Meselson and Stahl's Experiment

Question 1

What three principles was the experimental design based on?

Answer 1

1. DNA contains Nitrogen
2. Two isotopes of nitrogen = N15 (heavy) + N14 (light) –> can mark DNA strands for tracking replication
3. Bacteria incorporate nitrogen from their environment into their DNA.

Question 2

Why were 15N and14N used in this experiment?

Answer 2

The heavier 15N makes DNA denser, while 14N results in lighter DNA, allowing differentiation through centrifugation.

Question 3

What was the first step in Meselson and Stahl’s experiment?

Answer 3

Bacteria were grown in a medium containing 15N, so all their DNA incorporated the heavier nitrogen isotope.

Question 4

What happened after transferring bacteria to a 14N medium?

Answer 4

The bacteria replicated their DNA using 14N, resulting in newly synthesized DNA strands containing the lighter isotope.

Question 5

How was DNA extracted and analysed?

Answer 5

DNA was extracted and spun in a centrifuge. The distribution of DNA bands in the test tube was used to determine the replication pattern:
1. Heaver 15N-DNA sank lower
2. Intermediate-density DNA (one 15N strand and one 14N strand) appeared in the middle.
3. Lighter 14N-DNA stayed near the top.

Question 6

What was observed after one round of replication in 14N?

Answer 6

All DNA molecules were intermediate in density, containing one heavy strand (15N) and one light strand (14N), supporting semi-conservative replication.

Question 7

What was observed after two rounds of replication in 14N?

Answer 7

Two bands were seen:
1. One intermediate-density band (15N/14N)
2. One lighter band (14N/14N)

Question 8

How did the results disprove the conservative replication model?

Answer 8

The first round of replication produced only intermediate DNA, not separate heavy and light bands.

Question 9

How did the results disprove the dispersive replication model?

Answer 9

After two rounds, distinct bands (intermediate and light) appeared instead of a single, mixed-density band.

Question 10

How did the results confirm semi-conservative replication?

Answer 10

DNA replication produced one parental (heavy) strand and one newly synthesized (light) strand in each DNA molecule, consistent with semi-conservative replication.

Question 11

Why was the Meselson and Stahl experiment important?

Answer 11

It provided definitive evidence for the semi-conservative model of DNA replication, revolutionizing understanding of molecular biology.