What you need to know
- 2.10(i)DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. replication, including DNA polymeraseThe enzyme that joins free DNA nucleotides into a new strand, forming phosphodiester bonds, during replication..
- 2.10(ii)Meselson and Stahl’s evidence for semi-conservativeDNA replication in which each new DNA molecule has one original strand and one newly made strand. replication.
Detailed primer chemistry and leading/lagging-strand mechanisms are beyond the required explanation here. The density predictions assume complete, synchronised replication and the stated nitrogen source.
By the end, you should be able to…
- Explain how each DNA strand acts as a template.
- Distinguish hydrogen-bond separation from backbone synthesis.
- Predict density bands for three replication models.
Build your understanding
Each original strand becomes a template
Before cell division, DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. is copied. The double helixThe shape of DNA: two antiparallel polynucleotide strands joined by base pairs and twisted into a spiral. unwinds and hydrogen bonding between complementary bases is disrupted, separating the strands. Each original strand acts as a template: exposed bases determine which free DNA nucleotideThe monomer of nucleic acids: a pentose sugar, a phosphate group and a nitrogenous base. can pair with them.
Each original strand becomes a template (continued)
A pairs with T and C with G. DNA polymeraseThe enzyme that joins free DNA nucleotides into a new strand, forming phosphodiester bonds, during replication. catalyses joining adjacent nucleotides into the new strand by forming phosphodiester bondThe bond that links nucleotides: a phosphate group joins the sugar of one nucleotide to the sugar of the next. in its sugar–phosphate backbone. Complementary pairing specifies the sequence; polymerase makes the covalent links. The new strand is complementary to its template, rather than an identical sequence read in the same direction.
Why “semi-conservative”?
Each resulting DNA double helix contains one original strand and one newly synthesised strand. Half of each daughter molecule is conserved from the original molecule. The two daughter molecules normally carry the same base-sequence information as the parent, unless an error occurs.
Conservative replication would keep both original strands together and make a wholly new double helix. Dispersive replication would mix old and new DNA in segments throughout both strands. These alternative models make different predictions that can be tested.
Label DNA by density
Meselson and Stahl grew bacteria with the stable isotope ¹⁵N as their nitrogen source, so their DNA became relatively dense. They transferred the bacteria to a medium containing lighter ¹⁴N. New DNA then incorporated ¹⁴N. The labels distinguish old and new material; they do not make DNA radioactive.
DNA extracted after successive generations was separated by density-gradient centrifugation. Denser DNA forms a band lower in the gradient than lighter DNA. An intermediate-density band represents DNA with an intermediate overall isotope composition; it is not simply a mixture of separate heavy and light molecules.
Compare predictions with observations
After one generation, the DNA formed one intermediate band. semi-conservativeDNA replication in which each new DNA molecule has one original strand and one newly made strand. replication predicts hybridA mix of two different sources: the offspring of two different species (usually infertile), or a DNA molecule with one strand from each source. molecules with one heavy and one light strand. Conservative replication predicts separate heavy and light bands, so it is inconsistent with this observation. Dispersive replication also predicts an intermediate band after one generation: that observation alone does not distinguish it from semi-conservative replication.
After two generations, both intermediate and light bands were present. A hybrid molecule produces one hybrid and one fully light molecule when copied in ¹⁴N medium. Dispersive replication instead predicts one band moving progressively towards the light position. The two-band result therefore supports semi-conservative replication over the simple dispersive model.
With further generations in ¹⁴N, the hybrid fraction falls while the light fraction rises. In an ideal synchronised model the fractions are 1, 1/2 and 1/4 hybrid after generations one, two and three. No fully heavy molecules remain after the first round.
Diagram A
NOT: DNA polymerase does not join the two parent strands to each other.
- Hydrogen bonds break; phosphodiester bonds form in the new strand
- The new strand is complementary, not a copy of the same letters
- Conservative would keep both old strands together
- Dispersive would mix old and new DNA along both strands
Two traps this diagram blocks
- Do not say one daughter molecule is entirely old and the other entirely new.
- Do not say polymerase joins the parent strands back together.
From the diagram alone
Question
Explain why DNA replication is described as semi-conservative. [3]
Show a model answer
The two strands of the parent molecule separate and each acts as a template. DNA polymerase builds a new complementary strand on each one. Each daughter double helix therefore contains one original strand and one new strand, so half of the parent molecule is conserved in each.
Diagram B
NOT: 15N is a density label, not a radioactive label.
- Generation 1 alone also fits dispersive replication
- Generation 2 separates semi-conservative from simple dispersive
- One band can contain many molecules of the same density
- Ideal fractions of hybrid DNA: 1, then 1/2, then 1/4
Two traps this diagram blocks
- Do not claim the first intermediate band by itself disproves dispersive replication.
- Do not treat the number of bands as the number of DNA molecules.
From the diagram alone
Question
Explain why DNA after one generation in light nitrogen cannot distinguish semi-conservative replication from dispersive replication. [3]
Show a model answer
Both models predict DNA of intermediate density after one replication, so both predict one intermediate band. Conservative replication is already unlikely because it predicts two bands. A later generation is needed: semi-conservative replication then produces a separate light band, whereas simple dispersive replication keeps a single band.
| Model | After one generation | After two generations |
|---|---|---|
| Semi-conservative | One intermediate band | Intermediate and light bands |
| Conservative | Heavy and light bands | Heavy and light bands |
| Dispersive | One intermediate band | One band closer to light |
Make your reasoning visible
Give each component its own job
Template bases specify complementary nucleotideThe monomer of nucleic acids: a pentose sugar, a phosphate group and a nitrogenous base.; DNA polymeraseThe enzyme that joins free DNA nucleotides into a new strand, forming phosphodiester bonds, during replication. joins them through phosphodiester bondThe bond that links nucleotides: a phosphate group joins the sugar of one nucleotide to the sugar of the next.. Do not say polymerase joins the two parent strands together.
Use the second generation
A single intermediate band rejects the conservative model but still fits dispersive replication. Explain what the second-generation light band adds.
Do not confuse band number with molecule number
One band can contain many DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. molecules with the same density. Band position indicates density, not the length or number of molecules.
Key explanations
- Free DNA nucleotides pair with exposed bases: adenine with thymine and cytosine with guanine.
Avoid this misconception
The claim: semi-conservativeDNA replication in which each new DNA molecule has one original strand and one newly made strand. means one daughter DNA molecule is old and the other is new.
The correction: Each daughter molecule has one old strand and one new strand.
Heavy and light DNA: how we know DNA copies itself semi-conservatively
When DNA copies itself, what happens to the two old strands? In 1958 Matthew Meselson and Franklin Stahl found out by labelling DNA with a heavy form of nitrogen and tracking where it went. You can repeat their experiment in the model below.
What the model shows
Bacteria were grown on food containing heavy nitrogen (¹⁵N) until all their DNA was heavy. Then they were moved to food with normal, light nitrogen (¹⁴N), so every new DNA strand was light. A centrifuge separates DNA by density: heavy DNA forms a band low in the tube, light DNA a band higher up, and half-heavy, half-light DNA sits in between.
What to look for
Where the bands appear in each tube after each new generation. Before you run each generation, predict the bands, then compare your prediction with the model and with Meselson and Stahl’s real results.
What it tells us
Only semi-conservative copying fits every result: each new molecule keeps one old strand and gains one new one. One middle band after the first generation rules out conservative copying. Two bands, middle and light, after the second generation rule out dispersive copying.
Be careful what you claim
The model follows one starting molecule, and every molecule copies exactly once per generation; the real experiment used billions of bacteria. Generation 1 on its own cannot tell semi-conservative from dispersive copying, so you need generation 2 as well.
Try it in the model: choose the conservative hypothesis, predict generation 1, run it and compare with Meselson and Stahl’s results. Then try dispersive and semi-conservative.
Test three models of DNA replication
Bacteria grown in heavy ¹⁵N are moved to light ¹⁴N. Choose a hypothesis, predict the bands, run one generation at a time and compare with what Meselson and Stahl observed. An idealised model starting from one DNA molecule.
What you see
- DNA molecules as two strands: dark blue strands contain heavy ¹⁵N, pale amber strands contain light ¹⁴N. Dispersive strands are patchwork.
- A centrifuge tube for each generation. Denser DNA forms a band lower in the caesium chloride gradient; band darkness shows the share of DNA.
- Dashed reference lines for heavy (¹⁵N), intermediate (hybrid) and light (¹⁴N) DNA.
How to use it
- Choose a hypothesis, predict the bands, then run one generation at a time (up to four).
- Tick “Show Meselson and Stahl’s results” to compare a prediction with the data. Click a tube to see that generation’s molecules.
- Choose the nitrogen source for the next round to try the “back to ¹⁵N” stretch question.
What it shows
- Why one intermediate band in generation 1 rejects conservative replication.
- Why the intermediate + light bands in generation 2 reject simple dispersive replication.
- The semi-conservative pattern: the hybrid fraction halves each generation while two hybrid molecules remain.
Limits of the model
- Replication is idealised: every molecule copies exactly once per generation, and only newly supplied nitrogen is used.
- One starting molecule stands for the whole culture; the real experiment measured DNA from billions of cells.
- Band positions are spaced for clarity. Densities shown are typical values (about 1.710 and 1.724 g cm⁻³), not readings.
Guided tour · step 1 of 8
- Generation 0 grew in ¹⁵N, so its DNA is all heavy.
- In Run the experiment, just below these steps, set Hypothesis to Conservative (options: Semi-conservative, Conservative, Dispersive).
Each hypothesis makes its own prediction for every generation. Testing them one at a time is how the experiment was argued.
Run the experiment
Pick a prediction first (free explore lets you skip it).
Results
Bands in generation 0: heavy at 1.724 g cm⁻³ (100 %). Click a tube to see another generation.
Hybrid fraction by generation
| Gen | Grown in | Heavy | Hybrid | Light | Hybrid share |
|---|---|---|---|---|---|
| 0 | ¹⁵N | 1 | 0 | 0 | 0 |
Questions
Starting with one fully heavy molecule, how many molecules are expected after three complete rounds in light medium?
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Name the bonds disrupted when the strands separate and the bonds formed as each new strand is assembled.
Show suggested answer
Hydrogen bonding between complementary bases is disrupted. Phosphodiester bonds form along each new sugar–phosphate backbone.
Practice question
Retrieval and explanation · suggested answer
Why does a single intermediate band after one generation not establish semi-conservative replication by itself?
Show suggested answer
Dispersive replication also predicts DNA with intermediate density at this stage. Evidence after another generation distinguishes the models.
Practice question
Retrieval and explanation · suggested answer
Starting with one fully heavy DNA molecule, how many hybrid and fully light molecules are expected after three complete rounds in light medium?
Show suggested answer
There are eight molecules in total: two hybrid and six fully light. Each of the two original heavy strands remains in one hybrid molecule.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A scientist transfers hybrid DNA-producing cells from ¹⁴N medium back to ¹⁵N medium. Starting with one hybrid molecule, predict the products of one complete replication round. Assume only newly supplied nitrogen is incorporated.
Show a hint
Follow the heavy template and light template separately.
Explore the reasoning
The heavy template gains a new heavy strand, producing one fully heavy molecule. The light template gains a new heavy strand, producing one hybrid molecule. Expect heavy and intermediate bands.
Recall this lesson
Lesson recall questions
Question 1 of 5: Name the enzyme that joins complementary nucleotides during DNA replication.
Semi-conservative replication
Practice question
Retrieval and explanation · suggested answer
Name the enzyme that joins complementary nucleotides during DNA replication.
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
An experiment that separates explanations
2B · How DNA WorksMeselson and Stahl’s 1958 paper tested competing replication models by following nitrogen labels through generations. It remains an example of designing an observation that distinguishes explanations. The wider…Specification 2.10(ii)Read & use the scienceClose article
Why this matters
Meselson and Stahl’s 1958 paper tested competing replication models by following nitrogen labels through generations. It remains an example of designing an observation that distinguishes explanations. The wider research lesson is to look for a prediction on which rival models disagree. An observation can be consistent with an explanation without uniquely supporting it over every competitor. Sampling after another generation added information because the predicted distributions diverged. This is a principle students can carry into unfamiliar investigations: identify what each model predicts before deciding whether a result distinguishes them. The original paper is a landmark source; the replication mechanism itself remains part of the required specification.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.10(ii) understand how Meselson and Stahl’s classic experiment provided new data that supported the accepted theory of replication of DNA and refuted competing theories
Use the science
Why would sampling only before transfer and after one generation leave an important uncertainty?
Explore a possible response
The first intermediate band is compatible with both semi-conservative and dispersive models. Later sampling tests where their predictions diverge.
Explore the source
- Meselson and Stahl: original 1958 paperProceedings of the National Academy of Sciences (1958)
Molecular photocopying has a template problem
2B · How DNA WorksPCR lets laboratory scientists make many copies of a selected DNA region from a small starting sample. It uses repeated rounds of strand separation and DNA synthesis, with…Specification 2.10(i) · 2.9(ii)Read & use the scienceClose article
Why this matters
PCR lets laboratory scientists make many copies of a selected DNA region from a small starting sample. It uses repeated rounds of strand separation and DNA synthesis, with short primers helping select the target. This adapts the template principle and DNA polymerase activity you already know. It does not turn an unknown or missing sequence into the correct original, and copying a contaminant can create a misleading signal. The value of amplification therefore depends on sample handling and controls as well as chemistry. PCR’s detailed stages belong later in the course; here the extension is what template-directed copying can and cannot establish.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.10(i) understand the process of DNA replication, including the role of DNA polymerase
- 2.9(ii) know how complementary base pairing and the hydrogen bonding between two complementary strands are involved in the formation of the DNA double helix
Use the science
Why should a reaction containing all the reagents but no intended sample be included when searching for a particular DNA sequence?
Explore a possible response
It can reveal contamination or unintended amplification in reagents or handling. A positive signal is only interpretable against appropriate controls.
Explore the source
- Polymerase Chain Reaction (PCR) Fact SheetNational Human Genome Research Institute