Meselson and Stahl

Question 1

1st step of Meselson and Stahl experimental methods

Answer 1

Initially incubated E. coli in medium containing
“heavy” nitrogen (15N). This would be the only
source of nitrogen for the cell. All of the DNA
would be labeled with 15N.

Question 2

2nd step of Meselson and Stahl experimental methods

Answer 2

the cells were transferred into a medium
containing only “light” nitrogen (14N).

Question 3

3rd step of Meselson and Stahl experimental methods (3 points)

Answer 3

Samples were taken, DNA isolated, and separated on a
CsCl gradient.

Question 4

Meselson and Stahl results: Gen 0

Answer 4

100% N15 band showed up because it was pre-transfer, only N15

Question 5

Meselson and Stahl results: Gen 1

Answer 5

1) After one round of DNA replication, one band that’s hybrid between N14 and N15
2) Disproves conservative model

Question 6

Meselson and Stahl results: Gen 2

Answer 6

1) equal proportion of hybrid band and N14 band
2) Disproves dispersive model because it wouldn’t have 1:1 ratio

Question 7

Meselson and Stahl results: Gen 3

Answer 7

1) Hybrid band decreases in intensity, and N14 band increases in intensity as replication continues
2) Confirms semi-conservative model because hybrid represents smaller portion of total DNA and thus appears fainter, whereas N14 band represents greater portion and thus appears stronger