DNA replicationEdexcel International A Level Biology: Mind map

Exam questions on DNA replication

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.
   Describe the role of DNA polymerase in this test tube.2 marks
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.
   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
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.
   Explain how the two bands seen after two generations rule out dispersive replication and support semi-conservative replication.3 marks