DNA replication
AQA
A-Level
Semi-conservative replication ensures genetic continuity between generations of cells. During the S phase of interphase, before nuclear division, the DNA double helix unwinds. Each original strand acts as a template. Free nucleotides bind via complementary base pairing, and DNA polymerase joins them to form a new strand. Consequently, each new DNA molecule consists of one original parental strand and one newly synthesised strand. Because base pairing dictates the sequence, the genetic code is copied exactly. This provides genetic continuity: daughter cells produced by mitosis inherit an identical copy of the parent cell's genome, preserving essential genetic information across cell generations, barring rare mutations. Note that replication itself produces two identical DNA molecules (sister chromatids), not two daughter cells; mitosis then separates them into daughter cells.
9
Objectives
9
Exam Tips
14
Pitfalls
17
Key Terms
15
Mark Points
Subtopics in this area
The semi-conservative replication of DNA ensures genetic continuity between generations of cells.
The process of semi-conservative replication of DNA in terms of: unwinding of the double helix breakage of hydrogen bonds between complementary bases in the polynucleotide strands the role of DNA helicase in unwinding DNA and breaking its hydrogen bonds attraction of new DNA nucleotides to exposed bases on template strands and base pairing the role of DNA polymerase in the condensation reaction that joins adjacent nucleotides.
Students should be able to evaluate the work of scientists in validating the Watson–Crick model of DNA replication.
DNA replication Revision Guide
Learning Objectives
What you need to know and understand
- Explain how semi-conservative replication produces two identical DNA molecules (sister chromatids) with identical base sequences.
- State where in the cell cycle DNA replication occurs and describe the state of a chromosome immediately afterwards.
- Define genetic continuity and explain how DNA replication ensures it between generations of cells.
- Describe semi-conservative replication in sequence, naming both enzymes and the bonds each one breaks or makes.
- Explain why new nucleotides can be added in only one direction, referring to the specificity of DNA polymerase.
- Distinguish replication from transcription by naming three differences in enzymes, nucleotides and templates used.
- Predict the banding pattern expected after one and two replications for each of the conservative, dispersive and semi-conservative models.
- Interpret a density gradient result to say which model it eliminates and why.
- Evaluate the strength of the evidence for semi-conservative replication, referring to repetition and peer review.
Marking Points
Key points examiners look for in your answers
- Define semi-conservative replication as producing DNA molecules with one original template strand and one newly synthesised strand.
- Explain that complementary base pairing ensures the new DNA strand is an exact copy of the original sequence.
- State that DNA replication occurs during the S phase of interphase, before mitosis begins.
- Describe genetic continuity as the inheritance of identical genetic information from parent to daughter cells during mitosis.
- Distinguish that DNA replication produces two identical DNA molecules (sister chromatids), while mitosis separates them into daughter cells.
- one mark for DNA helicase breaking the hydrogen bonds between complementary bases so the strands separate
- one mark for each separated strand acting as a template, with free DNA nucleotides attracted to the exposed complementary bases
- one mark for specific base pairing, adenine with thymine and cytosine with guanine
- one mark for DNA polymerase catalysing the condensation reaction that joins adjacent nucleotides by phosphodiester bonds
- one mark for explaining one-directional synthesis by the specificity of DNA polymerase, whose active site is complementary to only one end of the developing strand
- one mark for growing cells in nitrogen-15 so that all their DNA contains the heavy isotope, then transferring them to nitrogen-14
- one mark for separating DNA according to density, with heavier DNA settling lower in the tube
- one mark for stating the single band of intermediate density after one replication rules out the conservative model, as it shows the DNA contains both heavy and light isotopes
- one mark for using the second generation result, equal amounts of intermediate and light DNA, to rule out the dispersive model
- one mark for stating that repetition by other scientists and peer review are needed before a model is accepted
Examiner Tips
Expert advice for maximising your marks
- 💡Always use the precise phrase 'one original strand and one newly synthesised strand' rather than vague terms like 'half old, half new'.
- 💡Link complementary base pairing directly to the concept of genetic continuity when explaining how exact copies are made.
- 💡When asked about the products of replication, specify DNA molecules or sister chromatids, not daughter cells, to avoid conflating replication with mitosis.
- 💡Name both enzymes and give each one a different job; a six-mark description that mentions only 'enzymes' cannot reach full marks.
- 💡Use 'attracted to exposed bases' and 'complementary base pairing' as separate points, because they are credited separately.
- 💡If the question gives you a diagram of the two strands, use it: say which end each new strand grows from before explaining why.
- 💡Sketch the three predicted band patterns for generation one and generation two before you write; the answer then almost writes itself.
- 💡An 'evaluate' command needs both what the evidence shows and a limitation or a reason for confidence, so include both.
- 💡Refer to the data given in the question by value or by band position rather than describing it in general terms.
Common Mistakes
Pitfalls to avoid in your exam answers
- Describing replication as conservative; correct by specifying that each new double helix contains one original strand and one new strand.
- Placing DNA replication during mitosis; correct by stating it occurs during the S phase of interphase, before chromosomes become visible.
- Confusing chromatids and chromosomes; correct by noting that after replication, one chromosome consists of two identical sister chromatids.
- Stating that semi-conservative replication directly produces two daughter cells; correct by clarifying that replication produces two DNA molecules (sister chromatids), and mitosis then divides the cell to form daughter cells.
- naming RNA polymerase, or RNA nucleotides, which describes transcription and not replication
- saying only one strand acts as a template, which is true of transcription but not of replication
- explaining the five prime to three prime rule by hydrogen bonding or base pairing, which examiners reject, instead of by the specific active site of DNA polymerase
- writing that helicase 'unzips the DNA' without saying that hydrogen bonds between bases are broken
- saying DNA polymerase forms the hydrogen bonds between bases, when it catalyses the condensation reaction that forms phosphodiester bonds
- describing nitrogen-15 as radioactive, when it is a stable heavy isotope and the bands are separated by density — correct this by referring to it as a heavy isotope
- treating the first generation result as proof of the semi-conservative model, when it only eliminates the conservative one — correct this by stating it is consistent with both semi-conservative and dispersive models
- saying the experiment proves the model true, rather than that it supports it and fails to disprove it — correct this by using terms like 'supports' or 'provides evidence for'
- forgetting that nitrogen is in the bases, and explaining the labelling as being in the sugar or the phosphate — correct this by specifying the nitrogenous bases
- evaluating with vague statements such as 'they did lots of repeats' with no reference to the actual data — correct this by referring to specific band positions and generations