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6.1Topic 6 · Immunity, Infection and Forensics

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

10 min read 16 parts Lesson 3 of 19 Unit quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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Forensics: Estimating Time of Death — EDEXCEL A LEVEL BIOLOGY A (SNAB) 9BI0 — REVISION NOTES notes: Explain how decomposition, insect succession and rigor mortis are used to estimate time of death.Forensics is the use of science in the investigation of criminal activities.

  • time of death (TOD)
  • decomposers
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Forensic entomology

Section 1 of 16Forensic science and time of death

Forensics is the use of science in the investigation of criminal activities.

It involves the collection and analysis of evidence from a potential crime scene, such as the place where a dead body is found.
Time of death (TOD)
Determining the time of death (TOD) is a very important part.
No single method is exact, so several are combined: body temperature, rigor mortis, decomposition, and the insects present.

Section 2 of 16Decomposition

After death, enzymes in the body's own cells begin to digest tissues (autolysis), and decomposer microorganisms such as bacteria and fungi then break the body down.

1
Decomposition continues over months or years until only a skeleton remains,
  • and after a few decades or centuries the skeleton disintegrates until nothing remains.
2
The rate is affected by temperature and availability of oxygen: decomposition is slower in anaerobic conditions and at lower temperatures,
  • and faster at high temperatures (until it is too hot and enzymes denature,
  • or too dry).
3
Moisture, burial depth and clothing also influence the rate.
4
The stage of decomposition therefore gives a rough guide to TOD, after allowing for the conditions.

Section 3 of 16Insect succession and life cycles

Succession refers to the change in the types of organisms found in a habitat over time.

1
A body is a temporary habitat, and different insect species colonise it at different times after death, which provides information about TOD.
2
Flies are found on the body within a few hours after death, whereas beetles only colonise later, when the body has dried.
3
The life-cycle stage of the insect is another clue.
4
For example, blowfly eggs hatch after about 24 hours,
  • so if larvae are present the person died more than 24 hours earlier;
    • if only eggs are present, less than about 24 hours have passed.
5
Other insects have longer life cycles, so finding them (pupae or adults) indicates a longer time since death.
6
A forensic entomologist compares these stages with known development rates at the recorded temperature.

Section 4 of 16Rigor mortis and decomposers

After death, cells stop making ATP by aerobic respiration and respire anaerobically, so lactic acid accumulates.

The fall in pH in the muscle cells denatures the enzymes that produce ATP.
Muscles contract because of two protein filaments, actin and myosin: myosin heads bind to actin and then bend,
  • causing contraction,
  • and ATP is required to allow the myosin heads to detach from the binding sites on actin.
Rigor mortis
Without ATP, the myosin heads stay attached to the actin filaments, locking the muscles in a contracted state: this is rigor mortis.
It begins within a few hours, is complete in about 12 hours, and passes after about 24 to 36 hours as the muscle proteins break down.
Warm conditions speed it up.

Dead plants and animals are broken down by decomposers (bacteria and fungi), which secrete enzymes onto the material and absorb the products (saprotrophic nutrition), recycling nutrients such as nitrogen and phosphorus back into the ecosystem.

Section 5 of 16Worked examples

  1. Explain why rigor mortis occurs after death. [4]4 marks

  2. Larvae of blowflies are found on a body. Suggest what this indicates about the time of death. [2]2 marks

  3. State two factors that affect the rate of decomposition. [2]2 marks

Section 6 of 16DNA profiling and PCR

Every person, except identical twins, has a unique DNA sequence, so a DNA profile can be produced and used to identify individuals, which is very useful in forensic science. Profiling can also determine genetic relationships between organisms. It targets regions of non-coding DNA that vary greatly between individuals (short tandem repeats).

Often only a tiny sample is available, so the DNA is amplified by the polymerase chain reaction (PCR).

The reaction mixture contains the DNA to be amplified, free nucleotides, primers, and a DNA polymerase.
Taq polymerase
The most commonly used polymerase is Taq polymerase from the thermophilic bacterium Thermus aquaticus, as it is stable at high temperatures.
Thermal cycling: heat to about 95 °C to separate the strands by breaking hydrogen bonds; cool to about 55 °C so primers anneal; heat to about 72 °C so Taq polymerase builds the new strands.
Each cycle doubles the DNA.
You do not need to know about forward and reverse primers.

Section 7 of 16Preparing and separating fragments

After PCR, the DNA is treated with restriction enzymes, which cut DNA at specific base sequences, breaking it into fragments of different lengths. A fluorescent tag can be added, in preparation for gel electrophoresis, a technique widely used in the analysis of DNA, RNA and proteins.

DNA fragments are loaded into wells at one end of a piece of agar gel in a buffer.
A current is passed through the gel. Molecules move through the gel because of the difference in charge across it.
DNA fragments move towards the anode (positive electrode) because of the attraction between the negatively charged phosphates of DNA and the anode.
Smaller fragments (shorter pieces) move faster and so travel further from the wells than larger fragments.
Probes
The fragments are transferred to a membrane. Probes, single-stranded DNA sequences complementary to regions of interest, are added; they bind to complementary fragments.
  • Probes carry a radioactive label (making X-ray film go dark) or a fluorescent label (visible under UV light).
  • The result is a pattern of bands that can be compared with control or standard fragments of known size.

Section 8 of 16Analysing the results

Gel electrophoresis produces a pattern of bands, each band being DNA fragments of one length.

Analysing the results
In DNA profiling the pattern from a crime scene sample is compared with those of suspects: if all the bands match, the suspect cannot be excluded; if bands differ, they are not the source.
The more bands that match between two profiles, the greater the genetic similarity.
Sources of error include contamination of samples with other DNA, degraded samples,
  • and the small probability of two individuals sharing a profile by chance.

Section 9 of 16Worked examples

  1. Describe the role of Taq polymerase in PCR. [2]2 marks

  2. Explain why shorter DNA fragments move further in gel electrophoresis. [2]2 marks

  3. Explain why DNA moves to the anode. [1]1 mark

Section 10 of 16Paternity testing

In a paternity test the DNA profile of a child is compared with those of a variety of candidates who could be the father.

1
A child receives half of its DNA from each parent through fertilisation,
  • so each band in the child's profile must come from the mother or from the father.
2
First, the bands that match the mother's profile are identified; the remaining bands must come from the father.
3
If many of those bands match with the bands of a candidate, this could indicate that he is the most likely biological father.
4
If a band in the child's profile that is not from the mother is absent from the candidate's profile, he is excluded.

Section 11 of 16Relationships in general

When comparing DNA profiles, the more bands that match between two individuals, the greater the genetic similarity and the closer the relationship.

Relationships in general
Siblings share about half of their bands, parents and children share half, grandparents and grandchildren share about a quarter,
  • while unrelated people share few.
This logic is used in forensic work (identifying relatives from a partial match), in identifying remains, and in immigration or inheritance disputes.

Section 12 of 16Breeding programmes and other uses

DNA profiling is also useful in selective or captive breeding programmes of animals and in the cultivation of plants.

Breeding programmes and other uses
In zoos, profiling identifies how closely related animals are,
  • so that pairs with the lowest relatedness can be mated to avoid inbreeding and keep genetic diversity high.
In farming it can identify animals or plants with desirable alleles to breed from.
It can also be used to identify species (for example to detect illegally traded wildlife products) and to study the relationships between populations.

Considerations

  • DNA databases raise ethical issues of privacy and consent, and an unexpected result in a paternity test can affect family relationships.
  • A profile match gives a statistical probability, not absolute certainty.

Section 13 of 16Limits and calculation

The probability that two unrelated people share the same profile at several loci is extremely small: if each locus has a 1 in 10 chance of a chance match and ten loci are used, the combined probability is 1 in 10 billion.

Limits and calculation
This is calculated by multiplying probabilities.
Nevertheless, statistical probabilities must be explained carefully in court, because misinterpretation can lead to wrongful conviction.
Identical twins have identical profiles, so cannot be distinguished by this method.

Section 14 of 16Worked examples

  1. A child has 8 bands; 4 match the mother. 4 of the remaining bands match candidate A and 1 matches candidate B. Which man is more likely to be the father? [2]2 marks

  2. Explain why a child shares half of its DNA with each parent. [2]2 marks

  3. Suggest how DNA profiling helps a captive breeding programme. [2]2 marks

Section 15 of 16Key terms

TermMeaning
Time of deathEstimated time at which a person died.
Rigor mortisStiffening of muscles after death due to lack of ATP.
SuccessionChange in the organisms in a habitat over time.
DecomposerMicroorganism that breaks down dead material.
Forensic entomologyUse of insects in legal investigations.
TermMeaning
DNA profilePattern of bands unique to an individual.
PCRTechnique for amplifying DNA.
Taq polymeraseHeat-stable DNA polymerase from Thermus aquaticus.
Restriction enzymeEnzyme that cuts DNA at specific sequences.
Gel electrophoresisSeparation of molecules by size and charge in a gel.
ProbeLabelled single-stranded DNA complementary to a target sequence.
TermMeaning
Paternity testDNA comparison to identify a child's father.
BandPosition on a gel where fragments of one length collect.
Captive breedingBreeding of endangered species in controlled conditions.
InbreedingMating between closely related individuals.

Section 16 of 16Exam technique

Avoid these mistakes
Saying ATP is needed for muscle contraction only — it is also needed to release myosin from actin.
Saying decomposition is faster in the cold.
Expecting a precise time of death from one method.
Avoid these mistakes
Saying DNA moves to the cathode.
Saying restriction enzymes join DNA.
Saying PCR needs a living organism.
Avoid these mistakes
Saying the child shares all bands with the father.
Treating a match as proof.
Forgetting to account for maternal bands.

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