Investigating time of death:
As soon as a person dies, chemical changes occur in order, so can be used to estimate the time of death.
Rigor mortis/degree of muscle contraction:
· Begins when muscle cells become deprived of oxygen
· Respiration is anaerobic, causing the build up of lactic acid in muscles
· The pH decreases due to the lactic acid, inhibiting enzymes to produce ATP
· ATP is required to keep muscles relaxed, so no ATP means the bonds become fixed, the muscles contract so the body stiffens
· Smaller muscles contract first
· Begins about 2 – 4 hours after death, full effect is about 6 – 8 hours. It passes at about 36 – 48 hours after death
· The bodies stages in rigor mortis can give a rough outline of time of death
· Metabolic reactions slow down so the core body temperature cools slowly from around 37oC to room temperature over time.
· If the temperature of the room is known, it is possible to create a cooling curve so discover the time of death
· The body reaches room temperature after about 18 hours
· Body temperature can be influenced by – body size, body position, clothing, air movement, humidity
Extent of decomposition
After death, tissues break down due to the action of enzymes.
Autolysis occurs – the loss of oxygen in tissues favours the growth of anaerobic bacteria.
Bodies usually follow a standard pattern of decay. Enzymes in the gut start to break down the wall of the gut and the surrounding area. As cells die, they release enzymes which help break down tissues.
The signs of decomposition such as decolouration of the skin (putrefication) and gas formation, combined with environmental conditions allow time of death to be estimated.
The warmer the environment, the faster the decay
Injuries also allow bacteria to enter, aiding decomposition
· After a few hours – cells and tissues are broken down by enzymes and bacteria present before death – turns skin green
· Few days-weeks – microorganisms decompose tissues and organs – produces gasses which cause bloating. Skin blisters and falls off
· Few weeks – tissues liquefy and seep into the area around the body
· Few months – years – skeleton remains
· Decades – centuries – skeleton disintegrates – nothing left
2 main ways – succession and life cycles of insects
As a body decays, the populations of insects on it change – there is a succession in species. The community of species present when the body is found allows the stage of succession to be determined and the time of death estimated.
· A dead body is a newly exposed habitat
· Anaerobic bacteria thrive in the no oxygen and acidic (lactic acid) conditions
· Certain flies, such as blowflies arrive, they are attracted to the moisture, smell and open wounds. They lay eggs on the carcass
· The eggs hatch, maggots eat the skin and tissue of the body, this liquidises certain parts which the adult flies feed on
· Beetles are attracted, they lay eggs and the grub that hatch eat the maggots
· Parasitic wasps lay eggs in the beetle and fly larvae
· Eventually the body dries out and species such as cheese and coffin flies are abundant
· Dehydration continues, maggots can’t survive. Beetles with strong mandibles, such as carcass beetles move in and eat the remaining muscles and connecting tissues
· Finally mites and moth larvae digest the hair
Forensic entomologists see what species are living on the body, therefore know how long down the line of succession of insects the body is – using this they can estimate the time of death.
The season, weather, size and location of the body will influence the type and number of species present.
· Insects go from eggs, which hatch to larvae, turn into a pupa and back into an adult
· When a forensic entomologist finds a body, they collect the eggs and larvae and pupa and let them grow into adults
· From the stage the insect was the found and the fact that insects have different life cycles for each stage, the entomologist can tell how long the insects have been there. This linked with succession can give a more accurate time of death
· Egg (1 day) à larva (9 days) à pupa ( 6 – 12 days)
Using forensic entomology, the date of death can be confirmed to a few days or theorised to a few months, but is mostly used for bodies 4 – 14 days old.
Polymerase Chain Reaction (PCR):
Allows small samples of DNA to be amplified for use during DNA profiling, meaning that forensics only need small DNA samples such as a strand of hair.
A cycle of temperature changes results in huge numbers of DNA fragments being produced.
1) Place a mixture of enzymes, primers, DNA and reactants in a vial
2) Place vial in a PCR machine/thermal cylinder
3) Heat to 90-95oC for about 30 seconds, this separates strands of DNA
4) Heat to 50 – 60oC for about 20 seconds, this binds/anneals primers (short DNA sequences complementary to DNA adjacent to STR) to DNA strands. The DNA primers are marked with a fluorescent tag.
5) Heat to 75oC for at least a minute, DNA builds complementary DNA strand
6) Repeat steps 3 – 5 as much as wanted
Each PCR cycle doubles the amount of DNA present
DNA profiling/DNA fingerprinting:
· To identify genetic information
· Everyone’s DNA is unique because of the variety of the DNA sections not used to code for proteins (introns)
· Look for short repeated sequences in introns (short tandem repeats)
· Mini satellites contain 20 – 50 base pairs
· Micro satellites contain 2 – 4 base pairs
· Can be several hundred copies of STR at a single locul – people vary
· The STR at many loci build up a unique pattern for that individual
Double stranded DNA and restriction enzymes.
1) DNA is cut into fragments of double stranded DNA using restriction enzymes (restriction endonucleases). They carry a negative charge
2) Fragments of double stranded DNA are loaded into the walls of an agarose gel in a tank – gel electrophoresis and is submerged in a buffer solution
3) Negatively charged DNA moves towards the positive electrode. These fragments separate into individual bands
4) Fragments move at different rates according to their size and charge. Small fragments with fewer satellite repeats travel faster (pass through gel quicker) and end up closer to the electrode after a set time
5) DNA is transferred to the nylon membrane using southern blotting
6) The membrane is washed with a buffer solution and labelled DNA probe which binds to the repeated sequence producing visible bands. Single stranded DNA probe binds to fragments with a complimentary sequence
7) Shown by X ray film or UV light
8) Only those that bound to the probe show up – this resulting image is called a DNA fingerprint
9) The DNA profile is then compared to a reference, e.g. a suspect or relative
Fingerprints can be taken, which are unique to an individual.
Fingerprints are taken using fine aluminium powders.
Once obtained, the main type of fingerprint can be classified – an arch, tented arch, whorl or a loop. Arch patterns are rare, the loop is most common.
Can be used if someone has no fingerprints on file or on the body.
Teeth and fillings decay slowly, they are resistant to burning. A forensic dentist makes a chart of the teeth, including dental work, fillings and missing teeth. This is compared to dental records. The forensic dentist may also look at the development of the teeth to determine the age.
Summary of forensics
· When: rigor mortis, decomposition, forensic entomology, temperature
· Who: personal ID, finger prints, dental records, DNA profiling