2: DNA Replication

Question 1

What did Crick and Watson’s model suggest?

Answer 1

a method of copying called semi-conservative replication

Question 2

Outline the process of semi-conservative replication.

Answer 2

• two strands of DNA molecule are separated by breaking h-bonds between their bases
• new polymers of nucleotides are assembled on each of the two single strands

Question 3

Define ‘template strand’.

Answer 3

a strand of DNA on which a new strand is assembled

Question 4

What is the significance of complementary base pairing in DNA replication?

Answer 4

it means that each new strand has the same base sequence as the old strand that was separated from the template strand

replicates = identical

Question 5

What is the result of semi-conservative replication?

Answer 5

both DNA molecules have one new strand and one strand conserved from parent molecule

Question 6

DNA replication is semi-conservative. What two alternative theories were suggested?

Answer 6

1. conservative replication - both strands of parent DNA remain together and another molecule is produced with 2 new strands
2. dispersive replication - every molecule produced by DNA replication has a mixture of old and new sections in both of its strands

Question 7

What did Meselson and Stahl do (re DNA replication)?

Answer 7

published strong evidence for semi-conservative replication

Question 8

Describe Meselson and Stahl’s experiment on DNA replication. (p29)

Answer 8

1. cultured many generations of E.coli bacteria in medium of ^15 N = nitrogen in bases of all bacteria was ^15 N
2. transferred bacteria to less dense ^14 N medium
3. solution of caesium chloride spun in ultracentrifuge at 45,000 revolutions per minute for 24 hours
4. caesium ions are heavy so tend to sink, establishing a gradient (greatest caesium concentration therefore density at bottom)
5. any substance centrifuged with caesium becomes concentrated at its level of density
6. M+S spun samples of DNA collected from bacterial culture at diff times after transfer to ^14 N medium
7. DNA shows up as dark band in UV light
8. after one generation (G1), DNA = intermediate density between ^15 N and ^14 N, as expected with one old and one new strand
9. G2: two equal bands, one still at ^15 N/ ^14 N and one at ^14 N
10. in following generations less dense ^14 N band became stronger and ^15 N/ ^14 N band became weaker.

Question 9

Describe stages of DNA replication.

Answer 9

Stage 1: helicase unwinds the double helix and separates the two strands by breaking hydrogen bonds

Stage 2: DNA polymerase links nucleotides together to form new strands, using pre-existing strands as templates

Stage 3: daughter DNA molecules each rewind into double helix

Question 10

Why are the two daughter DNA molecules in DNA replication identical in base sequence to one another and to the parent molecule?

Answer 10

complementary base pairing:
- adenine will only bond with thymine
- cytosine will only bond with guanine-
= each new strands is complementary to the template strand (on which it was made) and identical to the other template strand