DNA Replication - 21/12/23

Question 1

What is DNA Helicase?

Answer 1

An enzyme that breaks hydrogen bonds of DNA double helix and separates it

Question 2

What are the Stages of DNA Replication?
[5]

Answer 2

1. DNA Helicase break hydrogen bonds between complementary bases (unwinds it)
2. Both polynucleotides have exposed bases and act as templates
3. (Free DNA) nucleotides are attracted to exposed bases and hydrogen bonds form between complementary bases and strand of new nucleotides known as template strands
4. The new nucleotide have formed hydrogen bonds with bases of each polynucleotide strand thanks to DNA Polymerase
5. Then phosphodiester bonds form between nucleotides by condensation reaction

Question 3

What is DNA Polymerase?

Answer 3

An enzyme that joins free nucleotides together to form a new DNA strand

Question 4

How does DNA replication start?
[2]

Answer 4

• The DNA helicase breaks hydrogen bonds between bases on the two polynucleotide DNA strands
• Helix unwinds and forms two single strands

Question 5

Individual DNA nucleotides are attracted to exposed bases on polynucleotide strand that’s been unwind by DNA helicase. Where are these nucleotides made?

Answer 5

Nucleus

Question 6

What type of bonds does DNA Helicase break between complementary bases?

Answer 6

Hydrogen bonds

Question 7

What bonds form when DNA polymerase join free nucleotides?

Answer 7

Phosphodiester bonds

Question 8

What is one end of the DNA strand called?
[2]

Answer 8

3’ and 5’
(three prime and five prime)

Question 9

WIthin DNA replication, what ensures genetic continuity between generations of cells?

Answer 9

Semi-conservative replication

Question 10

Who came up with conservative and semi-conservative theories of DNA?

Answer 10

Watson and Crick

Question 11

Just a copy

Explain the Conservative Model
[3]

Answer 11

• Original DNA molecule is intact
• Seperate daughter DNA copy was built up with new molecules of deoxyribose, phosphate and organic bases
• Of two molecules produced, one is NEW material, one is entirely ORIGINAL material

Question 12

Explain the Semi-Conservative Model
[2]

Answer 12

• Original DNA molecule split into two seperate strands, each of which is replicated in mirror image
• One strand NEW, one strand ORIGINAL

Question 13

What did Meselston and Stahl do?

Answer 13

They did experiment to validate DNA is replicated using semi-conservative replication

Question 14

What did Meselson and Stahl’s experiment contain?

Answer 14

Two isotopes of nitrogen:

• Heavy Nitrogen 15N
• Light Nitrogen 14N

Question 14

Explain what Meselson and Stahl did to prove semi-conservative replication
[7]

Answer 14

1. Two samples of bacteria (light nitrogen + heavy nitrogen) are grown for many generations
2. As bacteria reproduced, it took up N to make nucleotides for new DNA. So, nitrogen slowly became part of bacteria’s DNA
3. Sample of DNA taken from each batch of bacteria and spun in centrifuge
4. DNA from heavy nitrogen settled lower down; DNA from light nitrogen settled higher up in centrifuge
5. Both broths are mixed
6. Bacteria left for 1 round of replication, then another DNA sample spun in centrifuge
7. DNA settled out in middle - mixture of heavy and light nitrogen. Therefore, semi-conservative, as it’s half and half

Question 15

If DNA replication was conservative, what would centrifuge look like?

Answer 15

• Original heavy DNA would be at bottom
• New light DNA would be at top

Question 16

How does the DNA sample being in the middle of centrifuge prove its semi-conservative?
[3]

Answer 16

• New bacteria DNA molecules contain one strand of heavy nitrogen (old DNA)
• and one strand of light nitrogen (new DNA)
• So it will be in middle