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2.4Topic 2 · Genes and Health

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

20 min read 27 parts Lesson 25 of 35 2 quizzes

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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Lesson summary

DNA Replication and the Meselson-Stahl Experiment — EDEXCEL A LEVEL BIOLOGY A (SNAB) 9BI0 — REVISION NOTES notes: Describe semi-conservative DNA replication, including leading and lagging strands, and explain the evidence from Meselson and Stahl.Before a parent cell divides it mu…

  • semi-conservative replication
  • DNA polymerase
  • leading strand
  • Worked example 1
  • Worked example 2
  • Worked example 3

Part 1 of 5Why and when DNA replicates

Before a parent cell divides it must copy its DNA, so that both daughter cells receive a full copy of the parental DNA.

1
The number of DNA molecules must be doubled before mitosis takes place.
2S phase
Replication occurs in the S phase of the cell cycle, which is part of interphase, when the cell is not dividing.
3Semi-conservative replication
The copying method is semi-conservative replication (semi = half): each new DNA molecule contains one original (conserved) strand and one newly made strand.

Part 2 of 5Mechanism of replication

The enzyme DNA helicase unwinds the double helix and breaks the hydrogen bonds between complementary base pairs, 'unzipping' the molecule at the replication fork.
  • Each original strand acts as a template.
In the nucleus there are free DNA nucleotides, which contain three phosphate groups.
  • They pair with the exposed bases on each template strand by complementary base pairing.
    • (A with T, C with G)
DNA polymerase
The enzyme DNA polymerase joins the new nucleotides together by condensation reactions,
  • forming phosphodiester bonds in the sugar-phosphate backbone of the new strand (the two extra phosphates are released,
  • which provides energy).
The original strand and its new strand are held together by hydrogen bonds between base pairs, forming a new DNA molecule.
  • Two identical molecules result, each half old and half new.

Part 3 of 5Leading and lagging strands

DNA polymerase can only build the new strand in one direction, the 5' to 3' direction.

Because the template strands are antiparallel and DNA is unzipped from the 3' towards the 5' end, DNA polymerase attaches to the 3' end of the original strand and moves towards the replication fork.
Leading strand
This lets it make the leading strand continuously.
Lagging strand
On the other template strand the polymerase has to work away from the fork,
  • so the lagging strand is made in short sections (fragments) that are later joined together by the enzyme DNA ligase.
The synthesis of the complementary strand therefore differs on the two template strands.

Part 4 of 5Meselson and Stahl: evidence for semi-conservative replication

Scientists were unsure whether replication was conservative (an entirely new molecule made from an intact original) or semi-conservative.

1
Matthew Meselson and Franklin Stahl used isotopes of nitrogen.
2
Bacteria were grown for many generations in a broth containing heavy nitrogen (15N),
  • so that they used it to make new nucleotides and, after some time, all their DNA contained only 15N.
3
DNA extracted from this culture and spun in a centrifuge settled near the bottom of the tube because it was heavy.
4
The bacteria were then transferred to a broth containing only the lighter 14N and left for one round of replication; DNA extracted and spun again settled in a band half-way up the tube, with intermediate density.

Interpretation

  • after one replication, each DNA molecule contained one heavy (15N) strand and one light (14N) strand.
  • Conservative replication would have produced one fully heavy and one fully light band, so it was ruled out.
  • After a second round, two bands appeared (light and intermediate), again as predicted for semi-conservative replication.

Part 5 of 5Worked examples

  1. Describe the role of DNA polymerase in replication. [3]3 marks

  2. In the Meselson-Stahl experiment, explain why the DNA after one generation in 14N formed a single band between the heavy and light positions. [3]3 marks

  3. State why replication occurs in the S phase. [1]1 mark

Key terms

TermMeaning
Semi-conservative replicationEach new DNA molecule keeps one original strand and has one new strand.
DNA polymeraseEnzyme that joins DNA nucleotides to form the new strand.
Replication forkPoint where the DNA is unwinding into two template strands.
Leading strandNew strand made continuously towards the replication fork.
Lagging strandNew strand made in short fragments away from the fork.
IsotopeForm of an element with a different number of neutrons, e.g. 15N.

Avoid these mistakes

1
Saying DNA polymerase unwinds the helix — helicase does that; polymerase joins nucleotides.
2
Saying hydrogen bonds join the new nucleotides in a strand — phosphodiester bonds do; hydrogen bonds join the two strands.
3
Saying replication produces one old molecule and one new — each new molecule is half old, half new.
4
Confusing RNA polymerase (transcription) with DNA polymerase (replication).

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