Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

6.1.3Topic 6 · Immunity, Infection and Forensics

DNA Profiling — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 9 parts Lesson 6 of 19 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read
Lesson summary

6.1.3 DNA Profiling — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe how DNA profiles are produced using PCR and gel electrophoresis, interpret profiles, and evaluate the uses and ethics of DNA profiling.Although any two humans share about 99.9% of their DNA sequence, the …

  • short tandem repeats (STRs)
  • Extraction
  • Amplification by PCR
  • Separation by gel electrophoresis
  • Visualisation
  • match probability

Section 1 of 9The basis of DNA profiling

Although any two humans share about 99.9% of their DNA sequence, the human genome contains many stretches of non-coding DNA made up of short sequences repeated one after the other.

1Short tandem repeats (STRs)
These are called short tandem repeats (STRs), or variable number tandem repeats.
2
The number of repeats at a given position (locus) varies between individuals, and each person has two alleles, one inherited from each parent.
3DNA profile
By examining several loci, the chance that two unrelated individuals have the same pattern becomes very small, so the result is a DNA profile (DNA fingerprint).
4
Identical twins share the same profile.

Section 2 of 9Producing a profile

1Extraction
DNA is obtained from blood, saliva, hair roots, semen or other tissue, and is purified.
2Amplification by PCR
(polymerase chain reaction): a very small sample is copied many times.
  • The mixture contains the DNA template, a heat-stable DNA polymerase (Taq polymerase from Thermus aquaticus), free nucleotides, buffer,
    • and short primers complementary to the sequences flanking the STR loci;
      • the primers are often labelled with fluorescent dye.
  • Cycles of temperature change are repeated about 25 to 30 times: heating to about 95 °C separates the strands by breaking hydrogen bonds; cooling to about 55 to 60 °C lets primers anneal;
    • and warming to about 72 °C lets the polymerase extend new strands.
  • Each cycle doubles the amount of DNA.
3Separation by gel electrophoresis
Separation by gel electrophoresis the DNA fragments are loaded into wells in an agarose gel covered by buffer, and an electric current is applied.
  • DNA is negatively charged because of its phosphate groups, so it moves towards the positive electrode.
  • Smaller fragments pass through the gel pores faster and move farther, so the fragments are separated by size.
  • Modern laboratories use capillary electrophoresis with fluorescent detection.
4Visualisation
Visualisation the fragments are shown by staining or fluorescence and the band pattern is compared with a known sample or a database.

Section 3 of 9Interpreting profiles

To compare profiles, look for bands that line up horizontally, meaning fragments of the same length.

Match probability
A match between a crime scene sample and a suspect means that the suspect cannot be excluded; the strength of the evidence depends on how many loci match and on how common the alleles are in the population, which gives a match probability.
For paternity tests, a child inherits half of each band pattern from the mother and half from the father, so every band in the child must be present in one of the parents.
Profiles are also used to identify victims of disasters, to detect inherited relationships and to study the genetic diversity of wild populations, and in some diseases to find mutations.
Feature of the profileMeaning
Bands at same position in two samplesSame fragment length, possible common source
Child band absent from both parentsParentage excluded (or a mutation)
More than two alleles at a locusMixture of DNA from more than one person

Section 4 of 9Limitations and ethical issues

1Contamination
Contamination because PCR is highly sensitive, a stray cell from an investigator can affect the result, so strict procedures and controls are needed.
2Degraded or mixed samples
Degraded or mixed samples may give partial profiles.
3Interpretation errors
Interpretation errors and misuse of statistics can mislead juries.
4Privacy
Privacy storing profiles of innocent people or of whole families on a database raises concerns about civil liberties, who can access the data and how long it is kept.
5Consent
Consent and the possible use of data to reveal information about disease risk or family relationships.
6
DNA is evidence of presence, not of guilt: it does not show how or when the material was deposited.

Section 5 of 9Wider uses

DNA profiling is used in forensic science, in paternity and immigration cases, in checking the identity of food and wildlife products, and in conservation, for example to identify illegally traded species or to select unrelated individuals for breeding programmes. Related techniques include DNA sequencing and the study of mitochondrial DNA, which is inherited from the mother and survives better in old remains.

Section 6 of 9Worked reasoning for paternity and crime scene profiles

When shown a gel, label the lanes: the crime scene sample, the victim, and each suspect.

1
First check the control or ladder of known lengths so the size of the bands can be estimated.
2
Next, compare each suspect lane with the crime scene lane, looking for every band at the same position.
3
If any band in the crime scene sample is missing from a suspect, that suspect is excluded; if all bands match, the suspect is not excluded, and the result must be put in the context of the match probability.
4
In a paternity case, mark each of the child's bands as maternal or paternal; those bands which are not in the mother must be present in the father if he is the biological parent.

Remember that the mixture of DNA from several people produces extra bands, and that low amounts of DNA may show allele dropout, where one allele fails to amplify. Careful experimental design, including negative controls with no DNA, helps to detect contamination. Quality standards and independent checking increase reliability.

Section 7 of 9Exam-style questions

  1. Describe the stages of PCR and explain why a heat-stable DNA polymerase is used. [5]5 marks

  2. Explain why DNA fragments move through an agarose gel and why smaller fragments move farther. [3]3 marks

  3. The probability that a given STR allele occurs in the population is 0.1 at locus A and 0.05 at locus B. Calculate the probability that a random person has both alleles and comment on its use. [3]3 marks

Section 8 of 9Key terms

TermMeaning
Short tandem repeatA short DNA sequence repeated several times in a row, varying in number between people.
DNA profileA pattern of DNA fragments that identifies an individual.
PCRPolymerase chain reaction, a method of copying DNA in vitro.
PrimerA short single-stranded DNA sequence that starts DNA synthesis.
Gel electrophoresisSeparation of molecules by size and charge in a gel under an electric field.
Taq polymeraseA heat-stable DNA polymerase used in PCR.
LocusThe position of a gene or marker on a chromosome.
Match probabilityThe chance that a random person has the same profile.

Section 9 of 9Exam technique

1
Saying DNA profiling shows the whole genome. It examines selected STR loci.
2
Stating that DNA moves to the negative electrode. DNA is negatively charged and moves to the positive.
3
Saying a match proves guilt. It shows that DNA could have come from that person, and that probability depends on the number of loci.
4
Forgetting that PCR requires primers, nucleotides and a heat-stable polymerase.

Last updated: