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2.3.1Unit 2: Cells, Development, Biodiversity and Conservation

DNA Replication — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 5 parts Lesson 17 of 26 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5Semi-Conservative Replication

Before a cell divides, its DNA must be copied so that each daughter cell receives a full set of genes.

Semi-conservative
  • Semi-conservative Each new DNA molecule contains one original strand and one newly made strand, so half of the parent molecule is conserved.
S phase
  • S phase Replication takes place in the S phase of interphase, before the cell divides by mitosis or meiosis.
Helicase
  • Helicase This enzyme unwinds the double helix by breaking the hydrogen bonds between complementary bases, exposing two single template strands.
Free nucleotides
  • Free nucleotides Activated nucleotides in the nucleus pair with the exposed bases by complementary base pairing, A with T and C with G.
DNA polymerase
  • DNA polymerase This enzyme joins adjacent nucleotides by phosphodiester bonds, building the new strand in the 5′ to 3′ direction.
Genetic continuity
  • Genetic continuity Keeping one original strand in every new molecule means daughter cells inherit exactly the same genes as the parent cell.

Part 2 of 5Stages of Replication

1
Helicase unwinds a region of the double helix and breaks hydrogen bonds, so that each separate strand can act as a template.
2
Free DNA nucleotides line up opposite their complementary bases on each template strand, held temporarily by hydrogen bonds.
3
DNA polymerase catalyses the formation of phosphodiester bonds between the nucleotides to make a continuous new strand.
4
The leading strand is made continuously, while the lagging strand is made in short fragments that are later joined together.
5
Each template strand and its new partner then re-form a double helix, giving two identical DNA molecules.

Part 3 of 5Evidence from Meselson and Stahl

Evidence from Meselson and Stahl
Heavy nitrogen
  • Heavy nitrogen Bacteria were grown for many generations in a medium containing ¹⁵N, so all of their DNA was heavy.
Transfer to light medium
  • Transfer to light medium The bacteria were moved to a medium containing the lighter isotope ¹⁴N, so new strands would be made from light nitrogen.
After one generation
  • After one generation A single band of intermediate density was seen,
    • which rules out conservative replication where heavy and light bands would appear.
After two generations
  • After two generations Two bands appeared, one intermediate and one light,
    • as expected if each hybrid molecule separates into a hybrid and a light molecule.
Generation in ¹⁴NHeavy DNAHybrid DNALight DNA
0AllNoneNone
1NoneAllNone
2NoneHalfHalf

Part 4 of 5Worked Example

Question

  • A DNA molecule with both strands labelled with ¹⁵N is allowed to replicate three times in a medium containing only ¹⁴N.
  • State how many DNA molecules result and how many are hybrid (¹⁵N/¹⁴N).

  1. Each round doubles the number of molecules, so after 3 rounds there are 2³ = 8 molecules.
  2. The two original ¹⁵N strands are conserved and never copied again, so they always remain in two separate molecules.
  3. Each of these two molecules also contains one new ¹⁴N strand, so they are hybrids.
  4. The other 8 − 2 = 6 molecules contain only ¹⁴N.

Answer 8 molecules in total: 2 are hybrid and 6 contain only ¹⁴N.

Part 5 of 5Exam Checklist & Tips

2.3.1 – Key Points for the Exam

  • Semi-conservative replication Copying of DNA so that each new molecule has one old strand and one new strand.
  • Helicase The enzyme that unwinds DNA and breaks the hydrogen bonds between bases.
  • DNA polymerase The enzyme that joins nucleotides together to form the new strand.
  • Template strand A single strand of DNA used as a pattern for making a complementary strand.
  • Isotope An atom of the same element with a different number of neutrons, such as ¹⁴N and ¹⁵N.

Common Mistakes

  • Saying that helicase breaks phosphodiester bonds, when it breaks only the hydrogen bonds between bases.
  • Writing that both strands of a new molecule are brand new, which would be conservative replication.
  • Forgetting that DNA polymerase joins nucleotides, but does not unwind the helix.
  • Stating that replication happens in mitosis rather than during the S phase of interphase.

Last updated:

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  • RNA: 5′ → 3′ 3′ → 5′.
  • : AUG UAC .
  • : UAA, UAG, UGA tRNA (Release Factor) .
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