1.1. 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). 

The bacteria started incorporating the nitrogen isotopes from the culture media into their DNA. As the bacterial colonies grew, the DNA of bacteria grown in a 15N culture became denser than bacteria grown in a normal (14N) culture.

Some bacteria were first grown in the 15N culture and then transferred to the 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 2.1 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

C Bacteria grown in a ¹⁴N culture

Figure 2.1

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. 

Suggest where a band would appear on this tube (B) in comparison to tubes A and C. 

1.2. Justify your answer to part (a). 

1.3. Explain how Meselson and Stahl’s experiment demonstrated the semi-conservative replication of DNA, concerning the results shown in Figure 2.1.