Manipulating genomes: (c) the principles of DNA profiling and its uses

The idea

DNA profiling exploits variation in repetitive, non-coding DNA to produce a pattern characteristic of an individual. In OCR's core method, DNA is extracted and amplified by PCR, cut with restriction enzymes, and the fragments separated by gel electrophoresis; shorter fragments travel further, so band positions reflect fragment lengths. Alternatively, short tandem repeat (STR) numbers at several loci are measured, giving a multi-locus profile. Because close relatives share more bands than unrelated people, profiles are compared rather than read absolutely. Uses include forensic identification from crime-scene samples, paternity and kinship testing, identifying tissue or blood samples, and monitoring genetic diversity in conservation. A match is probabilistic evidence, not proof of guilt.

What earns the marks

• DNA profiling examines variable repetitive non-coding regions (for example STRs) rather than whole genes, so individuals differ in fragment lengths or repeat numbers.
• The method involves extraction, PCR amplification, restriction digestion and gel electrophoresis, or STR locus sizing; shorter fragments migrate further in the gel.
• A profile is a pattern of bands or peaks compared between samples; a match indicates the samples likely came from the same individual or related individuals.
• Forensic use compares crime-scene DNA with a suspect or database sample; paternity testing compares a child's profile with parents.
• Interpretation is probabilistic: relatives share bands, contamination and degraded samples cause problems, so a match supports rather than proves identity.

Common mistakes and how to put them right

• Thinking a profile sequences the whole genome.

  Put it right by explaining that only variable repeat regions are compared.

• Assuming a single matching band proves identity.

  Put it right by noting that several loci must be compared and that relatives share many bands.

• Believing longer fragments travel furthest in electrophoresis.

  Put it right by stating that shorter fragments move further through the gel.

Exam tips

• Link each stage of the method to the reason for it, for example PCR increases DNA quantity before digestion.
• Use the phrase 'probability' or 'likelihood' when describing a match rather than claiming certainty.
• Practise comparing two band patterns and stating which loci differ and what that implies about relatedness.