4.1.3Additional lessons
Electrophoresis and PCR: Applications — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 101 of 142
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Making a DNA Profile
A DNA profile is the pattern of bands produced from regions of the genome that vary greatly between individuals.
1
DNA is collected from a source such as blood, saliva or hair roots, and may be a very small amount.
2
PCR amplifies the regions of interest, which contain variable numbers of short repeated sequences.
3
Restriction enzymes may be used to cut the DNA into fragments of different lengths.
4
The fragments are separated by gel electrophoresis according to length.
5
The bands are made visible with fluorescent dye or labelled probes, giving a pattern that is compared with other samples.
Section 2 of 5Why Profiles Differ
Tandem repeats
- Tandem repeats Non-coding regions contain short sequences repeated end to end, and the number of repeats differs between people.
Inheritance
- Inheritance Each person inherits one set of repeat lengths from each parent, so half of a child's bands match each parent.
Uniqueness
- Uniqueness Apart from identical twins, the combination of repeat lengths is expected to be unique to an individual.
Reliability
- Reliability Using more repeat regions makes the chance of two unrelated people matching by accident extremely small.
Section 3 of 5Applications
Applications
Forensics
- Forensics Profiles from a crime scene are compared with those of suspects, victims and controls to include or exclude individuals.
Paternity
- Paternity Every band in a child's profile must come from the mother or father, so a man missing a paternal band is excluded.
Disaster victims
- Disaster victims Profiles from remains can be compared with relatives' profiles to identify people who cannot be recognised otherwise.
Diagnosis
- Diagnosis PCR can detect tiny amounts of pathogen genetic material;
- viral RNA is first converted to DNA before amplification.
Care needed
- Care needed Samples must be protected from contamination, because PCR can amplify stray DNA as readily as the intended sample.
Section 4 of 5Worked Example
Question
- A child's profile has bands at positions 1, 2, 3 and 4.
- The mother has bands 1 and 3.
- Man X has bands 2, 4 and 5;
- man Y has 2 and 3;
- man Z has 4 and 6.
- Which man could be the father?
- Bands 1 and 3 can come from the mother, so they give no information about the father.
- Bands 2 and 4 are not in the mother, so both must come from the father.
- Man Y lacks band 4 and man Z lacks band 2, so both are excluded.
- Man X has both bands 2 and 4.
Answer Man X is the only possible father, because he has both paternal bands.
Section 5 of 5Exam Checklist & Tips
4.1.3 – Key Points for the Exam
- DNA profile A pattern of bands showing the lengths of variable repeat regions in an individual's DNA.
- Tandem repeat A short sequence repeated end to end whose number varies between individuals.
- Forensic Relating to the use of scientific evidence in legal investigations.
- Contamination Unwanted DNA entering a sample, which can lead to false results.
Common Mistakes
- Believing a single shared band proves identity or parentage; several matching bands are needed.
- Forgetting that a child's non-maternal bands must match the father.
- Saying PCR makes profiles; PCR amplifies, and electrophoresis separates.
- Ignoring the controls and contamination precautions in forensic work.
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