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8.4.12The Control of Gene Expression (A Level only)

Genetic Fingerprinting — AQA A Level Biology 7402 Revision Notes & Topical Questions

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📚 AQA A Level Biology 7402 Hub / Topical Revision
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8.4.12 Genetic Fingerprinting — AQA A Level Biology 7402 notes: Describe the stages of producing a genetic fingerprint using PCR and electrophoresis, and interpret the results.A genetic fingerprint is a pattern of bands produced from the VNTRs (or STRs) of an individual's DNA.Exc…

  • Extract DNA
  • polymerase chain reaction (PCR)
  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3
  • Taq polymerase

Section 1 of 9What is a genetic fingerprint?

A genetic fingerprint is a pattern of bands produced from the VNTRs (or STRs) of an individual's DNA.

What is a genetic fingerprint?
Except for identical twins, the pattern is unique, because the number of repeats at many loci is highly variable.
It is used to compare DNA from different sources, such as a crime scene sample and a suspect, or a child and its possible parents.

Section 2 of 9Stages

1Extract DNA
Extract DNA from a sample (blood, saliva, hair root, semen or tissue). Even a tiny sample is enough because the DNA can be amplified.
2Amplify
Amplify the VNTR regions using the polymerase chain reaction (PCR), with primers complementary to sequences flanking the repeats, free nucleotides and a heat-stable DNA polymerase.
  • Each cycle
    • heat to about 95 °C to separate the strands;
      • cool to about 55 °C so that primers anneal;
      • warm to about 72 °C so that the polymerase extends from the primers.
  • The amount of DNA doubles each cycle.
3Separate
Separate the fragments by gel electrophoresis.
  • Fragments are loaded into wells at the negative end and move towards the positive end, smaller ones moving farther.
4Visualise
Visualise the fragments.
  • In older methods the DNA is transferred to a membrane and a labelled probe added;
    • in modern methods the primers carry fluorescent labels and a laser detects the fragments.
5Compare
Compare the pattern of bands with those from other samples.

Section 3 of 9Understanding PCR

ComponentPurpose
PrimersShort single-stranded DNA; bind to the ends of the target and give DNA polymerase a starting point
Taq polymeraseHeat-stable, so not denatured at 95 °C
Free nucleotidesRaw material for new strands
ThermocyclerAutomatically changes temperature

After n cycles there are 2n copies per original molecule, so 30 cycles gives over a billion copies. PCR works in vitro, with no cells needed, and is fast. Contamination is a risk because even stray DNA from the operator may be amplified, so strict sterile technique is needed.

Section 4 of 9Interpreting a fingerprint

Two samples with the same pattern of bands at all loci tested probably came from the same person.

Interpreting a fingerprint
Bands in a child's profile must each match a band from the mother or the father.
A band in the child that matches neither parent suggests that the man is not a biological parent (or that a mutation has occurred).
Forensic profiles use many STR loci so that the probability of a coincidental match is extremely small, for example less than one in a billion.

Section 5 of 9Limitations

Mixtures of DNA from several people give complex patterns that are hard to interpret. Degraded samples may give only partial profiles. Identical twins have the same profile. Relatives share many bands, so close relatives can give misleadingly similar profiles. The quality of evidence also depends on how the sample was collected and stored, since contamination can lead to false matches.

Section 6 of 9Calculation practice and evaluating the evidence

Number of copies after n cycles = 2n (if the starting amount is one molecule). If a sample starts with 50 molecules and undergoes 10 cycles, the number is 50 × 210 = 51 200. Note that real reactions are not perfectly efficient so this is a maximum.

When a fingerprint from a crime scene matches a suspect, the court needs to know how likely it is that an unrelated person would match by chance.

1
This is the random match probability, calculated by multiplying the frequency of each allele across all of the loci tested.
2
A large number of loci reduces the probability, but not to zero.
3
In addition, the match is only as good as the sample: a mixed sample from several people may include alleles from the victim,
  • and a very small sample may give allele drop-out,
  • where one of the two alleles at a locus fails to amplify and a heterozygote looks like a homozygote.
4
For these reasons, scientists use controls, repeat the test, and report the result with its limitations.
5
The technique supports other evidence, such as witness accounts, and does not replace it.

Section 7 of 9Exam-style questions

  1. Explain why a heat-stable DNA polymerase is used in PCR. [2]2 marks

  2. A PCR starts with one DNA molecule. Calculate the number of copies after 12 cycles. [1]1 mark

  3. Describe how the bands in a genetic fingerprint of a child relate to those of the parents. [2]2 marks

Section 8 of 9Key terms

TermMeaning
Genetic fingerprintPattern of bands from an individual's repeat sequences
PCRPolymerase chain reaction: in vitro amplification of DNA
PrimerShort single-stranded DNA that starts replication
Taq polymeraseHeat-stable DNA polymerase from Thermus aquaticus
ElectrophoresisSeparation of molecules by size in an electric field
ThermocyclerMachine that cycles temperature for PCR

Section 9 of 9Exam technique

1
Giving the wrong order for PCR stages: separate the strands, anneal primers, extend.
2
Saying primers are made by the polymerase: they are supplied, and are complementary to flanking sequences.
3
Forgetting that the bands move towards the positive electrode.
4
Claiming a fingerprint is always uniqueidentical twins match.

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