1.5.2 DNA replication

Question 1

Why is semi-conservative replication important

Answer 1

• ensures genetic continuity between generations of cells

Question 2

Describe the process of semi-conservative DNA replication

Answer 2

1. DNA helicase breaks hydrogen bonds between complementary bases unwinding the double helix
2. Both strands act as templates
3. Free DNA nucleotides attracted to exposed bases and join by specific complementary base pairing
4. Hydrogen bonds form between adenine-cytosine and guanine-cytosine
5. DNA polymerase joins adjacent nucleotides on new strand by condensation reactions
6. Forming phosphodiester bonds

Question 3

Use your knowledge of enzyme action to suggest why DNA polymerase move in opposite directions along DNA strands

Answer 3

• DNA has anti parallel strands
• so shapes / arrangements of nucleotides on two ends are different
• DNA polymerase is an enzyme with a specific shaped active site
• so can only bind to substrate with complementary shape

Question 4

Two scientists who proposed it

Answer 4

Watson and Crick

Question 5

Describe the work of Meselson and Stahl in validating the Watson-crick model of semi-conservative DNA replication

Answer 5

1. Bacteria grown in medium containing heaving nitrogen (15N) so nitrogen is incorporated into DNA bases
   - DNA extracted & centrifuged - settles near bottom, as all DNA molecules contain 2 heaving strands
2. Bacteria transferred to medium containing light nitrogen and allowed to divide once
   - DNA is extracted and centrifuged - settles in the middle, as all DNA molecules contain 1 original heavy and 1 new light strand
3. Bacteria in light nitrogen is allowed to divide again
   - DNA extracted and centrifuged - half settles in middle, as containing 1x original heavy and 1 new light strand; half settles near top as contains 2 light strands