Lesson 22 - Meselson & Stahl experiment

Question 1

what are the 3 models for how DNA is replicated?

Answer 1

conservative
semi-conservative
dispersive

Question 2

What 3 facts did Meselson & Stahl base their work on?

Answer 2

1.) All DNA contains nitrogen - bases
2.) Nitrogen has different isotopes
3.) When bacteria are grown in a medium they will absorb any Nitrogen from the growth medium and incorporate it into any new DNA they make during DNA replication (use it to make new free activated nucleotides)

Question 3

what are the 3 isotopes of nitrogen?

Answer 3

Nitrogen-14 7E, 7P, 7N
Nitrogen-15 7E, 7P, 8N
Nitrogen-16 7E, 7P, 9N

Question 4

what does replication in bacteria look like?

Answer 4

1.) DNA is replicated
2.) Cell elongates
3.) Cell pinches off in the middle
4.) daughter cells separate

Question 5

Stages of the Meselson and Stahl experiment

Answer 5

1) bacteria are grown for several generations in a growth medium containing heavy nitrogen-15
2) the bacteria from stage 1 are transferred to another growth medium containing light nitrogen-14
3) the bacteria from stage 2 are grown in another growth medium containing light nitogen-14

Question 6

How could meselston and stahl tell what Nitrogen isotope the DNA contained when extracted from the bacteria?

Answer 6

The extracted DNA is spun in a centrifuge in a solution of cesium chloride. The DNA molecules move in the test tube based on their density/ mass. Molecules with a greater mass/density will settle further towards the bottom of the test tube and visa versa
The results from the centrifugation are then analysed

Question 7

what will the extracted dna look like in the tubes after centrifugation?
generation 0
generation 1
generation 2

Answer 7

generation 0 = both strands are heavy, so the band is at the bottom of the test tube
generation 1 = 1 heavy and 1 light strand of DNA, so the band is in the middle of the test tube
generation 2 = 1 light and 1 mixed DNA molecule, so there are 2 bands, 1 at the top and 1 in the middle