Assembling and Amplifying DNA Revision Notes | A Level AQA Biology

1. Why Assembling and Amplifying DNA Matters

Imagine you have identified the gene responsible for producing human insulin. That gene is one tiny sequence among three billion base pairs. To study it, modify it, or use it to manufacture a protein, you first need to isolate it, build it into a working construct, and then copy it millions of times. That is precisely what this topic covers.

Gene technologies — cutting, assembling, and amplifying DNA — underpin everything from insulin production to forensic profiling to cancer research. The AQA specification requires you to understand both how DNA fragments are assembled into functional constructs and how those fragments are amplified, either inside living cells (in vivo) or in a test tube (in vitro).

2. Building a Functional DNA Construct: Promoters and Terminators

A raw coding sequence on its own is biologically silent. For a host cell to transcribe and translate a gene of interest, that gene must be flanked by the correct regulatory signals. Think of it like a sentence: the gene is the words, but without a capital letter at the start and a full stop at the end, the sentence cannot be read correctly.

The terminator region sits downstream (3') of the coding sequence. It signals to RNA polymerase that transcription should stop, ensuring a clean, complete mRNA transcript is produced. When engineers assemble a recombinant DNA construct, both the promoter and the terminator must be added to the fragment of DNA so that the host cell's transcriptional machinery can recognise and act on it correctly.

In practice, the promoter chosen must be compatible with the host organism. A promoter that functions in Escherichia coli may not function in a yeast cell, so the choice of regulatory sequences is matched to the intended host. This is a detail examiners sometimes probe in 'suggest' questions. Evaluating the consequences of using a mismatched promoter could lead to transcriptional failure in the host organism or produce undesired effects in gene expression.

3. Cutting and Joining: Restriction Endonucleases, Ligases, and Vectors

Now that the construct has its regulatory regions, the next challenge is getting it into a host cell. This is a two-stage process: first, cut the DNA fragment and the vector with the same restriction endonuclease; second, join them together with DNA ligase.

Here is the logic of the process. The same restriction endonuclease cuts both the vector (often a plasmid) and the DNA fragment containing the gene of interest. Because both cuts are made by the same enzyme, they produce complementary sticky ends. The fragment and the linearised vector can then anneal — their sticky ends pair up by complementary base pairing — and DNA ligase catalyses the formation of phosphodiester bonds to seal the sugar-phosphate backbone, producing a recombinant vector.

Worked example: Assembling a Recombinant Plasmid: Step-by-Step

Describe, in order, how a gene of interest (with its promoter and terminator) is inserted into a plasmid vector using restriction endonucleases and ligase.

1. Attach the promoter region upstream and the terminator region downstream of the gene of interest. — Without these, the gene cannot be transcribed in the host cell. This is done before cutting.
2. Treat both the DNA fragment (gene + regulatory regions) and the plasmid vector with the SAME restriction endonuclease. — Using the same enzyme ensures both the fragment and the vector have complementary sticky ends. The plasmid is linearised at its restriction site.
3. Mix the fragment and linearised plasmid. Complementary sticky ends pair by hydrogen bonding between complementary bases. — The fragment 'slots into' the gap in the plasmid. This is temporary — hydrogen bonds alone are not enough to hold the construct together.
4. Add DNA ligase, which catalyses the formation of phosphodiester bonds between the sugar-phosphate backbones. — This permanently joins the fragment into the plasmid, producing a stable recombinant vector.
5. Mix the recombinant plasmid with host cells (e.g. E. coli) under conditions that increase membrane permeability (e.g. heat shock or electroporation). — The host cell takes up the recombinant plasmid — this is transformation. The cell can now replicate the plasmid and express the gene of interest.

Answer: Promoter and terminator added → same restriction endonuclease cuts fragment and plasmid → complementary sticky ends anneal → DNA ligase seals phosphodiester bonds → recombinant plasmid introduced into host cell by transformation.

Transformation is the term for a host cell taking up foreign DNA. Not every cell in a culture will successfully take up the recombinant plasmid, which is why transformed cells must be identified and selected — often using antibiotic resistance genes carried on the plasmid as a selectable marker.

4. Amplifying DNA: In Vitro vs In Vivo

Once a DNA fragment has been assembled, it often needs to be copied in large quantities. The AQA specification distinguishes two routes: in vitro amplification (outside a living organism, in a tube) and in vivo amplification (inside a living organism). Each has distinct principles, advantages, and limitations.

4a. PCR: The In Vitro Method

PCR — the polymerase chain reaction — is the workhorse of molecular biology. It can take a single copy of a target DNA sequence and produce billions of copies in a matter of hours, all without a living cell. The key insight is that DNA replication can be driven artificially by cycling temperature.

The reaction mixture contains: the template DNA, two short oligonucleotide primers (one complementary to each strand, flanking the target sequence), free DNA nucleotides (dNTPs), and a thermostable DNA polymerase — most commonly Taq polymerase, originally isolated from the bacterium Thermus aquaticus which lives in hot springs.

Worked example: The Three Stages of One PCR Cycle

Describe what happens during a single cycle of PCR, explaining the purpose of each temperature stage.

1. The reaction mixture is heated to approximately 95 °C. — High temperature breaks the hydrogen bonds between complementary base pairs, separating the double-stranded DNA into two single strands. This exposes the template sequences for primer binding.
2. The temperature is lowered to approximately 50–65 °C (the exact temperature depends on the primer sequence). — Primers bind (anneal) to their complementary sequences on each single-stranded template by hydrogen bonding. The two primers flank the target region on opposite strands, defining the sequence that will be amplified.
3. The temperature is raised to approximately 72 °C, the optimum for Taq polymerase. — Taq polymerase extends each primer in the 5' to 3' direction, adding complementary nucleotides from the free dNTP pool. This synthesises a new complementary strand, doubling the number of copies of the target sequence.
4. Steps 1–3 are repeated, typically 25–35 times, in an automated thermocycler. — Each cycle doubles the number of target DNA molecules. After n cycles, the theoretical yield is 2^n copies. After 30 cycles: 2^30 = over one billion copies from a single starting molecule.

Answer: Denaturation (95 °C) separates strands → Annealing (50–65 °C) allows primers to bind → Extension (72 °C) allows Taq polymerase to synthesise new strands. Each cycle doubles the target sequence; 30 cycles yields over one billion copies.

A critical feature of PCR is that Taq polymerase is thermostable — it is not denatured at 95 °C. Earlier attempts at in vitro amplification used heat-sensitive polymerases that were destroyed during denaturation and had to be replenished each cycle. Taq polymerase survives repeated heating, making the process fully automatable in a thermocycler.

4b. In Vivo Amplification: Culturing Transformed Host Cells

Recall from Section 3 that transformation introduces a recombinant vector into a host cell. Once inside, the plasmid is replicated every time the host cell divides. Growing a large culture of transformed bacteria therefore amplifies the DNA fragment in vivo — inside living cells.

The host cells are cultured under appropriate conditions (nutrient medium, correct temperature, aeration for aerobic bacteria). As the bacterial population doubles with each generation, so does the number of copies of the recombinant plasmid — and therefore the number of copies of the gene of interest. This is the in vivo route to DNA amplification.

An important advantage of in vivo amplification over PCR is that if the gene is expressed, the host cells also produce the protein product. This is how recombinant human insulin is manufactured: the insulin gene is expressed in bacterial or yeast host cells, which secrete functional insulin protein. PCR only copies the DNA; it does not produce protein.

5. Comparing and Evaluating the Two Amplification Methods

Examiners frequently ask you to compare or evaluate PCR and in vivo amplification. The key is to go beyond listing features and instead justify which method is more appropriate for a given context.

PCR is the method of choice when the starting material is extremely limited — for example, a trace of DNA from a crime scene, an ancient specimen, or a biopsy. It requires no living cells and can be completed in a few hours. However, because Taq polymerase lacks a proofreading mechanism, errors introduced during extension are not corrected, so the copies may contain mutations. This matters less for diagnostic purposes but is significant if the amplified DNA will be used to produce a therapeutic protein.

In vivo amplification via transformed host cells is slower and requires the prior construction of a recombinant vector, but it offers the advantage of protein production and, in many host organisms, proofreading during replication reduces the error rate. For large-scale manufacture of a gene product, in vivo amplification is generally the preferred route.

6. Exam Technique: Getting Full Marks on This Topic

Questions on this topic tend to fall into three categories: describe/explain questions about the mechanism of PCR or vector construction; compare questions asking you to contrast in vitro and in vivo methods; and evaluate or suggest questions asking you to justify a choice of method for a given scenario. Each requires a slightly different approach.

For describe questions about PCR, examiners expect you to name all three temperature stages, state the approximate temperature for each, and state what happens at each stage — including which molecule acts at each step. Saying 'the DNA is copied' at extension is insufficient; you need to state that Taq polymerase extends the primer by adding complementary nucleotides in the 5' to 3' direction.

For compare questions, structure your answer around a clear criterion: speed, starting material required, error rate, whether protein is produced, and scale. Avoid writing two separate descriptions — a comparison must explicitly link the two methods using language such as 'whereas', 'in contrast to', or 'unlike PCR'.

Across this topic, the unifying principle is that gene technologies give scientists precise control over DNA: which sequences are cut, how they are assembled with regulatory regions, how they are introduced into host cells, and how they are copied at scale. Mastering the mechanistic detail — the enzymes, the temperatures, the bonds formed — is what separates a good answer from a full-mark answer at A Level.