DNA replicationEdexcel International A Level Biology: Subtopic test

1. 1
   A researcher sets up a test tube containing a single-stranded DNA template with the base sequence ATTGCC, a supply of free DNA nucleotides of all four types, DNA polymerase and suitable conditions.

   (a)

   Which base sequence is synthesised on the template?

   [1 mark]

   • AUAACGG
   • BATTGCC
   • CTAACCG
   • DTAACGG

   (b)

   What holds each free nucleotide in position on the template before it is joined to the growing strand?

   [1 mark]

   • APhosphodiester bonds
   • BHydrogen bonds between complementary bases
   • CPeptide bonds
   • DIonic bonds between phosphate groups

   (c)

   Describe the role of DNA polymerase in this test tube.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Bacteria were grown for many generations in a medium containing only the heavy isotope 15N, so that all of their DNA contained 15N. They were then transferred to a medium containing only 14N and allowed to divide once every 20 minutes. DNA was extracted from samples taken at intervals and centrifuged in a density gradient, where each DNA molecule settles at a position that depends on its density.

   (a)

   If replication is semi-conservative, which bands would be seen in the sample taken after 40 minutes (two generations)?

   [1 mark]

   • AAn intermediate band and a light band, in equal amounts
   • BA heavy band and a light band
   • CA single intermediate band
   • DA single light band

   (b)

   If replication is semi-conservative, what percentage of the DNA molecules would contain any 15N after 60 minutes (three generations)?

   [1 mark]

   • A50%
   • B12.5%
   • C75%
   • D25%

   (c)

   After 20 minutes the DNA formed a single band at the intermediate position. Explain why this result is not expected if replication is conservative.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student is reading about the experiment of Meselson and Stahl and wants to understand how it was designed. In the experiment, bacteria grown in 15N were transferred to 14N. After two generations in 14N the extracted DNA formed two bands in equal amounts, one at the intermediate position and one at the light position.

   (a)

   Explain how the two bands seen after two generations rule out dispersive replication and support semi-conservative replication.

   [3 marks]

   (b)

   Suggest why the bacteria were grown for many generations in 15N before being transferred, and why they were then transferred to 14N rather than kept in 15N.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A culture of Escherichia coli has been grown for many generations in a medium containing only 15N and is then transferred to a medium containing only 14N. Each cell replicates its DNA once before each division and the culture is left to divide for four generations.

   (a)

   Describe how DNA is replicated in one of these bacterial cells before it divides.

   [6 marks]

   (b)

   Predict the proportions of heavy, intermediate and light DNA molecules after four generations in 14N if replication is semi-conservative, conservative or dispersive. Explain how the result would show which is correct.

   [6 marks]

   Total for question 4: 12 marks

End of questions