA polymerase joins the DNA nucleotides
• forming phosphodiester bonds.

Question 11

During replication, the two DNA strands separate and each acts as a template for the production of a new strand. As new DNA strands are produced, nucleotides can only be added in the 5’ to 3’ direction.
Explain why new nucleotides can only be added in a 5’ to 3’ direction. (4)

Answer 11

DNA polymerase;
is specific;
Only complementary with 3’ end of strand
Shapes of 5’ end and 3’ end are different

Question 12

what binds to the exposed bases on the template strands during DNA replication

Answer 12

free DNA nucleotides