Meselson and Stahl were two scientists who investigated DNA replication during the 1950s.
They grew E. coli bacteria in dishes containing culture media.
- Some bacteria were grown in a culture containing an isotope of nitrogen with a higher molecular weight (15N) than normal nitrogen (14N)
- Some bacteria were first grown in the (15N) culture and then transferred to the normal (14N) culture
- Some bacteria were grown in the normal (14N) culture
The bacteria started incorporating the nitrogen isotopes from the culture media into their DNA. Bacterial colonies that incorporated (15N) culture became denser than bacteria incorporating normal (14N) culture.
After letting the bacteria grow for several generations, the scientists extracted and centrifuged the DNA. During centrifugation, the heavier, denser molecules settled towards the bottom of the tube. Figure 1 shows some of their results.
Key:
A Bacteria grown in a (15N) culture
B Bacteria first grown in a (15N) culture and then transferred to the normal (14N) culture (contents not shown)
C Bacteria grown in a (14N) culture
Fig 1
Some bacteria were first grown in the (15N) culture and then transferred to the normal (14N) culture (tube B). They were left there with enough time to replicate once before the DNA was extracted.
i)
Suggest where a band would appear on this tube (B) in comparison to tubes A and C.
[2]
ii)
Suggest how the appearance of tube B would change if the E. coli bacteria were left in the (14N) culture to reproduce for one additional generation.
[2]