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6 · Nucleic Acids and Protein Synthesis

Cambridge A Level Biology — Full F-drive source: 6 · Nucleic Acids and Protein Synthesis — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 8 parts Lesson 13 of 13 Unit quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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Lesson summary

6.1 Semi-Conservative DNA Replication: Summary — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain how DNA is copied accurately before cell division. Preparation for mitosis — replication occurs so that a parent cell can divide to produce two genetically identical daughter cells.

  • Chromosome number
  • Active site
  • Leading strand
  • Lagging strand
  • DNA ligase
  • Experiment

Part 1 of 6Why DNA replicates

Preparation for mitosis
  • Preparation for mitosis replication occurs so that a parent cell can divide to produce two genetically identical daughter cells.
Chromosome number
  • Chromosome number each daughter needs the same number of chromosomes as the parent, so the number of DNA molecules must be doubled before mitosis.
Semi-conservative
  • Semi-conservative half of the original DNA molecule is conserved in each new molecule, because each has one old (template) strand and one new strand.

Part 2 of 6Stages of replication

1
Helicase unwinds the double helix and breaks the hydrogen bonds between bases, exposing the two template strands.
2
Free activated nucleotides (nucleoside triphosphates with two extra phosphates) pair with the exposed complementary bases, A with T and C with G.
3
DNA polymerase catalyses condensation reactions between the deoxyribose and phosphate of adjacent nucleotides, forming the sugar–phosphate backbone.
4
It cleaves off the two extra phosphates and uses the energy released to make the phosphodiester bonds.
5
Hydrogen bonds form between the complementary base pairs of the template and the new strand.

Part 3 of 6Direction, leading and lagging strands

Direction
  • DNA polymerase adds nucleotides only in the 5′ to 3′ direction, so a new strand is built from its 5′ end towards its 3′ end.
Active site
  • Active site the enzyme has a specific active site and attaches to the 3′ end of the growing strand.
Leading strand
  • Leading strand because the template strands are antiparallel, one strand is made continuously towards the replication fork.
Lagging strand
  • Lagging strand the other is made in short Okazaki fragments away from the fork.
DNA ligase
  • DNA ligase it joins the fragments into a continuous strand.

Part 4 of 6Evidence for semi-conservative replication

Experiment
  • Meselson and Stahl grew bacteria in heavy nitrogen (15N) and then transferred them to light nitrogen (14N).
Generation 1
  • Generation 1 all DNA had intermediate density.
Generation 2
  • Generation 2 there were equal amounts of intermediate and light DNA.
Conclusion
  • Conclusion this pattern fits only semi-conservative replication.

Part 5 of 6Complementary strands, bonds and accuracy

New strand
  • New strand it is the complement of the template and runs antiparallel to it.
Hydrogen bonds
  • Hydrogen bonds each A–T pair has two and each C–G pair has three, and helicase breaks them using ATP.
Accuracy
  • Accuracy complementary base pairing lets the template be read accurately.
Proof-reading
  • DNA polymerase corrects occasional errors, and any remaining errors are mutations.

Part 6 of 6More from the specification

Sugar-phosphate backbone
  • The continuous chain of alternating sugars and phosphates in a polynucleotide.
Polynucleotide
  • A chain of many nucleotides.
Pentose
  • A monosaccharide with five carbon atoms.
Carbon
  • Element with four valence electrons that forms the backbone of organic molecules.
Purine
  • A double-ring baseadenine or guanine.
Pyrimidine
  • A single-ring basecytosine, thymine or uracil.

Key terms

TermMeaning
Semi-conservativeEach new DNA molecule has one old and one new strand.
DNA polymeraseEnzyme that builds new DNA strands.
DNA ligaseEnzyme that joins DNA fragments.
Activated nucleotideNucleoside triphosphate.
Leading strandStrand made continuously.
Lagging strandStrand made in fragments.

Avoid these mistakes

1
Saying the new strand grows 3′ to 5′ — it grows 5′ to 3′.
2
Saying ligase makes hydrogen bonds — it forms phosphodiester bonds between fragments.
3
Forgetting that activated nucleotides provide the energy for bond formation.
4
Describing replication as occurring in mitosis — it occurs in S phase before mitosis.

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