Gene technology techniques: WACE Year 12 Biology
“Describe the techniques of gene technology including restriction enzymes, PCR, gel electrophoresis and DNA profiling”
Gene technology relies on a toolkit of techniques: restriction enzymes cut DNA at specific sequences leaving sticky ends; ligase joins DNA fragments to make recombinant DNA; the polymerase chain reaction (PCR) makes millions of copies of a DNA region; gel electrophoresis separates DNA fragments by size; and DNA profiling compares the resulting patterns to identify individuals.
What this dot point is asking
SCSA wants you to describe each core technique, explain what it achieves, and understand how they combine in applications such as genetic engineering and forensic DNA profiling. A strong answer states the purpose and the principle of each method.
Restriction enzymes and recombinant DNA
Restriction enzymes are proteins that cut DNA at specific recognition sequences. Many cut in a staggered way, leaving short single-stranded overhangs called sticky ends. Because the same enzyme leaves complementary sticky ends on any DNA it cuts, fragments from different sources can be joined.
DNA ligase then seals the fragments together, producing recombinant DNA: a single DNA molecule combining sequences from more than one source, such as a human gene inserted into a bacterial plasmid.
Because a given restriction enzyme always cuts the same sequence, it leaves matching sticky ends on every fragment. This lets a gene from one organism slot neatly into a vector from another, which is the foundation of genetic engineering.
The polymerase chain reaction (PCR)
PCR makes millions of copies of a target DNA region in a few hours. It repeats a three-step cycle:
- Denaturation: heating to around 95 degrees Celsius separates the two DNA strands.
- Annealing: cooling lets short primers bind to the ends of the target sequence.
- Extension: a heat-stable DNA polymerase builds new complementary strands at around 72 degrees Celsius.
Each cycle doubles the amount of DNA, so the number of copies grows exponentially. PCR is essential when only a tiny sample of DNA is available, such as at a crime scene.
Gel electrophoresis
Gel electrophoresis separates DNA fragments by size. DNA samples are loaded into wells in a gel, and an electric current is applied. Because DNA is negatively charged, it moves toward the positive electrode. Smaller fragments move faster and travel further, so the fragments separate into bands by size. The pattern of bands can then be compared between samples.
Work through the forensic pipeline
A small bloodstain is found at a scene. PCR is used to amplify specific regions of the DNA so there is enough to analyse. The amplified DNA is then run through gel electrophoresis, which separates the fragments by size into a banding pattern. This pattern is compared with patterns from suspects. A match between the crime-scene profile and a suspect's profile is strong evidence the suspect's DNA was present.
DNA profiling
DNA profiling (DNA fingerprinting) compares the banding patterns produced from regions of DNA that vary greatly between individuals. Because these regions differ from person to person, the resulting profile is highly individual (except for identical twins). It is used in forensic identification, paternity testing, and identifying remains.
How the techniques work together
The techniques are usually combined. PCR provides enough DNA to work with; restriction enzymes and ligase build recombinant molecules; gel electrophoresis separates fragments so they can be visualised; and the patterns support profiling or confirm that a gene has been inserted correctly. Understanding the purpose of each lets you follow a whole procedure from sample to result.
Restriction enzyme: an enzyme that cuts DNA at a specific sequence. Recombinant DNA: DNA combining sequences from more than one source. PCR: a technique that amplifies a DNA region exponentially. Gel electrophoresis: separation of DNA fragments by size using an electric field.
Why this matters for continuity
These techniques let humans read, copy and rearrange the genetic material that underlies continuity of species. They make it possible to study genes directly, diagnose genetic conditions, produce useful proteins such as insulin in bacteria, and identify individuals from trace DNA. They are the practical tools behind the applications and ethical debates covered elsewhere in the unit.
- Saying PCR separates DNA
- PCR copies and amplifies DNA; gel electrophoresis separates it by size.
- Forgetting why DNA moves to the positive electrode
- DNA is negatively charged because of its phosphate groups.
- Saying larger fragments travel further in a gel
- Smaller fragments move faster and travel further.
- Confusing restriction enzymes and ligase
- Restriction enzymes cut DNA; ligase joins it.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2021-style6 marksDescribe the three steps of one cycle of the polymerase chain reaction (PCR), stating the approximate temperature and purpose of each step, and explain why the number of DNA copies grows exponentially.Show worked answer →
A 6 mark answer needs the three steps with temperatures and purposes plus the exponential reasoning.
- Denaturation ()
- Heating breaks the hydrogen bonds and separates the double-stranded DNA into two single strands (templates).
- Annealing ( to )
- Cooling lets short primers bind to the complementary ends of the target sequence on each template.
- Extension ()
- A heat-stable DNA polymerase (e.g. Taq) adds complementary nucleotides from the primers, building a new strand on each template.
- Why exponential
- Each cycle doubles the amount of target DNA because both strands are copied. After cycles there are roughly copies, so the quantity grows exponentially.
Markers reward each step with its temperature and purpose and the doubling-per-cycle reasoning.
WACE 2023-style5 marksA forensic scientist runs DNA from a crime scene and three suspects on a gel. Explain how gel electrophoresis separates the DNA and how the resulting banding patterns are used to identify a suspect.Show worked answer →
A 5 mark answer needs the separation principle plus the identification logic.
Separation. DNA samples are loaded into wells in an agarose gel and an electric current is applied. DNA is negatively charged (due to its phosphate groups), so it moves toward the positive electrode. Smaller fragments move through the gel more easily and travel further, larger fragments lag behind, so fragments separate into bands by size.
Identification. The banding pattern (profile) reflects the sizes of the variable DNA regions, which differ between individuals. The crime-scene profile is compared with each suspect's profile. A suspect whose bands match the crime-scene pattern at every position is implicated, because an identical pattern by chance is extremely unlikely; a mismatch at any band excludes that suspect.
Markers reward the negative charge and size-based movement, smaller fragments travelling further, and matching banding patterns to identify or exclude a suspect.