What you'll learn
- How the structure of DNA makes accurate copying possible.
- What semi-conservative replication means.
- The roles of DNA helicase and DNA polymerase.
- How Meselson and Stahl’s evidence supported the Watson–Crick model of DNA replication.
Why DNA needs to be copied
Before a cell divides, it must copy its DNA. This is called DNA replication. The copied DNA can then be passed into new cells, so each daughter cell receives the same genetic information.
DNA replication
DNA replication is the process by which a DNA molecule is copied to produce two DNA molecules with the same base sequence.
Genetic continuity
Genetic continuity means maintaining the same genetic information between generations of cells, so daughter cells inherit the same DNA instructions as the parent cell.
The DNA structure you need first
DNA is a polymer, which means it is made from many smaller repeating units joined together. The small units are called nucleotides.
DNA nucleotide
A DNA nucleotide is made of three parts: a deoxyribose sugar, a phosphate group, and one nitrogen-containing base: adenine, thymine, cytosine, or guanine.
DNA has two polynucleotide strands. A polynucleotide strand is a chain of nucleotides joined by strong covalent bonds between the sugar of one nucleotide and the phosphate of the next. This forms the sugar-phosphate backbone.
The two strands are held together by hydrogen bonds, which are weak attractions between complementary bases.
The base-pairing rules are:
- Adenine (A) pairs with thymine (T).
- Cytosine (C) pairs with guanine (G).
Base pairing makes copying possible
Each DNA strand can act as a template, because the exposed bases determine which new nucleotides must be added.
Predicting a complementary DNA strand
A section of one DNA strand has the sequence:
T A C G G A
What sequence would be added to form the complementary strand?
- Apply the base-pairing rules: A pairs with T, and C pairs with G.
- Match each original base with its complementary base: T needs A, A needs T, C needs G, G needs C, G needs C, and A needs T.
- The complementary sequence is therefore:
A T G C C T
What semi-conservative replication means
DNA replication is described as semi-conservative.
Semi-conservative replication
Semi-conservative replication means that each new DNA molecule contains one original parental strand and one newly synthesised strand.
“Conservative” means “kept”. So in semi-conservative replication, half of each original DNA molecule is conserved in each new molecule.
This matters because the original strands guide the formation of the new strands. As long as complementary base pairing happens correctly, the new DNA molecules have the same base sequence as the original.
The stages of DNA replication
The diagram shows the main stages you need for AQA A-Level Biology: unwinding, hydrogen bond breakage, complementary base pairing, and joining of nucleotides.
1. The double helix unwinds
DNA begins as a double helix. During replication, the two strands must be separated so their bases are exposed.
This is done by DNA helicase.
DNA helicase
DNA helicase is an enzyme that unwinds the DNA double helix and breaks the hydrogen bonds between complementary base pairs.
Once the hydrogen bonds are broken, the two strands separate. Each original strand can now act as a template.
Breaking the wrong bonds
DNA helicase breaks hydrogen bonds between bases, not the strong covalent bonds in the sugar-phosphate backbone.
2. Free nucleotides are attracted to exposed bases
Free DNA nucleotides are present in the nucleus. Once the template bases are exposed, these free nucleotides line up by complementary base pairing.
For example:
- If the template has A, a free T nucleotide lines up.
- If the template has G, a free C nucleotide lines up.
This makes replication accurate because each base only pairs with its complementary partner.
3. DNA polymerase joins the nucleotides
The new nucleotides must be joined together to form a new polynucleotide strand.
DNA polymerase
DNA polymerase is an enzyme that joins adjacent DNA nucleotides together by catalysing condensation reactions.
A condensation reaction is a reaction that joins molecules together and releases water. In DNA replication, condensation reactions form covalent bonds in the sugar-phosphate backbone of the new strand.
New DNA is not made from bases alone
DNA polymerase joins whole DNA nucleotides, not just bases. Each nucleotide includes a sugar, phosphate, and base.
4. Two identical DNA molecules are formed
At the end of replication, there are two DNA molecules. Each one contains:
- one original parental strand
- one newly synthesised complementary strand
This is why the process is semi-conservative.
Why replication gives genetic continuity
Because complementary base pairing preserves the base sequence, semi-conservative replication produces DNA molecules carrying the same genetic information as the original molecule.
Keeping the enzyme roles clear
DNA helicase and DNA polymerase are both essential, but they do different jobs.
- DNA helicase separates the strands by breaking hydrogen bonds.
- DNA polymerase joins new nucleotides together to build the new strands.
Two enzymes, two verbs
Think: helicase separates, polymerase synthesises.
How scientists validated the Watson–Crick model
Watson and Crick proposed the double-helix structure of DNA. Their model suggested a possible copying mechanism: if the two strands separate, each can act as a template for a new complementary strand.
However, scientists needed evidence to test whether DNA replication really was semi-conservative.
There were three possible models:
- Conservative replication: the original DNA molecule stays completely intact, and a completely new DNA molecule is made.
- Semi-conservative replication: each new DNA molecule contains one old strand and one new strand.
- Dispersive replication: old and new DNA are mixed in fragments throughout both strands.
Meselson and Stahl tested these models using nitrogen isotopes.
Isotope
An isotope is a form of an element with the same number of protons but a different number of neutrons. Nitrogen-15 is heavier than nitrogen-14.
They grew bacteria in a medium containing heavy nitrogen, nitrogen-15, so the bacteria made heavy DNA. They then transferred the bacteria to a medium containing light nitrogen, nitrogen-14. As the bacteria replicated, any newly made DNA contained light nitrogen.
They used density-gradient centrifugation to separate DNA by density.
Density-gradient centrifugation
Density-gradient centrifugation is a technique where molecules are spun at high speed and form bands at positions matching their density.
What Meselson and Stahl observed
After one round of replication in light nitrogen, all the DNA formed one intermediate-density band. This showed that the DNA was hybrid: one heavy strand and one light strand.
After two rounds of replication, there were two bands:
- one intermediate-density band
- one light-density band
This matched the prediction for semi-conservative replication.
Interpreting density-gradient results
A sample of bacterial DNA is first grown in nitrogen-15, then transferred to nitrogen-14. After one round of replication, centrifugation shows one intermediate-density band. After two rounds, it shows one intermediate band and one light band. Which replication model is supported?
- Compare the first result with the conservative model. Conservative replication predicts one heavy band and one light band after the first round, so one intermediate band rules this out.
- Compare the second result with the dispersive model. Dispersive replication predicts only intermediate DNA after repeated rounds, not a separate light band, so this rules out dispersive replication.
- The results match semi-conservative replication: after one round all DNA is hybrid, and after two rounds half is hybrid and half is light.
Evaluating the evidence
Meselson and Stahl’s work was strong because the three models made clearly different predictions. Their results did not just “fit” semi-conservative replication; they also ruled out the main alternative models.
This is good scientific validation: a model is tested by making predictions and comparing those predictions with evidence.
However, be careful with wording. The experiment supported the Watson–Crick model of replication very strongly, but science does not usually say one experiment “proves” a model forever. Instead, it provides evidence that supports the model and rejects alternatives.
In the exam
- Use the exact phrase semi-conservative replication and state that each new DNA molecule contains one original strand and one new strand.
- Keep enzyme roles separate: DNA helicase breaks hydrogen bonds, while DNA polymerase joins adjacent nucleotides by condensation reactions.
- For Meselson and Stahl questions, compare the observed bands with the predictions of conservative, semi-conservative, and dispersive replication.
Check yourself
- Why does complementary base pairing allow each original DNA strand to act as a template?
- What bonds are broken by DNA helicase, and what bonds are formed when DNA polymerase joins nucleotides?
- How do the Meselson and Stahl results after two rounds of replication rule out dispersive replication?
