Interpret amplification and fragment evidence with appropriate limitations.
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Method and worked examples
PCR amplifies selected DNA. Heating separates strands; cooling lets complementary primers bind; heat-stable polymerase extends them using free nucleotide building blocks. Ideal copy number equals starting copies ×2 to the power of completed doubling cycles. Amplification increases quantity rather than reporting every base.
Electrophoresis separates fragments: shorter DNA generally travels farther through a gel. Read labelled Sanger fragments from shortest to longest to obtain the newly synthesised strand 5′ to 3′. Profiles compare variable markers such as short tandem repeats. Under a single-source, error-free model, a missing required allele can exclude a candidate. A matching limited profile means compatibility, not certainty of identity. Contamination, mixtures and relatedness can complicate results.
Example 1
An ideal PCR starts with 3 double-stranded target molecules and completes 4 doubling cycles. How many result?
3×24=48 target molecules. This assumes perfect amplification and no limiting reagents.
Example 2
A sample has marker alleles 118 and 1412. Candidate A is 118 and 1412; B is 108 and 1412. Interpret.
A is compatible at these markers; B is excluded under the stated single-source, error-free model. More loci and quality checks are needed for stronger identification evidence.
Watch out:Profiles have statistical and sampling limitations; contamination, mixtures and relatedness must be considered.
PCR, sequencing and DNA profiles practice | Year 12 Biology | Ascent Arcade