Building New DNA Strands Revision Notes | A Level AQA Biology

1. Why Building New DNA Strands Matters

Every time a cell divides, it must pass on an exact copy of its genetic information. One copying error in billions of base pairs could alter a protein, disrupt a cellular process, or contribute to disease. The mechanism that prevents this is precise, elegant, and worth understanding at the molecular level.

This lesson focuses on two tightly linked events: how free nucleotides find their correct positions on a template strand, and how DNA polymerase locks them permanently into a new strand. These two spec requirements are not separate topics — they are consecutive steps in the same molecular story.

2. Setting the Scene: The Template Strand

Picture the double helix after the enzyme helicase has unwound and separated the two strands. Each strand now has its bases exposed — adenine, thymine, guanine, and cytosine pointing outward into the surrounding nucleoplasm. These exposed bases are the template: a molecular instruction set for building a new complementary strand.

The nucleoplasm contains a pool of free deoxyribonucleotide triphosphates — individual nucleotides, each carrying three phosphate groups (a triphosphate), a deoxyribose sugar, and one of the four bases. These free nucleotides are the raw materials. The template strand determines which ones get used and in what order.

With the template exposed and free nucleotides available, the question becomes: how does each nucleotide end up in exactly the right position? The answer lies in the chemistry of the bases themselves.

3. Attraction of Nucleotides: Complementary Base Pairing

Free nucleotides in the nucleoplasm move around by diffusion. When a free nucleotide drifts close to the exposed template strand, something specific happens: if its base is complementary to the exposed template base, hydrogen bonds form between them. This is the attraction that draws each nucleotide into position.

The pairing rules are fixed by molecular geometry. Adenine on the template attracts a free nucleotide carrying thymine; guanine on the template attracts a free nucleotide carrying cytosine — and vice versa. A nucleotide with the wrong base cannot form stable hydrogen bonds and drifts away. This selectivity is what makes the process accurate.

Notice that G-C pairs form three hydrogen bonds while A-T pairs form only two. This means G-C base pairs are slightly more stable. Regions of DNA with a higher proportion of G-C pairs require more energy to separate — a detail that can appear in exam questions about the stability of different DNA sequences.

At this stage, the free nucleotides are held in place by hydrogen bonds only. They are correctly positioned but not yet permanently joined to each other. That requires the next step — and a specific enzyme.

4. DNA Polymerase and the Condensation Reaction

Once free nucleotides are aligned along the template by hydrogen bonding, DNA polymerase moves along and catalyses the reaction that joins them into a continuous strand. This is a condensation reaction: a covalent phosphodiester bond forms between the phosphate group of the incoming nucleotide and the deoxyribose sugar of the nucleotide already in the chain, with a molecule of water released as a by-product.

Each condensation reaction adds one nucleotide to the growing chain. DNA polymerase works progressively along the template, catalysing bond after bond, building the new strand nucleotide by nucleotide. The result is a sugar-phosphate backbone held together by strong covalent phosphodiester bonds, with the bases projecting inward to hydrogen-bond with the template.

It is worth being precise about what DNA polymerase does and does not do. It catalyses the formation of the phosphodiester bond between adjacent nucleotides — it does not position the nucleotides (that is done by base pairing) and it does not break the hydrogen bonds holding the strands together (that is helicase). Each enzyme has a distinct, non-overlapping role.

Worked example: Tracing the Sequence: From Template Base to Phosphodiester Bond

Describe, in order, the molecular events that result in a new nucleotide being permanently added to a growing DNA strand.

1. Helicase unwinds the double helix; hydrogen bonds between the two strands break, exposing the bases on each template strand. — This is a prerequisite step — without exposed bases, there is nothing for incoming nucleotides to align against.
2. A free deoxyribonucleotide triphosphate from the nucleoplasm moves by diffusion towards the exposed template strand. — The nucleoplasm contains all four types of free nucleotide. Only the one with the complementary base will remain in position.
3. If the base on the free nucleotide is complementary to the template base (A-T or G-C), hydrogen bonds form and the nucleotide is held in the correct position. — This is the attraction step specified in the AQA spec. Hydrogen bonding provides specificity — wrong-base nucleotides cannot form stable bonds and drift away.
4. DNA polymerase catalyses a condensation reaction between the phosphate group of the newly aligned nucleotide and the deoxyribose sugar of the previous nucleotide in the chain. — A phosphodiester bond forms and a water molecule is released. This is the covalent bond that permanently incorporates the nucleotide into the new strand.
5. DNA polymerase moves along the template in the 5' to 3' direction, repeating steps 2-4 for each subsequent nucleotide position. — The new strand grows progressively. The hydrogen bonds to the template remain intact, holding the new strand in register with the original.

Answer: Exposed template base → complementary free nucleotide attracted by hydrogen bonding → DNA polymerase catalyses condensation reaction → phosphodiester bond forms, water released → nucleotide permanently incorporated into new strand.

5. Connecting the Two Mechanisms

Base pairing and the condensation reaction are sequential, not independent. Base pairing positions each nucleotide correctly; the condensation reaction then locks it in place. Remove either step and the process fails: without base pairing, nucleotides would join in a random, non-complementary sequence; without DNA polymerase, correctly positioned nucleotides would remain loosely held by hydrogen bonds and never form a stable strand.

This sequential dependency is exactly what examiners test. A question asking you to 'explain how a new DNA strand is built' requires both mechanisms in the correct order, with the correct bond types named at each stage.

6. Visualising the Process

A useful mental model: picture the template strand as a strip of Velcro with a specific pattern. Free nucleotides are small pieces that can only stick if their pattern matches. Once stuck (hydrogen bonded), a molecular stapler — DNA polymerase — comes along and permanently rivets each piece to the next with a covalent bond. The Velcro holds things in place temporarily; the rivet makes it permanent.

The diagram below represents the key molecular relationships. The template strand runs from top to bottom. Free nucleotides align opposite their complementary template bases. DNA polymerase travels along, catalysing each condensation reaction in turn, building the new strand from the 5' end to the 3' end, while reading the template strand in the 3' to 5' direction.

The chart reinforces a key conceptual point: hydrogen bonds are individually weak (though collectively significant), while phosphodiester bonds are strong covalent bonds. This is why the new strand, once synthesised, is stable — it is held together by covalent chemistry, not just the temporary attraction of base pairing.

7. Exam Technique and Common Mistakes

Questions on this topic tend to use the command words 'describe' or 'explain'. A 'describe' answer needs the sequence of events stated clearly; an 'explain' answer needs the molecular reasons — why hydrogen bonds form, why DNA polymerase is needed, why a condensation reaction produces a phosphodiester bond. Practise switching between these modes.

Mark schemes for extended-answer questions on DNA replication almost always award separate marks for: (1) naming complementary base pairing as the mechanism of nucleotide attraction, (2) naming DNA polymerase as the enzyme involved, and (3) naming the condensation reaction and/or phosphodiester bond. All three must appear in your answer to secure full marks.

Together, complementary base pairing and the DNA polymerase-catalysed condensation reaction ensure that every new DNA strand is an accurate, covalently bonded copy of the original template — the molecular foundation of genetic inheritance.