DNA replicationAQA A-Level Biology: Mind map

Exam questions on DNA replication

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.
   Explain why enzyme X must act before enzyme Y can join nucleotides on the template strands.2 marks
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.
   Explain how the result after two divisions supports the semi-conservative model but not the dispersive model.2 marks
3. One strand of a DNA molecule that is about to be replicated has the base sequence ATTGCCGATC.
   Explain how the new strand is formed on this template strand.3 marks