DNA replicationAQA A-Level Biology: Revision notes
Section 1
Why DNA replicates
Before a cell divides, its DNA must be copied so that each daughter cell receives the same genetic information. Semi-conservative replication ensures genetic continuity between generations of cells: each new DNA molecule is made of one original (template) strand and one newly made strand.
Section 2
Unwinding and separating the strands
DNA helicase unwinds the double helix and breaks the hydrogen bonds between the complementary bases. The two polynucleotide strands separate, exposing their bases. Each strand now acts as a template for a new strand.
Helicase breaks hydrogen bonds between the strands. It does not break the phosphodiester bonds in the backbone.
Section 3
Forming the new strands
Free DNA nucleotides are attracted to the exposed bases of each template strand. They line up by complementary base pairing: A with T, and C with G, held by hydrogen bonds. DNA polymerase catalyses condensation reactions that join adjacent nucleotides, forming phosphodiester bonds in the sugar-phosphate backbone of the new strand.
The result is two identical DNA molecules, each with one old and one new strand.
Worked example: template ATTGCCGATC gives new strand TAACGGCTAG, with 5 A-T pairs (10 hydrogen bonds) and 5 C-G pairs (15 hydrogen bonds), so 25 hydrogen bonds in total.
Write the order: helicase, base pairing, polymerase. The new strand contains thymine, never uracil.
Section 4
Testing the models
Three models were possible. Conservative: the parent molecule stays intact and a completely new copy is made. Dispersive: both strands of each daughter are a mixture of old and new DNA. Semi-conservative: each daughter has one old and one new strand, as proposed by Watson and Crick (1953).
Meselson and Stahl (1958) grew bacteria in ¹⁵N medium, then in ¹⁴N medium, and separated DNA by density. After one division all DNA was intermediate, which ruled out the conservative model. After two divisions half was intermediate and half light, which ruled out the dispersive model and fitted semi-conservative replication.
Section 5
Evaluating the evidence
Strengths: the isotope labels let old and new DNA be distinguished; the results were quantitative; each model made a different prediction; bacteria divide quickly, so several generations could be studied; controls (DNA from ¹⁵N only and ¹⁴N only bacteria) allowed comparison.
Limitations: the evidence is indirect (density, not the strands themselves), it was gathered from bacteria only, and it does not reveal the mechanism or enzymes involved.
Overall, the work validated the Watson-Crick model by eliminating the alternatives, and further work in other organisms has been consistent with it.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on DNA replication
- 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
- 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
- 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
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).