What you need to know
- 6.16–6.20Original mixed practice. Numbers are illustrative.
No mark allocations and no invented match probabilities.
By the end, you should be able to…
- Link extracellular digestionDigestion outside the cells: enzymes are secreted onto food and the products are absorbed. to carbon dioxide.
- Qualify every time-of-death claim.
- Calculate a theoretical polymerase chain reactionA method that makes many copies of a DNA sequence by repeated cycles of heating (denaturing), cooling (annealing primers) and extending with Taq polymerase. yield and read which fragment travelled further.
Build your understanding
Keep the topics in their lanes
Microbes and CO₂ are 6.16. Temperature and rigor are early 6.20. Decay, successionThe gradual change in the community of an area over time, as each set of species changes the conditions for the next. and insects are later 6.20. polymerase chain reactionA method that makes many copies of a DNA sequence by repeated cycles of heating (denaturing), cooling (annealing primers) and extending with Taq polymerase. is amplification. The gel is length. The profile is the pattern used for identity or relatedness, including plants and animals.
One calculation
If efficiency is 100 percent, copies = start × 2^n. Illustrative: 1 molecule, 8 cycles, 256 copies. A 100-base-pair fragment travels further than a 1000-base-pair fragment toward the positive electrode.
Make your reasoning visible
Say at least
Insect evidence is a minimum time since colonisation unless arrival at death is given.
Anode
Positive electrode. DNA is negative.
Avoid this misconception
The claim: polymerase chain reactionA method that makes many copies of a DNA sequence by repeated cycles of heating (denaturing), cooling (annealing primers) and extending with Taq polymerase., a gel and a cold body together prove a named person died at a named minute.
The correction: They are three different measurements, each with a limit. Cooling is a range, PCR can be contaminated, and a profile needs many clean loci.
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
State the role of extracellular enzymes in decomposition.
Show suggested answer
They hydrolyse large molecules in dead matter into soluble products that the microorganism can absorb.
Practice question
Retrieval and explanation · suggested answer
Give one reason rigor mortis is eventually lost.
Show suggested answer
Muscle proteins are broken down by enzymes and microorganisms, so the locked bridges no longer hold the muscle stiff.
Practice question
Retrieval and explanation · suggested answer
Theoretical PCR: 2 molecules, 6 cycles, full efficiency. How many copies?
Show suggested answer
2 × 2^6 = 128.
Practice question
Retrieval and explanation · suggested answer
A child’s band is longer than both bands of one proposed parent and matches the other parent only at a second locus. Comment in one sentence.
Show suggested answer
One matching locus is not enough, and a band that fits neither allele of a parent at the first locus weakens the claim that this adult is a parent.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Illustrative scene: ambient temperature was not recorded, blowfly larvae are present, and a plant profile from a leaf in the hand matches a rare cultivar at eight loci with clean controls. What can you say about time, and what can you say about the plant? What can you not join into one claim?
Show a hint
Split the questions the data can answer.
Explore the reasoning
Time of death is poorly constrained: no ambient temperature for cooling or for insect development, so the larvae are only a sign that flies colonised, not a number of days. The leaf can be linked to that cultivar because eight clean loci match and the controls worked. You cannot say the cultivar tells you the time, or that the larvae identify the plant. Figures and the cultivar are a teaching scenario, not a case.
Recall this lesson
Lesson recall questions
Question 1 of 5: State the role of microorganisms in the decomposition of organic matter.
Microbes, decay and carbon
Practice question
Retrieval and explanation · suggested answer
State the role of microorganisms in the decomposition of organic matter.
Lesson: 6C 1: Microorganisms and Decomposition
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Methods first
6C · Exam PracticeNIST frames forensic science as scientific work on evidence. Use that as the tone of a 6C answer: state the measurement, state the limit, do not invent a…Specification 6.20Read & use the scienceClose article
Why this matters
NIST frames forensic science as scientific work on evidence. Use that as the tone of a 6C answer: state the measurement, state the limit, do not invent a certainty the data do not have. A method becomes evidence only when the sample, controls and limits are described. Separating identity from time-since-death questions prevents a plausible-sounding but invalid conclusion. A test can be technically correct while answering the wrong biological question entirely.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.20 understand how to determine the time of death of a mammal by examining the extent of decomposition, stage of succession, forensic entomology, body temperature and degree of muscle contraction
Use the science
Which of PCR, cooling and an insect stage is an identity test?
Explore a possible response
None of them. Identity is the profile from fragment lengths. PCR only makes copies. Cooling and insects speak to time, and only as ranges.
Explore the source
A time-of-death estimate is a range
6C · Forensic Science and the Time of DeathA review of forensic entomology stresses that insect evidence is influenced by temperature, species and the time when insects first reached a body. Investigators may estimate a minimum…Specification 6.20Read & use the scienceClose article
Why this matters
A review of forensic entomology stresses that insect evidence is influenced by temperature, species and the time when insects first reached a body. Investigators may estimate a minimum interval since colonisation, which is not automatically the time since death. Cooling and rigor mortis offer other clues, but both depend on surroundings and individual variation. The forensic scientist’s job is to combine methods and state uncertainty honestly. Television often compresses this into a precise hour; a responsible report explains assumptions and a supported range. The specification names several indicators because no single biological clock is perfect.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.20 understand how to determine the time of death of a mammal by examining the extent of decomposition, stage of succession, forensic entomology, body temperature and degree of muscle contraction
Use the science
Why can the oldest larva provide a minimum interval since colonisation without proving the exact hour of death?
Explore a possible response
Insects may arrive after death, and their development rate depends on species and temperature; the larva dates its own growth, not the death itself.
Explore the source
- A review of postmortem intervals and forensic entomologyMedicine, Science and the Law · Abstract
A DNA band is not a verdict
6C · DNA ProfilingNIST explains why a forensic DNA mixture is harder to interpret than a clean textbook gel. Several people may contribute DNA, a small amount can produce weak signals,…Specification 6.18 · 6.19Read & use the scienceClose article
Why this matters
NIST explains why a forensic DNA mixture is harder to interpret than a clean textbook gel. Several people may contribute DNA, a small amount can produce weak signals, and some alleles can drop out during testing. Electrophoresis still separates fragments by length; interpretation then compares several genetic markers and considers uncertainty. A crime-drama image of one matching band cannot identify a person by itself. The analyst must also ask whether the sample is relevant, whether contamination is possible and how strong the comparison is. A profile can support an inference about identity or relationship, not a direct conclusion about what action occurred.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.18 know how gel electrophoresis can be used to separate DNA fragments of different length
- 6.19 understand how DNA profiling is used for identification and determining genetic relationships between organisms (plants and animals)
Use the science
Why is one band of matching length much weaker evidence than a well-controlled multi-locus profile?
Explore a possible response
Many people can share one fragment length. Multiple independent markers and controls reduce coincidence and reveal mixture or contamination issues.