Students should be able to: explain the biological principles that…

The idea

Genetic fingerprinting analyses Variable Number Tandem Repeats (VNTRs), non-coding DNA sequences where repeat numbers differ between individuals. DNA is extracted and cut with restriction endonucleases, or specific VNTR loci are amplified by PCR. Fragment lengths vary depending on the repeat number. Gel electrophoresis separates these fragments: DNA is negatively charged due to phosphate groups, so it migrates through the gel towards the anode. The gel acts as a sieve, meaning shorter fragments travel further. Fragments are transferred to a membrane by Southern blotting and detected with labelled DNA probes, producing a banding pattern. Scientists use this to match crime scene DNA in forensics, identify pathogen strains in medical diagnosis, and prevent inbreeding in animal and plant breeding by selecting genetically diverse individuals to cross.

What earns the marks

• Explain separation: DNA is negatively charged due to phosphate groups and moves towards the anode.
• Explain separation: the gel acts as a sieve, so shorter DNA fragments travel faster and further.
• Explain principles: DNA is cut with restriction endonucleases, or specific VNTR loci are amplified by PCR, producing fragments whose length depends on the number of repeats.
• Explain detection: fragments are transferred to a membrane by Southern blotting and detected using labelled DNA probes.
• Explain uses: in breeding, comparing VNTR patterns identifies genetically diverse individuals to cross, preventing inbreeding.
• Interpret gels: estimate the length of an unknown DNA fragment by comparing its position against a DNA ladder of known standard lengths.

Common mistakes and how to put them right

• Stating that PCR alone replaces restriction endonucleases in all genetic fingerprinting.

  Put it right: Classical genetic fingerprinting uses restriction endonucleases to cut DNA, while PCR amplification of specific VNTR loci is a modern alternative, so describe the method the question asks about.

• Concluding a definitive match from a single shared band, rather than recognising that a match across multiple VNTR loci is required for a high probability of identification.

  Put it right: State that several loci must match before a match is treated as strong evidence.

• Stating that genetic fingerprinting sequences the whole genome, rather than remembering that it only analyses the length of specific non-coding VNTR fragments.

  Put it right: Describe it as comparing fragment lengths at specific VNTR loci, not whole-genome sequencing.

Exam tips

• When interpreting a gel, explicitly quote the figure's labels (e.g., 'the band at 3 kb in lane 2') rather than making vague comparisons.
• For 'explain why' questions on fingerprinting uses, always provide both the specific field application (e.g., forensics) and the biological reason (e.g., matching VNTR profiles to place a suspect at the scene).