3.1.5.2 DNA replication

Question 1

What is the first step of DNA replication?

Answer 1

The double helix unwinds.

Question 2

What enzyme is responsible for unwinding the DNA double helix?

Answer 2

DNA helicase.

Question 3

What does DNA helicase do during DNA replication?

Answer 3

DNA helicase breaks hydrogen bonds between complementary base pairs.

Question 4

What happens after the hydrogen bonds between DNA strands are broken?

Answer 4

Each strand acts as a template for complementary base pairing.

Question 5

What is the role of free DNA nucleotides in DNA replication?

Answer 5

They are attracted to exposed bases on the template strands by complementary base pairing.

Question 6

What is complementary base pairing?

Answer 6

Adenine pairs with thymine and cytosine pairs with guanine.

Question 7

What enzyme joins adjacent nucleotides in the newly forming DNA strand?

Answer 7

DNA polymerase.

Question 8

How does DNA polymerase join nucleotides together?

Answer 8

DNA polymerase catalyses a condensation reaction forming phosphodiester bonds.

Question 9

What is the result of semi-conservative replication?

Answer 9

Each new DNA molecule consists of one original strand and one newly synthesised strand.

Question 10

Why is DNA replication described as semi-conservative?

Answer 10

Each new DNA molecule retains one original strand and one newly synthesised strand.

Question 11

What was the role of Meselson and Stahl’s experiment?

Answer 11

They provided experimental evidence supporting the semi-conservative model of DNA replication.

Question 12

How did Meselson and Stahl confirm semi-conservative replication?

Answer 12

They used nitrogen isotopes (15N and 14N) and density gradient centrifugation to show that new DNA molecules contained one old and one new strand.

Question 13

Why was Watson and Crick’s model of DNA replication initially doubted?

Answer 13

Other models such as conservative and dispersive replication had not yet been disproven.

Question 14

What scientific method did Watson and Crick use to develop their model?

Answer 14

They used X-ray diffraction images and model building based on Chargaff’s base-pairing rules.

Question 15

What is the significance of the 3’ and 5’ ends of a DNA strand?

Answer 15

DNA strands are antiparallel meaning one runs 5’ to 3’ while the other runs 3’ to 5’.

Question 16

Why does DNA polymerase only add nucleotides to the 3’ end?

Answer 16

DNA polymerase can only work in the 5’ to 3’ direction.

Question 17

How does DNA polymerase synthesise the leading strand?

Answer 17

It continuously adds nucleotides in the 5’ to 3’ direction towards the replication fork.

Question 18

How does DNA polymerase synthesise the lagging strand?

Answer 18

It synthesises short Okazaki fragments discontinuously in the 5’ to 3’ direction away from the replication fork.

Question 19

What enzyme joins Okazaki fragments together?

Answer 19

DNA ligase.

Question 20

Why is DNA ligase needed in DNA replication?

Answer 20

It forms phosphodiester bonds between Okazaki fragments on the lagging strand.

Question 21

Describe the structure of DNA.

Answer 21

1. Polymer of nucleotides;

Accept ‘Polynucleotide’
Accept for ‘phosphate’. phosphoric acid

2. Each nucleotide formed from deoxyribose, a phosphate (group) and
   an organic/nitrogenous base;
3. Phosphodiester bonds (between nucleotides);
4. Double helix/2 strands held by hydrogen bonds;
5. (Hydrogen bonds/pairing) between adenine, thymine and
   cytosine, guanine;

Question 22

Describe how a phosphodiester bond is formed between two nucleotides
within a DNA molecule.

Answer 22

1. Condensation (reaction)/loss of water;
2. (Between) phosphate and deoxyribose;
3. (Catalysed by) DNA polymerase;

Reject if DNA polymerase joins AT/GC OR
complementary nucleotides/bases OR forms
hydrogen bonds

Question 23

Name the protein associated with DNA in a chromosome.

Answer 23

Histone;

Question 24

Describe how the separation of strands occurs.

Answer 24

1. DNA helicase;
2. Breaks hydrogen bonds between base pairs/ AT and
   GC/complementary bases

Question 25

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

Answer 25

1. Joins (adjacent DNA) nucleotides; Reject suggestions that it forms hydrogen bonds or joins complementary bases. Reject ‘nucleotide bases’. 2. (Catalyses) condensation (reactions); 3. (Catalyses formation of) phosphodiester bonds (between adjacent nucleotides);

Question 26

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

Answer 26

Watson and Crick

Question 27

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

Answer 27

1. Weak / easily broken hydrogen bonds between bases allow two strands to separate / unzip; may appear in the same feature 2. Two strands, so both can act as templates; may appear in the same feature 3. Complementary base pairing allows accurate replication; Allow description of complementary base pairing and accurate replication.

Question 28

Contrast the structures of ATP and a nucleotide found in DNA to give two differences.

Answer 28

1. ATP has ribose and DNA nucleotide has deoxyribose; 2. ATP has 3 phosphate (groups) and DNA nucleotide has 1 phosphate (group); 3. ATP – base always adenine and in DNA nucleotide base can be different / varies;