Decay, DNA techniques and forensics — Edexcel International A Level Biology
Decomposition and carbon recycling, PCR, gel electrophoresis, DNA profiling, and the evidence used to estimate time of death.
Revision notes
5 short notes, in the order of the specification. Each one in short:
Saprotrophic bacteria and fungi decompose dead organisms, faeces and waste by secreting extracellular enzymes that digest organic matter and absorbing the soluble products. Their respiration releases carbon dioxide to the atmosphere, recycling carbon so it is available to producers for photosynthesis.
The polymerase chain reaction amplifies a DNA sequence in a thermal cycler. Each cycle has denaturation at about 95 °C, annealing of primers at about 55 to 65 °C and extension by Taq polymerase at about 72 °C. The number of copies doubles each cycle.
Gel electrophoresis separates DNA fragments by length. DNA is negatively charged because of its phosphate groups, so in an electric field it moves through an agarose gel towards the positive electrode. Shorter fragments move faster and further, and fragments are made visible with a fluorescent dye or probe.
DNA profiling compares variable-length repeated sequences in non-coding DNA (short tandem repeats). They are amplified by PCR and separated by gel electrophoresis, giving a band pattern. It identifies individuals and shows relationships between organisms, because offspring inherit half their bands from each parent.
Time since death of a mammal can be estimated from body temperature, rigor mortis, the extent of decomposition, the stage of succession of organisms colonising the body, and the age and species of insects such as blowfly larvae. Each is affected by environmental conditions, so several are used together.
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Quick check questions
How do saprotrophs digest dead matter?
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They secrete extracellular enzymes on to it and absorb the soluble products.
Why is Taq polymerase used in PCR?
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It is heat-stable, so it is not denatured at about 95 °C and works at 72 °C.
Why do shorter DNA fragments travel further in gel electrophoresis?
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They move more easily through the mesh of the gel.
What is an STR?
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A short tandem repeat: a short sequence of non-coding DNA repeated many times, with a different number of repeats in different people.
Why does rigor mortis happen?
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ATP is no longer made, so myosin heads cannot detach from actin and muscles stay contracted.
Frequently asked questions
How do microorganisms recycle carbon?
Saprotrophic bacteria and fungi secrete extracellular enzymes that digest dead organisms and waste, then absorb the soluble products. They respire some of this, releasing carbon dioxide into the atmosphere. Producers can then take up this carbon dioxide in photosynthesis, so carbon returns to the living world.
How does PCR work?
PCR copies a section of DNA using repeated cycles. At about 95 °C the strands separate, at about 55 to 65 °C primers anneal, and at about 72 °C Taq polymerase builds new strands from free nucleotides. Each cycle doubles the amount of DNA, giving millions of copies after 30 cycles.
How does gel electrophoresis separate DNA?
DNA is negatively charged because of its phosphate groups. In an electric field it moves through an agarose gel towards the positive electrode. The gel is a mesh, so shorter fragments move faster and further than longer ones, separating the fragments by length into bands.
How is DNA profiling used to identify people?
DNA profiling compares the number of repeats in non-coding short tandem repeats. These are amplified by PCR and separated by gel electrophoresis, giving a pattern of bands. A match across several loci makes it very unlikely that two samples come from different people, so it can link a suspect to a crime scene.
How is the time of death of a body estimated?
Forensic scientists use several methods together: body temperature, rigor mortis, the extent of decomposition, the stage of succession of organisms on the body, and the species and age of insects such as blowfly larvae. Each depends on environmental conditions, so the result is an estimated range.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026