Chapter 4: 4.2 Meselson-Stahl Experiment

Question 1

In 1953, there were 3 theories as to how DNA might replicate, what were they?

Answer 1

1. Conservative
2. Semi-conservative
3. Dispersive

Question 2

Define:

Conservative Theory of DNA Replication

Answer 2

The entire DNA molecule is copied making a second, completely new DNA molecule

Question 3

Define:

Semi-Conservative Theory of DNA Replication

Answer 3

The two strands of DNA separate, with each strand acting as a template to copy a new strand

Question 4

Define:

Dispersive Theory of DNA Replication

Answer 4

The DNA was cleaved into small fragments, the fragments were copied and then re-attached

Question 5

In the Meselson-Stahl Experiment:

What was the test organism?

Answer 5

Escherichia Coli (E. coli) bacteria

Question 6

In the Meselson-Stahl Experiment:

What was the first part of the experiment?

Answer 6

Grow E. col in “heavy nitrogen (N^15) so that all the DNA in the bacterial was labelled with N^15

Question 7

In the Meselson-Stahl Experiment:

When the DNA from the first part of the experiment was extracted and centrifuged, what happened?

Answer 7

All DNA = “heavy”

Question 8

Define:

Centrifuged

Answer 8

Separated by weight

Question 9

In the Meselson-Stahl Experiment:

What was the second part of the experiment?

Answer 9

Transfering the cells to “light” nitrogen (N^14) and let DNA replicate once

Question 10

In the Meselson-Stahl Experiment:

When the DNA from the second part of the experiment was extracted and centrifuged, what happened?

Answer 10

All DNA = 50% “heavy” / 50% “light”

Question 11

In the Meselson-Stahl Experiment:

What does the results from the second part of the experiment prove?

Answer 11

Rules out the conservative theory
* Would mean that template DNA molecule would be all “heavy” and new DNA would be all “light”

Question 12

In the Meselson-Stahl Experiment:

What was the third part of the experiment?

Answer 12

Let the DNA replicate in “light” nitrogen (N^14) one more time

Question 13

In the Meselson-Stahl Experiment:

When the DNA from the third part of the experiment was centrifuged, what happened?

Answer 13

DNA was either:
* 50% “heavy” / 50% “light”
* OR all “light”

Question 14

In the Meselson-Stahl Experiment:

What does the results from the third part of experiment prove?

Answer 14

Rules out the dispersive theory
* Would mean that the DNA would continue to be 50% “heavy” / 50% “light”

Question 15

What did the Meselson-Stahl Experiment show?

Answer 15

DNA replicates using the semi-conservative mechanism