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2.4.1Topic 2 · Genes and Health

DNA Replication — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

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📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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2.4.1 DNA Replication — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe the semi-conservative replication of DNA, naming the enzymes involved, and explain the evidence for the mechanism.Before a cell divides by mitosis or meiosis, its DNA must be copied so that each daughte…

  • Unwinding and unzipping
  • DNA helicase
  • Base pairing
  • DNA polymerase
  • Hydrogen bonding
  • leading strand

Section 1 of 9Why DNA replicates

Before a cell divides by mitosis or meiosis, its DNA must be copied so that each daughter cell receives a complete set of genetic information.

Replication must be accurate, because errors become mutations.
It takes place during the S phase of interphase in the cell cycle.
Semi-conservative
The process is semi-conservative: each new DNA molecule consists of one original (parental) strand and one newly synthesised strand.

Section 2 of 9Stages of replication

1Unwinding and unzipping.
  • The enzyme DNA helicase breaks the hydrogen bonds between complementary bases at a site called the origin of replication.
  • The two strands separate, forming a replication fork.
  • Each strand acts as a template.
2Base pairing.
  • Free DNA nucleotides in the nucleus (present as triphosphates,
  • which supply the energy for bond formation) line up opposite their complementary bases on each template strand: A with T, C with G.
3Joining.
DNA polymerase catalyses condensation reactions that form phosphodiester bonds between adjacent nucleotides, building the new strand.
  • It can only add nucleotides to the 3' end of a growing strand, so it works in the 5' to 3' direction.
4Hydrogen bonding.
Hydrogen bonds form between the bases of the template and the new strand, and the two coil into a double helix.

Section 3 of 9Leading and lagging strands

The two template strands are antiparallel.

Leading strand
On one template the new strand, the leading strand, is made continuously towards the replication fork.
Lagging strand
On the other template, polymerase must work away from the fork,
  • so the lagging strand is made in short sections called Okazaki fragments, each starting with a short RNA primer.
DNA ligase
The fragments are later joined by the enzyme DNA ligase, which seals gaps between them in the sugar-phosphate backbone.
Although the lagging-strand detail goes beyond the basic requirements, naming helicase, polymerase and ligase is useful.

Section 4 of 9The Meselson-Stahl experiment

Meselson and Stahl provided evidence for semi-conservative replication in 1958. They grew bacteria for many generations in a medium containing heavy nitrogen (15N), so all of their DNA was heavy. They transferred the bacteria to a medium with light nitrogen (14N) and extracted DNA from each generation, then spun it in a centrifuge using a density gradient.

GenerationResult (position in tube)Conclusion
0 (parental)One heavy bandAll DNA contains 15N
1One band, intermediate densityEach molecule has one heavy and one light strand; rules out conservative replication
2One intermediate and one light bandConsistent with semi-conservative; rules out dispersive replication

Conservative replication would have given a heavy band and a light band in generation 1. The observed result supported the semi-conservative model.

Section 5 of 9Accuracy and importance

Complementary base pairing makes replication accurate, and DNA polymerase has proofreading activity that removes incorrectly inserted nucleotides.

Accuracy and importance
Other repair enzymes correct errors afterwards.
Even so, errors occur at a very low rate, producing mutations.
The significance of semi-conservative replication is that genetic information is passed on faithfully from cell to cell and from parent to offspring, keeping genetic continuity,
  • while rare errors provide the raw material of variation.

Prokaryotic DNA is circular and replicates from a single origin, while eukaryotic chromosomes have many origins so that replication of a long chromosome is completed in the time available.

Section 6 of 9Calculations and links

Number calculations are possible.

1
After n rounds of replication of one DNA molecule, there are 2n molecules.
2
Of these, only two contain a strand from the original molecule, so the proportion carrying original DNA is 2 / 2n.
3
After four rounds there are 16 molecules, 2 of which (12.5%) contain an original strand.
4
This links to the cell cycle, mitosis, meiosis and PCR, which copies DNA in a similar way using a heat-stable polymerase.

Common exam traps concern the roles of the energy source and the enzymes.

1
The free nucleotides are activated triphosphates, and the energy released when two phosphates are removed drives formation of the phosphodiester bond,
  • so no separate ATP is needed for each bond.
2
Remember that helicase acts at the start of the process and polymerase after it.
3
If a question asks why replication is described as semi-conservative, state that each daughter molecule keeps one of the original strands,
  • which is conserved, and gains one new strand.
4
Also state when replication occurs, in the S phase before mitosis,
  • and why it is needed: so that each daughter cell has an identical copy of the genome.
5
Finally, link accuracy to proofreading, because DNA polymerase checks each added base.

Section 7 of 9Exam-style questions

  1. Describe how DNA is replicated semi-conservatively. [5]5 marks

  2. In the Meselson-Stahl experiment, explain why the DNA of the first generation grown in 14N formed a single band of intermediate density. [3]3 marks

  3. A DNA molecule replicates five times. Calculate the total number of molecules and the number containing an original strand. [2]2 marks

Section 8 of 9Key terms

TermMeaning
Semi-conservative replicationEach new DNA molecule has one original and one new strand.
DNA helicaseEnzyme that breaks hydrogen bonds between bases to unwind DNA.
DNA polymeraseEnzyme that joins nucleotides to form a new strand.
DNA ligaseEnzyme that joins DNA fragments by phosphodiester bonds.
Template strandA strand of DNA used as a pattern for a new strand.
Okazaki fragmentA short section of the lagging strand.
Replication forkThe Y-shaped region where DNA strands separate.

Section 9 of 9Exam technique

1
Confusing helicase with polymerase. Helicase unzips; polymerase joins nucleotides.
2
Stating that hydrogen bonds join the nucleotides in the new strand. Phosphodiester bonds do.
3
Describing replication as conservative. Each molecule keeps one original strand.
4
Forgetting to mention that both strands act as templates.

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