3.1.6 Semi-Conservative Replication

Question 1

Describe the process of DNA replication.

Answer 1

1. DNA Helicase breaks hydrogen bonds between bases, unwinds double helix. 🡪 Two strands which both act as templates.
2. Free floating DNA nucleotides attracted to exposed bases via specific complementary base pairing, hydrogen bonds form (adenine-guanine; guanine-cytosine).
3. DNA polymerase joins adjacent nucleotides on new strand by condensation, forming phosphodiester bonds (🡪 sugar phosphate backbone).

Question 2

What is semi-conservative replication and why is it important?

Answer 2

1. Replication is semi-conservative – each new strand formed contains one original / template strand and one new strand.
2. Ensures genetic continuity between generations of cells.

Question 3

Describe the experiment carried out by Meselson & Stahl and how it was used to provide evidence for semi-conservative replication.

Answer 3

1. Bacteria grown in a nutrient solution containing heavy nitrogen (15N) for several generations.
2. Nitrogen incorporated into bacterial DNA bases.
3. Bacteria then transferred to a nutrient solution containing light nitrogen (14N) and allowed to grow and divide twice.
4. During this process, DNA from different samples of bacteria was extracted, suspended in a solution in separate tubes and spun in a centrifuge.

Question 4

Samples in the experiment:

Answer 4

1. Sample 1. DNA from bacteria grown for several generations in a nutrient solution containing 15N → DNA molecules contain 2 ‘heavy’ strands.
2. Sample 2. DNA from bacteria grown originally in a nutrient solution containing 15N, then transferred for one division to a solution containing 14N → DNA molecules contain 1 original ‘heavy’ and 1 new ‘light strand.
3. Sample 3. DNA from bacteria grown originally in a nutrient solution 15N, then transferred for two divisions to a solution containing 14N → 50% DNA molecules contain 1 original ‘heavy’ and 1 new ‘light’ strand, 50% contain both ‘light’ strands.