Genetic fingerprinting - AL only (3.8.4.3)
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DNA profiling (also called DNA fingerprinting) analyses non-coding, variable regions of DNA to compare individuals.
DNA profiling most commonly uses short tandem repeats (STRs), also known as microsatellites, which consist of short repeated DNA sequences. The number of repeats at each STR locus varies between individuals, producing a unique DNA profile (except in identical twins).
DNA profiling follows these steps:
- Sample collection – a biological sample containing DNA is obtained, such as blood, saliva, hair roots or plant tissue.
- DNA extraction – DNA is isolated from the cells in the sample.
- PCR – the short tandem repeat regions (STR) are amplified using PCR to produce many copies for analysis.
- Electrophoresis – the amplified DNA fragments are separated according to their length (smaller fragments move further), producing a pattern of bands.
- Profile comparison – resulting DNA profiles are compared between samples. Matching banding patterns indicate that the DNA is likely to have originated from the same individual.
Relatives share a predictable proportion of STR patterns:
- Parents and offspring share approximately of their STR alleles / bands.
- Siblings share many STR alleles / bands, but usually have different overall DNA profiles.
DNA profiling can be used to confirm parentage, breeding lineages, or inheritance disputes. In plants, profiling is used to confirm breeding crosses and verify cultivar purity. In animals, it is used to manage captive breeding programmes (zoos) and maintain genetic diversity.
Uses of DNA profiling include:
- Matching suspects to crime scene samples.
- Identifying victims in disasters.
- Distinguishing between similar-looking organisms (rare species, plant cultivars).
- Preventing inbreeding by checking relatedness.
- Tracking illegal wildlife trade (timber, ivory).
- Assessing the genetic risk of disease.
DNA fingerprinting can be used for determining genetic relationships.
All the bands shown in a child’s DNA fingerprint must correspond to bands from one of the parents’ fingerprints. It is important to note that not all the bands from either parent will be observed in the child.
PCR (Polymerase Chain Reaction) is a technique used to rapidly amplify specific fragments of DNA.
The reaction mixture contains:
- Template DNA – the DNA containing the target sequence to be copied.
- Primers – short single-stranded DNA sequences that bind to the ends of the target region.
- Free DNA nucleotides – used to build new DNA strands.
- DNA polymerase – a heat-stable enzyme that synthesises new DNA strands.
- Buffer – maintains the optimum and conditions for the enzyme.
The PCR Cycle (repeated 25–35×):
- Denaturation (~95): hydrogen bonds between complementary DNA strands break.
- Annealing (~50–65): primers bind (anneal) to complementary sequences on the template (target) DNA.
- Extension (~72): DNA polymerase adds complementary nucleotides to the primers creating new DNA strands.
Each cycle approximately doubles the amount of target DNA resulting in exponential amplification.
After n cycles, the number of DNA molecules ≈ 2n (assuming 100 efficiency).
PCR–based DNA replication has many advantages. PCR:
- Can replicate DNA extremely quickly.
- Does not require any living organisms or cells, so has no ethical issues.
- Can be used for a variety of applications, such as forensics.
The main disadvantage of PCR is that if the sample is contaminated, then the contaminated DNA will also be replicated.