DNA structure and replicationEdexcel A-Level Biology B: Subtopic test

1. 1
   A scientist extracts double-stranded DNA from a bacterium and analyses its base composition. She finds that 28% of the bases are adenine.

   (a)

   What percentage of the bases in this DNA is guanine?

   [1 mark]

   • A22%
   • B28%
   • C44%
   • D56%

   (b)

   Which type of bond joins adjacent nucleotides in the same strand of the DNA?

   [1 mark]

   • AHydrogen bond
   • BGlycosidic bond
   • CPhosphodiester bond
   • DPeptide bond

   (c)

   Explain why the percentage of adenine in double-stranded DNA is equal to the percentage of thymine.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Bacteria were grown for many generations in a medium containing the heavy isotope ¹⁵N, so all the nitrogen in their DNA was ¹⁵N. They were then transferred to a medium containing only the lighter isotope ¹⁴N and allowed to divide. DNA from each generation was extracted and separated by density to show whether each molecule was heavy (both strands ¹⁵N), intermediate (one strand ¹⁵N and one ¹⁴N) or light (both strands ¹⁴N).

   (a)

   After one division in the ¹⁴N medium, the DNA of all the bacteria is of the same density. If DNA replication is semi-conservative, which density is it?

   [1 mark]

   • AHeavy
   • BIntermediate
   • CLight
   • DHalf the DNA is heavy and half is light

   (b)

   After a second division in the ¹⁴N medium, what fraction of the DNA molecules contains any ¹⁵N?

   [1 mark]

   • AAll of them
   • BNone of them
   • COne quarter
   • DOne half

   (c)

   Explain why the result after one division in the ¹⁴N medium supports semi-conservative replication and not conservative replication.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A cell is preparing to divide. Before division, every DNA molecule in the nucleus must be copied so that each daughter cell receives a complete set of genetic information.

   (a)

   Describe the roles of DNA helicase, DNA polymerase and DNA ligase in DNA replication.

   [3 marks]

   (b)

   Explain how semi-conservative replication ensures that the two daughter cells receive identical DNA.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student is building a model of a DNA molecule to show how its structure allows it to carry out two functions: storing genetic information and being copied accurately before cell division.

   (a)

   Describe the structure of DNA that the student's model must show, including the structure of a nucleotide.

   [6 marks]

   (b)

   Explain how the structure of DNA makes it suited to storing genetic information and to being replicated accurately. In your answer, state what is meant by a gene.

   [6 marks]

   Total for question 4: 12 marks

End of questions