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DNA replicationEdexcel International A Level Biology: Revision notes

Section 1

Semi-conservative replication

Before a cell divides, its DNA must be copied. DNA replication is semi-conservative: each new DNA molecule is made of one original (template) strand and one newly synthesised strand.

This works because of complementary base pairing: A pairs with T and C pairs with G. Each original strand therefore fixes the base sequence of the strand built on it, and the two new molecules are identical to the original.

Key termssemi-conservative replicationtemplate
Exam tip

If asked why the copies are identical, say that complementary base pairing makes each template strand fix the sequence of the new strand.

Section 2

The process of replication

  1. DNA helicase unwinds the double helix and breaks the hydrogen bonds between the base pairs, so the two strands separate and the bases are exposed.
  2. Each original strand acts as a template.
  3. Free DNA nucleotides from the cell pair with the exposed bases by complementary base pairing, held by hydrogen bonds.
  4. DNA polymerase joins the adjacent nucleotides of the new strand together.
  5. Each new molecule rewinds into a double helix containing one old and one new strand.
Key termsDNA helicasefree nucleotides

Section 3

The role of DNA polymerase

DNA polymerase is the enzyme that catalyses the joining of adjacent nucleotides on the new strand. It forms phosphodiester bonds between the phosphate group of one nucleotide and the sugar of the next, in condensation reactions.

The hydrogen bonds between bases only hold each free nucleotide in position; DNA polymerase forms the strong covalent backbone of the new strand.

Key termsDNA polymerase
Common mistake

DNA polymerase does not unwind DNA or break hydrogen bonds; that is DNA helicase. Polymerase builds the new strand.

Section 4

The Meselson and Stahl experiment: method

Meselson and Stahl tested how DNA is copied using bacteria and two nitrogen isotopes.

  1. Bacteria were grown for many generations in a medium containing heavy nitrogen, 15^{15}N, so all their DNA was heavy.
  2. They were transferred to a medium containing only 14^{14}N (light), so all newly made strands were built from 14^{14}N.
  3. Samples were taken after each generation and the DNA was extracted.
  4. The DNA was centrifuged in a density gradient, where each molecule settles at a position set by its density. Heavy DNA lies lower than hybrid DNA, and hybrid DNA lower than light DNA.
Key termsisotopedensity gradient centrifugation

Section 5

Interpreting the results

Generation 0: one heavy band (both strands 15^{15}N).

Generation 1: one intermediate band. Every molecule has one 15^{15}N and one 14^{14}N strand. This rules out conservative replication, which would give one heavy and one light band.

Generation 2: two bands, intermediate and light, in equal amounts. This rules out dispersive replication, which would give one band at a lighter position. It matches semi-conservative replication.

After nn generations there are 2n2^n molecules, and only 2 of them contain 15^{15}N. After three generations that is 2 out of 8 molecules, or 25%.

Key termsconservative replicationdispersive replication
Exam tip

In a prediction question, work out the number of molecules (2n2^n) first, then say how many contain the original strands.

Must know

  • Replication is semi-conservative: one old strand and one new strand in each molecule
  • Helicase unwinds and breaks hydrogen bonds; free nucleotides pair by complementary base pairing
  • DNA polymerase joins adjacent nucleotides with phosphodiester bonds
  • Meselson and Stahl: 15^{15}N then 14^{14}N, centrifuge, bands
  • One intermediate band at generation 1 rules out conservative; intermediate plus light at generation 2 rules out dispersive

That's the notes covered.

Carry on to the next subtopic.

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
See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).