DNA Replication

Question 1

What is semi-conservative replication? (1)

Answer 1

When half of each new double helix contains one strand from the original parent DNA molecule

Question 2

What are the 4 requirements for semi-conservative replication? (4)

Answer 2

• 4 types of nucleotides (with the 4 bases: ATCG)
• 2 polynucleotide strands to become template strands
• The enzyme DNA polymerase
• A source of chemical energy to drive the process

Question 3

Explain the process of semi-conservative replication with KEYTERMS (7)

Answer 3

• The enzyme DNA HELICASE UNWINDS the double-helix structure
• by BREAKING THE HYDROGEN BONDS between the bases of the two strands
• and each strand becomes a TEMPLATE STRAND
• for FREE DNA NUCLEOTIDES to bind to via condensation reaction
• using COMPLEMENTARY BASE PAIRING to attach to the correct base
• The reaction is catalysed by the enzyme DNA POLYMERASE
• so HYDROGEN BONDS FORM between the bases
• Each new dna molecule contains one strand from the original dna molecule and one new strand

Question 4

Each DNA has a directional structure. Why? (2)

Answer 4

Due to the ends of each strand being:
- 3’ (3 prime end) - A hydroxyl group attached to the 3rd carbon
- 5’ (5 prime end) - A sugar attached to the 5th carbon

Question 5

What end of the template is DNA complementary to and what does this mean? (2)

Answer 5

• DNA is complementary to the 3’ end
• so free nucleotides are attached the template strand in the 3’ to 5’ direction

Question 6

If the template strand has a 3’ to 5’ direction, what direction will the new strand built in? (2)

Answer 6

• In a 5’ to 3’ direction
• because the strands are antiparallel

Question 7

Why does the 5’ to 3’ strand take longer to form? (1)

Answer 7

• The DNA polymerase has to detach and reattach in chunks as it works in a 3’ to 5’ direction

Question 8

Which researchers came up with evidence for semi-conservative replication? (1)

Answer 8

Meselson and Stahl

Question 9

What fact did the semi-conservative replication experiment rely on? (1)

Answer 9

The fact that nucleotides contain nitrogen

Question 10

How many isotopes of nitrogen were there and what do they bands for these look like if spun in a centrifuge? (2)

Answer 10

• nitrogen-14 (higher up in the centrifuge because it is lighter)
• nitrogen-15 (lower down in the centrifuge because it is heavier)

Question 11

Explain how the semi-conservative replication experiment was carried out ()

Answer 11

• Scientists took nitrogen-15 and placed it in a growth medium of nitrogen-14 (so theres only nitrogen-14 nucleotides available)
• They let the nitrogen-15 DNA replicate once
• They then spun it in a centrifuge to observe the bands
• They put the results back into the growth medium of nitrogen-14 and allowed it to replicate once more
• They then took it out and spun it in a centrifuge to observe the bands of the second observation

Question 12

Draw the findings of the semi-conservative replication experiment after the first and second replication:
- Draw the bands (2)
- Draw the strands (2)

Answer 12

Check answer in notes

Question 13

How would DNA replicate if it replicated conservatively? (1)

Answer 13

There would only be DNA molecules containing strands of only nitrogen-14 or nitrogen-15 but not both

Question 14

Draw how nitrogen-15 would replicate conservatively twice in a nitrogen-14 growth medium:
- Draw the bands (2)
-Draw the strands (2)

Answer 14

Check answer in notes