DNA structure and replicationEdexcel A-Level Biology B: Mind map

Exam questions on DNA structure and replication

1. A scientist extracts double-stranded DNA from a bacterium and analyses its base composition. She finds that 28% of the bases are adenine.
   Explain why the percentage of adenine in double-stranded DNA is equal to the percentage of thymine.2 marks
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).
   Explain why the result after one division in the ¹⁴N medium supports semi-conservative replication and not conservative replication.2 marks
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.
   Describe the roles of DNA helicase, DNA polymerase and DNA ligase in DNA replication.3 marks