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1.5.6Biological Molecules

The Process of Semi-Conservative Replication — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 35 of 49 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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1.5.6 The Process of Semi-Conservative Replication — AQA A Level Biology 7402 notes: Describe the stages of semi-conservative DNA replication and interpret the Meselson-Stahl evidence.Before a cell divides by mitosis or meiosis, DNA is copied in the S phase of interphase so that …

  • one original (parental) strand and one newly made strand
  • DNA helicase
  • DNA nucleotides
  • DNA polymerase
  • leading strand
  • lagging strand

Part 1 of 6Why DNA replicates

Before a cell divides by mitosis or meiosis, DNA is copied in the S phase of interphase so that each daughter cell receives a full set of genetic information.

Semi-conservative
Replication is semi-conservative: each new DNA molecule consists of one original (parental) strand and one newly made strand.
This helps to keep copying accurate, because each old strand acts as a template.

Part 2 of 6The steps of replication

1DNA helicase
DNA helicase breaks the hydrogen bonds between complementary bases, unwinding the double helix and separating the two strands at the replication fork.
  • The strands are now exposed as templates.
2DNA nucleotides
Free DNA nucleotides in the nucleus are attracted to exposed bases by complementary base pairing (A with T, C with G).
3DNA polymerase
DNA polymerase joins the adjacent nucleotides by condensation reactions, forming phosphodiester bonds.
  • It can only add nucleotides to the 3' end, so the new strand always grows 5' to 3'.
4Leading strand
The leading strand is synthesised continuously, while the lagging strand is made in short sections (Okazaki fragments) that are joined by DNA ligase.
5
Hydrogen bonds form between the bases of each template and its new strand, and each pair of strands recoils into a double helix,
  • giving two identical DNA molecules.

Replication happens at many points along a eukaryotic chromosome at once, speeding up copying. Rare mistakes are corrected by proofreading, but uncorrected errors are mutations.

Part 3 of 6Links

Replication relies on enzyme specificity (helicase, polymerase), base pairing (1.5.2) and phosphodiester bond formation (1.5.1). Cell-cycle timing is covered in 2.3.1, and PCR (a laboratory copy of DNA) uses the same principle at high temperature. Many students lose marks by omitting the role of each enzyme, so learn the job of each one precisely.

Part 4 of 6Details that gain marks

Examiners often look for named enzymes and precise bond language.

Hydrogen bonds
Note the contrast between the two kinds of bond: hydrogen bonds are broken by helicase and re-formed between each template and its new strand,
  • whereas phosphodiester bonds are made by DNA polymerase within the new strand.
Replication needs a supply of all four deoxyribonucleoside triphosphates and ATP.
In eukaryotes, replication is started at many origins at once,
  • producing replication bubbles that join,
  • which is why the long chromosomes are copied within a few hours of S phase.

Accuracy matters because errors become mutations. DNA polymerase checks each new base as it is added and can remove a mismatch (proofreading). Further repair enzymes fix errors afterwards. Despite this, a very small number of errors escape, which is the source of new alleles and so of genetic variation and, occasionally, of diseases such as cancer.

Part 5 of 6Interpreting density-gradient data

When reading Meselson-Stahl style results, draw the tube for each generation: a band's position tells you the density, and its thickness tells you how much DNA is there.

1
After one generation all the DNA lies between heavy and light.
2
After two generations, half is intermediate and half light.
3
After three generations the intermediate band becomes a quarter of the total while the light band is three quarters.
4
In general, the number of hybrid molecules stays at two, so the fraction that is hybrid is 2 divided by 2n for n generations.

Part 6 of 6Exam-style questions

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

  2. In the Meselson-Stahl experiment, state and explain the result after one generation in 14N that rules out conservative replication. [3]3 marks

  3. After three generations in 14N, what proportion of the DNA molecules contain some 15N? [2]2 marks

Key terms

TermMeaning
Semi-conservative replicationEach new DNA molecule contains one original and one new strand.
DNA helicaseEnzyme that unwinds and separates DNA strands by breaking hydrogen bonds.
DNA polymeraseEnzyme that joins nucleotides to form a new strand.
DNA ligaseEnzyme that joins DNA fragments.
Template strandA strand that acts as a pattern for a new strand.
Replication forkThe Y-shaped region where DNA is being unwound and copied.
Okazaki fragmentShort length of DNA made on the lagging strand.

Avoid these mistakes

1
Thinking DNA polymerase unwinds DNA. Helicase does.
2
Saying new strands are made 3' to 5'. Synthesis is 5' to 3'.
3
Describing the semi-conservative result as 'half new DNA molecules and half old'. Each molecule is half old and half new.
4
Forgetting that DNA ligase joins fragments on the lagging strand.
5
Writing that nucleotides join using hydrogen bonds. They are joined by phosphodiester bonds; hydrogen bonds join bases.

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