How do we read, copy and edit genomes, and how are these technologies used?
Application of reproduction and genetics: recombinant DNA technology; PCR; gel electrophoresis; DNA profiling and sequencing; genetic screening; and the ethical issues raised.
A focused answer to the Eduqas Component 2 statement on the applications of genetics. Covers recombinant DNA technology, the polymerase chain reaction, gel electrophoresis, DNA profiling and sequencing, genetic screening, and the ethical issues.
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What this dot point is asking
Eduqas wants you to describe recombinant DNA technology, the polymerase chain reaction, gel electrophoresis, DNA profiling and sequencing, and genetic screening, and to discuss the ethical issues. This applies the genetics of Component 2 to modern biotechnology.
Recombinant DNA technology
To make an organism produce a useful protein (for example bacteria making human insulin):
- Isolate the gene using a restriction enzyme (which cuts DNA at a specific recognition sequence, leaving sticky ends) or make it from mRNA using reverse transcriptase.
- Cut a vector (usually a plasmid) with the same restriction enzyme, giving complementary sticky ends.
- Join the gene into the plasmid with DNA ligase, forming recombinant DNA.
- Transform the host cell with the recombinant plasmid (for example by heat shock).
- Use a marker gene (for example antibiotic resistance or a fluorescent marker) to identify cells that took up the plasmid, then culture them.
PCR and gel electrophoresis
DNA profiling and sequencing
DNA profiling uses regions of non-coding, repetitive DNA (short tandem repeats) that vary between individuals. The DNA is amplified by PCR, cut or copied, separated by electrophoresis, and the banding pattern compared. It is used in forensics, paternity testing and studying relatedness. DNA sequencing reads the exact base order of a length of DNA, underpinning genomics and personalised medicine.
Genetic screening and ethics
Genetic screening tests individuals (or embryos) for disease-causing alleles, for example carrier testing or prenatal diagnosis. This raises ethical issues: consent and the right not to know, privacy of genetic data, possible discrimination (by insurers or employers), and difficult decisions about embryos or pregnancies. Eduqas may ask you to weigh the benefits against these concerns.
Examples in context
Example 1. Bacteria making human insulin. The human insulin gene is inserted into bacteria, which then make identical human insulin in large quantities, a cheaper, more reliable supply than animal insulin and a classic recombinant DNA application.
Example 2. Forensic DNA evidence. A tiny sample from a crime scene is amplified by PCR and profiled, then compared with suspects, illustrating why PCR (amplification of small samples) and electrophoresis (separation by size) are central to forensics.
Try this
Q1. Name the enzyme used to cut DNA at a specific sequence and the enzyme used to join DNA fragments. [2 marks]
- Cue. Restriction enzyme (cuts) and DNA ligase (joins).
Q2. State the three temperature steps of one PCR cycle and what happens at each. [3 marks]
- Cue. About 95 degrees Celsius (denaturation, strands separate); about 55 degrees Celsius (annealing, primers bind); about 72 degrees Celsius (extension, polymerase builds new strands).
Q3. Explain why smaller DNA fragments travel further during gel electrophoresis. [2 marks]
- Cue. DNA is negatively charged and moves towards the positive electrode; smaller fragments move more easily through the gel pores, so they travel further in the same time.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20196 marksDescribe how a gene for a useful protein, such as human insulin, can be transferred into a bacterium so that the bacterium produces the protein.Show worked answer →
Isolate the gene: cut it out using a restriction enzyme (which cuts at a specific base sequence), or make it from mRNA using reverse transcriptase, leaving sticky ends.
Cut a plasmid (vector) with the same restriction enzyme, so it has complementary sticky ends.
Join the gene into the plasmid using DNA ligase, forming recombinant DNA.
Insert the recombinant plasmid into the bacterium (transformation), for example by heat shock or electroporation.
Use a marker gene to identify which bacteria took up the plasmid, then culture those bacteria; they transcribe and translate the gene to make the protein.
Markers reward isolating the gene with a restriction enzyme (or reverse transcriptase), the same enzyme cutting the plasmid, DNA ligase joining them, transformation into the bacterium, and a marker to select successful cells.
Eduqas 20214 marksDescribe the polymerase chain reaction (PCR) and explain why it is useful in producing a DNA profile.Show worked answer →
PCR amplifies DNA. The DNA is heated to about 95 degrees Celsius to separate (denature) the two strands by breaking hydrogen bonds.
It is cooled to about 55 degrees Celsius so that primers anneal (bind) to the ends of the target sequence.
It is warmed to about 72 degrees Celsius, the optimum for a heat-stable DNA polymerase (Taq), which extends the primers, building new complementary strands.
The cycle is repeated many times, doubling the DNA each cycle, so the amount increases exponentially.
This is useful for DNA profiling because only a tiny sample (for example from a crime scene) is needed; PCR amplifies it to a quantity large enough to analyse.
Markers reward denaturation by heating, primers annealing on cooling, extension by a heat-stable polymerase, repeated cycles doubling the DNA, and the use of amplifying a small sample.
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Sources & how we know this
- Eduqas A Level Biology Specification (A400) — Eduqas (2015)