Forensics (Edexcel A Level Biology (A) SNAB): Flashcards
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Define forensics.
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Define forensics.
Forensics is the use of science in the investigation of criminal activities, including the collection and analysis of evidence.
Name four factors used to estimate the time of death.
Any four of:
Extent of decomposition
Stage of succession and forensic entomology
Body temperature (algor mortis)
Degree of muscle contraction (rigor mortis)
Define algor mortis.
Algor mortis is the cooling of a body after death, as metabolic reactions stop and no more heat is produced.
Roughly how fast does a body cool after death, and why is this useful?
About 1.5–2.0 °C per hour.
Measuring body temperature lets forensic scientists estimate the time of death.
Define rigor mortis.
Rigor mortis is the stiffening of the muscles after death, caused by the muscles becoming locked in a contracted state.
Explain why rigor mortis occurs at the biochemical level.
Without oxygen, muscles respire anaerobically, producing lactic acid that lowers pH and denatures ATP-producing enzymes.
Without ATP, myosin heads cannot detach from actin, locking muscles contracted.
Define forensic entomology.
Forensic entomology is the study of the insect colonies on a dead body to help estimate the time of death.
How can blowfly larvae indicate a body has been dead more than 24 hours?
Blowfly eggs take about 24 hours to hatch.
So the presence of larvae means at least 24 hours have passed.
How does the succession of insects differ from ecological succession?
In a body, all newly arriving species remain as decomposition progresses.
In an ecosystem, early pioneer species are out-competed and disappear.
Why do flies leave a body but beetles remain over time?
As tissue dries out, flies leave because they prefer moisture.
Beetles can decompose dry tissue, so they stay.
The stiffening of muscles after death is called mortis.
The stiffening of muscles after death is called rigor mortis.
True or False?
The time of death can be determined exactly.
False.
Time of death is only an estimate — many factors affect a body, so it cannot be pinpointed precisely.
Define decomposer.
A decomposer is a microorganism, such as a bacterium or fungus, that breaks down dead plant and animal material.
How do decomposers break down dead material?
They secrete enzymes.
These break large organic molecules down into smaller ones such as glucose.
What gases do decomposers release into the atmosphere?
Carbon dioxide and methane.
How is the carbon released by decomposers returned to living organisms?
Carbon dioxide is absorbed by plants.
They fix it into carbohydrates during photosynthesis.
Which process do decomposers use to release carbon dioxide?
Respiration.
They respire the small molecules obtained from dead tissue.
Give two examples of decomposer organisms.
Bacteria and fungi.
Why is decomposition important for nutrient recycling?
It releases nutrients locked in dead organisms.
These are recycled back into the ecosystem, e.g. carbon returned to the atmosphere for plants.
Decomposers secrete to break large organic molecules into smaller ones.
Decomposers secrete enzymes to break large organic molecules into smaller ones.
True or False?
Decomposers digest their food inside their bodies before absorbing it.
False.
They secrete enzymes to digest food externally, then absorb the small molecules produced.
What happens to the small molecules, such as glucose, that decomposers produce?
They are broken down further during respiration to release energy.
Define decomposition.
Decomposition is the breakdown of dead organisms by decomposers, which recycles nutrients back into the ecosystem.
True or False?
Decomposition returns carbon to the atmosphere as carbon dioxide.
True.
As decomposers respire, they release carbon dioxide back into the atmosphere.
Define polymerase chain reaction (PCR).
PCR is an in vitro method of amplifying DNA — making many identical copies of a DNA fragment.
Name the three stages of one PCR cycle.
Denaturation, annealing and elongation (extension).
What happens during the denaturation stage of PCR?
The DNA is heated to 95 °C.
This breaks the hydrogen bonds, separating the two strands.
What is the role of primers in PCR?
Short single-stranded DNA sequences that are complementary to the ends of the target region.
They mark where DNA polymerase should start building the new strand.
Why is Taq polymerase used in PCR?
It comes from a thermophilic bacterium, so it does not denature at the high temperatures used.
What happens to the amount of DNA in each PCR cycle?
It doubles.
So 20 cycles produce about a million copies.
In gel electrophoresis, why does DNA move towards the anode?
DNA is negatively charged due to its phosphate groups.
So it is attracted to the positive electrode (anode).
In gel electrophoresis, how are DNA fragments separated?
By size.
Smaller fragments move faster and further through the gel pores than larger ones.
What is the purpose of the DNA standard loaded in the first well?
It gives a set of known results.
New samples can be compared against it.
How are the separated DNA bands made visible?
Probes are added — radioactive labels (seen on X-ray film) or fluorescent dyes (seen under UV light).
Why is each person's DNA profile (banding pattern) unique?
People have different numbers of repeats in their VNTR regions.
This gives fragments of different lengths, and so a unique pattern of bands.
How is a DNA profile used in a paternity test?
The child's profile is compared with candidate fathers'.
A child shares half their bands with each parent, so the candidate with many matching bands is the likely father.
PCR amplifies DNA, doubling the amount in each .
PCR amplifies DNA, doubling the amount in each cycle.
True or False?
In DNA profiling, the more bands that match between two profiles, the closer the genetic relationship.
True.
More matching bands means greater genetic similarity and a closer relationship.
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