| Date | May 2016 | Marks available | 3 | Reference code | 16M.3.SL.TZ0.2 |
| Level | Standard level | Paper | Paper 3 | Time zone | TZ0 |
| Command term | Explain | Question number | 2 | Adapted from | N/A |
Question
Over 50 years ago, Meselson and Stahl investigated the mechanism of DNA replication. They transferred a rapidly growing population of Escherichia coli from a growth medium containing only 15N to a growth medium with only 14N. DNA samples were centrifuged at high speed in a salt density gradient. In the original published research, DNA molecules of the same density appear as a band in the UV absorption photographs as shown.
The density of the DNA band at generation 0 is 1.724 and the density of the dark band of DNA at generation 4.1 is 1.710. Estimate the density of the DNA band at generation 1.0.
Describe the nitrogen composition of the DNA band in the E. coli at generation 1.0.
Explain the pattern shown in generation 3.0.
This experiment was designed to demonstrate whether replication was semi-conservative or conservative. Distinguish between semi-conservative replication and conservative replication.
Markscheme
1.717
Half 14N and half 15N
OR
one/new strand 14N and one/old strand 15N
OR
half labelled
Must indicate equal quantities eg: 50 % of each or 1 strand of each
«As replication is semi-conservative» each new strand is built on parental/old/template strand (Do not give a mark for “semi-conservative”)
Generation 3 shows DNA that is mostly made of 14N
When E. coli replicates, half of its new DNA must always contain 14N when growing in a 14N growth medium
Every new generation of E. coli always has a smaller proportion of «labelled» 15N in its DNA «than the previous generation»
Each new generation has half the amount of 15N in previous generation
Accept answers in an annotated diagram
Table format not required
Examiners report
Question 2 on the Meselson and Stahl investigation was generally problematic and very poorly answered though a few candidates did score high marks.
Question 2 on the Meselson and Stahl investigation was generally problematic and very poorly answered though a few candidates did score high marks.
The majority of candidates described the shading and densities as opposed to explaining the pattern in terms of the N present in the DNA.
Many candidates scored 2 marks distinguishing between semi-conservative and conservative replication.
