DNA replicationAQA A-Level Biology: Subtopic test

1. 1
   Before a cell divides, its DNA is replicated. Two enzymes are involved. Enzyme X separates the two polynucleotide strands of the double helix. Enzyme Y catalyses the joining of nucleotides on each separated strand.

   (a)

   Which row correctly identifies enzymes X and Y?

   [1 mark]

   • AX is DNA helicase and Y is DNA polymerase
   • BX is DNA polymerase and Y is DNA helicase
   • CX is RNA polymerase and Y is DNA helicase
   • DX is ATP synthase and Y is DNA polymerase

   (b)

   Which statement describes how new nucleotides are positioned on a separated template strand?

   [1 mark]

   • AThey attach in a random order, and the order is corrected later
   • BThey form covalent bonds with the bases on the template strand
   • CFree nucleotides with complementary bases hydrogen bond to the exposed bases on the template
   • DFree nucleotides bond to the phosphate groups of the template strand

   (c)

   Explain why enzyme X must act before enzyme Y can join nucleotides on the template strands.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   E. coli bacteria were grown for many generations in a medium containing the heavy isotope ¹⁵N, so that all their DNA contained ¹⁵N. They were then transferred to a medium containing only the lighter isotope ¹⁴N and allowed to divide. DNA was extracted and separated by density using centrifugation. After one division all the DNA had a single density, between that of ¹⁵N DNA and ¹⁴N DNA. After two divisions half the DNA had this intermediate density and half had the density of ¹⁴N DNA.

   (a)

   Which conclusion can be drawn from the result after one division alone?

   [1 mark]

   • AReplication must be semi-conservative, because no other model is possible
   • BThe conservative model is not correct, as no DNA had the density of ¹⁵N DNA alone
   • CThe dispersive model is correct, because the DNA had an intermediate density
   • DDNA does not replicate in bacteria in ¹⁴N medium

   (b)

   After a third division in ¹⁴N medium, what proportion of the DNA molecules would have the density of ¹⁴N DNA, according to the semi-conservative model?

   [1 mark]

   • ANone
   • B50 %
   • C25 %
   • D75 %

   (c)

   Explain how the result after two divisions supports the semi-conservative model but not the dispersive model.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   One strand of a DNA molecule that is about to be replicated has the base sequence ATTGCCGATC.

   (a)

   Explain how the new strand is formed on this template strand.

   [3 marks]

   (b)

   Give the base sequence of the new strand, calculate the number of hydrogen bonds between the template and the new strand, and explain why semi-conservative replication ensures genetic continuity.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In 1953 Watson and Crick suggested that DNA replicates semi-conservatively. Two other models were possible: a conservative model, in which the parent molecule stays intact and a completely new copy is made, and a dispersive model, in which both strands of each daughter molecule are a mixture of old and new DNA. In 1958 Meselson and Stahl tested the three models by growing bacteria in media containing the isotopes ¹⁵N and ¹⁴N, extracting the DNA after each division and separating it by density. Their results matched only the semi-conservative model.

   (a)

   Describe how DNA is replicated, referring to the enzymes involved.

   [6 marks]

   (b)

   Evaluate the work of Meselson and Stahl in validating the Watson-Crick model of DNA replication.

   [6 marks]

   Total for question 4: 12 marks

End of questions