QCE Biology - Unit 4 - Genetics and heredity

Recombinant DNA, PCR and DNA Profiling | QCE Biology

Learn recombinant DNA, restriction enzymes, plasmids, DNA ligase, PCR, gel electrophoresis and DNA profile interpretation.

Updated 2026-05-18 - 7 min read

Recombinant DNA, PCR, gel electrophoresis and DNA profiling is part of the way QCE Biology turns living systems into evidence students can describe, analyse and evaluate. The safest way to study it is to connect each term to a data pattern, a biological mechanism and a limitation.

Core explanation

Recombinant DNA

Restriction enzymes cut DNA at specific sequences. A plasmid can be cut with the same enzyme, a target gene inserted, and DNA ligase used to join the fragments.

PCR

PCR amplifies a target DNA region through repeated cycles of denaturation, primer annealing and extension. This produces enough DNA for analysis.

Gel electrophoresis

DNA fragments move through a gel when an electric field is applied. Smaller fragments usually travel further, creating a banding pattern.

DNA profiling

DNA profiles compare banding patterns. A match does not mean every base is identical; it means the tested regions produced the same profile.

Gene cloning, PCR and sequencing detail

Recombinant DNA technology can be used for gene cloning: producing many copies of a gene or producing a gene product in a host organism.

| Step | What happens | Why it matters | | --- | --- | --- | | Isolate target DNA | The gene or DNA region of interest is obtained | Defines what will be copied or expressed | | Cut DNA and plasmid | The same restriction enzyme creates compatible ends | Allows target DNA to fit into the vector | | Join fragments | DNA ligase seals the sugar-phosphate backbone | Produces recombinant plasmids | | Transform host cells | Plasmids enter bacterial cells | Host cells copy the plasmid as they divide | | Select successful cells | Antibiotic resistance or markers identify transformed cells | Separates cells with plasmids from cells without plasmids | | Express or harvest | Cells produce protein or copied DNA is collected | Creates the useful product or sample |

PCR is different from gene cloning because it copies DNA in a tube rather than inside living cells. Denaturation separates DNA strands, annealing allows primers to bind, and extension uses a heat-stable DNA polymerase to build new strands. Repeating the cycle causes exponential amplification of the target region.

Gel electrophoresis separates fragments by length because DNA is negatively charged and moves toward the positive electrode. A DNA ladder provides known fragment sizes, so unknown bands can be estimated by comparison.

Sanger DNA sequencing uses chain-terminating nucleotides to generate fragments ending at each base. Reading the separated fragments allows the base sequence to be inferred. Sequencing is more detailed than a DNA profile because it identifies the order of bases rather than just comparing fragment patterns.

Appraising biotechnology data

When evaluating DNA evidence, consider contamination, sample degradation, primer specificity, incomplete digestion, band resolution, sample size, whether the tested loci are informative and whether the reference database is appropriate. In forensic contexts, a DNA profile can strongly support identity, but it must be interpreted with collection method and probability, not treated as automatic proof.

What each biotechnology tool is best for

Recombinant DNA technology is best when the goal is to insert, copy or express a gene. PCR is best when the goal is to amplify a known target region from a small sample. Gel electrophoresis is best when the goal is to separate DNA fragments by size. DNA profiling is best when the goal is to compare individuals at selected variable loci. DNA sequencing is best when the exact base order is needed.

Transformation is the uptake of foreign DNA by a cell. In bacterial gene cloning, only some cells take up the recombinant plasmid, so selection is needed. A selectable marker such as antibiotic resistance allows transformed cells to survive while non-transformed cells do not. A reporter marker can help identify whether the target insert disrupted or activated a visible gene.

PCR requires primers that flank the target sequence. If primers bind non-specifically, the wrong region may be amplified. If the sample is contaminated, PCR can amplify contaminant DNA because the method is highly sensitive. Controls are therefore essential: a negative control checks contamination, while a positive control checks that the PCR reagents and cycling conditions work.

In gels, thick or smeared bands can suggest too much DNA, degraded DNA or poor running conditions. Bands of similar size may not separate clearly, so a "match" must be interpreted with resolution limits.

Insulin production is a common recombinant DNA example. A human insulin gene can be inserted into a bacterial plasmid, transformed into bacterial cells and expressed so the bacteria produce human insulin protein. The important biological idea is that the genetic code is shared across organisms, so a bacterial ribosome can translate a human coding sequence if the gene is placed in an expression system the bacterium can use.

PCR can be used to detect pathogens when primers target a pathogen-specific DNA sequence. A positive result suggests that the target sequence was present in the sample, but interpretation depends on controls, contamination risk and whether the test detects living pathogens or leftover genetic material.

Biotechnology workflows often use microtubes or reaction tubes, but the container is not the key concept. The key concept is what is placed in the reaction: template DNA, primers, free nucleotides, buffer and heat-stable DNA polymerase for PCR; or cut plasmid, target DNA and ligase for recombinant DNA construction.

How to use this in data questions

Start by identifying what has been measured. In Biology, a graph or table is rarely just asking for a trend; it is asking whether you can connect the trend to a process. Quote enough data to show the pattern, then use the concept language from the syllabus. If the evidence is limited, name the limitation precisely: sample size, sampling method, uncontrolled variables, measurement precision, population choice or the time scale of the data.

A useful study habit is to turn each heading into a data prompt. Ask what you would expect to happen if the relevant variable increased, decreased or was removed. For ecology topics, think about abundance, distribution, biodiversity, biomass and carrying capacity. For genetics topics, think about genotype, phenotype, gene expression, allele frequency and inheritance pattern. For evolution topics, think about variation, selection pressure, gene flow, isolation and relatedness.

When a question asks you to evaluate, do not just list problems with the experiment. Link the limitation to the confidence of the conclusion. For example, a small sample size matters because a few unusual individuals can distort the pattern. An uncontrolled abiotic factor matters because it gives another possible explanation for the same biological trend. This is the difference between naming a limitation and using it scientifically.

Worked example

Common exam traps

Other traps to watch for:

• using a general word when a syllabus term is available
• ignoring units, sample size or time scale
• treating a model as a perfect copy of the real ecosystem or cell
• writing a memorised paragraph that does not use the given data

Quick check

Sources

• QCAA Biology subject page
• QCAA Biology 2025 syllabus
• OpenStax Biology 2e: Biotechnology
• OpenStax Biology 2e: Mapping Genomes
• National Human Genome Research Institute: Polymerase Chain Reaction
• Khan Academy: DNA cloning and recombinant DNA