Notes: 6.1 - 6.4 + 6.10 Forensics

Question 1

How can we determine time of death ?

Answer 1

1. Body temperature
2. Degree or muscle contraction (rigamortis)
3. Stage of succession
4. Level of decomposition
5. Forensics entomology (types of insects in the body)

Question 2

Blurt everything you know about Decomposition.

Answer 2

• Digestive enzymes are released (e.g bacteria and fungi) when an organism dies.
• Enzymes begin to break down body tissue into smaller molecules.

Question 3

Name the stages of decomposition

Answer 3

1. Autolysis - Body’s own enzymes, break down cells.
2. Bloating and putrefaction - Skin turns a greenish discoloration. When the enzymes respire they release carbon dioxide and methane that cause bloating.
3. Active decay - breaking down of soft tissue
4. Dry decay - When all the soft tissue has decayed, so you have the skin and cartilage left and worms and other organisms come and break them down.
5. Skeletonisation - The body tissues have been completely broken down and only the bones remain.

Question 4

What factors affect the rate of decomposition

Answer 4

• Temperature
• Oxygen availability : E.g. aerobic microorganisms need oxygen, so decomposition could be slower if there’s a lack of oxygen

Question 5

Blurt everything you know about Body temperature

Answer 5

• Human body has an internal temp of around 37°C.
• When a person dies metabolic reactions slow down.
• This will cause body temperature to fall until it equals the temperature of its surroundings.
• Human body cools at a rate of 1.5°C to 2°C.
• so from the temperature of a dead body they can work out the approximate TOD.

Question 6

What factors affect body temperature.

Answer 6

• Body size : More body fat = More insulation = Cooling slower.
• Body position : Compact = Cools slower. Spread out = Faster.
• Clothing : More clothing = Cooling slower
• Air movement : More wind = cools faster
• Humidity : More humid = Cools slower
• Temperature of surroundings : Hotter = Cools slower.

Question 7

How do we measure internal body temperature.

Answer 7

Using a long thermometre in the rectum.

Question 8

What is the reduction in body temp also know as ?

Answer 8

The reduction in body temperature is known as *algor mortis**.

Question 9

Blurt everything you know about Rigor mortis

Answer 9

• After death muscle cells become deprived of oxygen.
• Cells start to respire via aerobic respiration
• This respiration produces lactic acid
• Lactic acid is acidic = Produces H⁺ ions = pH of cells fall.
• Inhibiting enzymes that produce ATP to produce ATP.
• No ATP means the bonds between the myosin and actin in the muscle cells become
  fixed and the body stiffens.

Question 10

What does rigor mortis mean ?

Answer 10

The stiffening of the body.

Question 11

Time stamps of rigor mortis

Answer 11

• 4 to 6 hours after death, rigor mortis takes place.
• 12 to 18 hours every muscles will be stiff
• 24 to 36 hours muscle will loosen.

Question 12

What factors affect rigor mortis

Answer 12

• Degree of muscle development
• Temperature : Higher temps will speed up the rate of rigor mortis, because the chemical reaction in the body are faster.

Question 13

Blurt everything you know about Forensics entomology

Answer 13

• When a person dies they will be colonised by insects.
• TOD can be estimated by identifying the type of insect present on the body : Flies will be found on the body within a few hours after death, while beetles will only colonise the body later.
• TOD can also be estimated by identifying the stage of life cycle the insect is in :
  
  • Blowfly eggs will hatch after about 24 hours so if larvae are present on the body it indicates that the person died more than 24 hours ago.

Question 14

Blurt everything you know about Stage of succession

Answer 14

• The types of organism found in a dead body change over time, going through a number of stages
  — this is called succession.
• Bacteria are the first type of organism present on the human body - immediately after death the conditions in a dead body are most favourable to bacteria
• As bacteria decompose and digest body tissues, the conditions become more favourable for flies. The flies will lay eggs in the body which will hatch into larvae.
• The fly larvae break down tissues of the corpse further, resulting in conditions which are suitable for beetles.
• The body begins to dry out, which results in flies leaving the body as they prefer more moist environments. Beetles remain on the body since they can digest dry tissue.
• When the body has been fully decomposed, there is no material for the insects to feed on so any remaining insects will leave.

Question 15

How is succession for plants different for succession to animals.

Answer 15

Succession in a dead body is similar to plant succession. — the only difference is
that most of the early insects (e.g. beetles) remain on the body as other insects colonise it.

Question 16

Factors affecting animal succession.

Answer 16

• Location of the body : different stages of succession will be seen depending on whether the body is in open air or submerged in water. This affects weather or not insects can access the body.

Question 17

Blurt points from DNA theory.

Answer 17

• A DNA profile is a genetic fingerprint.
• Everyone’s DNA is different except identical twins.
• DNA profiling can be used to identify people and determine genetic relationships between humans, between animals, between plants.

Question 18

How can a DNA sample be obtained ?

Answer 18

Blood or saliva

Question 19

What is PCR used for ?

Answer 19

To amplify DNA

Question 20

Explain the stages of PCR

Answer 20

1. A reaction mixture is prepared that contains : DNA Sample, Free DNA nucleotides, Primers, DNA polymerase.
2. The DNA mixture is heated to 95°C - this is to break the hydrogen bonds between the 2 strands of DNA.
3. The mixture is then cooled to between 50 - 65 °C. - This is so that the primers can bind (anneal) to strands.
4. The reaction mixture is headed to 72°C, which is optimum temp for DNA polymerase.
5. DNA polymerase forms a new DNA strand from catalysing the formation of phosphodiester bonds between the free DNA nucleotides which align along the DNA template strand by complementary base pairing rules.
6. The cycle is then repeated

Question 21

For each cycle of PCR how much DNA is produced ?

Answer 21

Each PCR cycle doubles the amount id DNA.

Question 22

What happens after PCR ?

Answer 22

A fluorescents tag is added to all the DNA fragments so they can be viewed under UV light.

Question 23

Blurt everything you know about gel electrophoresis.

Answer 23

• Is used to separate out DNA fragments according to their length. This works as DNA is neg charged meaning it moves towards a positive charge when placed in an electric field.
• Shorter DNA fragments travel through gel more quickly, They will travel longer distance than the DNA fragments.

Question 24

Explain how you would carry out gel electrophoresis.

Answer 24

• An agarose gel is prepared. Which contains a row of wells at the top of the gel.
• The gel tray is then placed into a tank. Ensure that the wells are closest to the cathode the gel box
• Add buffer solution into the tank to ensure that the gel is covered in the buffer solution.
• Mix the DNA sample with a loading dye.
• Then set your micropipette to a set volume.
• Using a micropipette load the wells with the DNA sample. ensure you use a clean micropipette time each time you load a well with a new sample.
• Put the lid on the gel box x and connect the leads from the gel box to the power supply.
• Turn the power supply on. - this causes an electrical current to be passed through the gel.
• The current will cause the DNA fragments to separate according to length
• Wait for 30 minutes
• Turn the power supply off.
• Remove the gel tray and stain the DNA fragments..

Question 25

How do we determine how closely related species are ?

Answer 25

• Through gel electrophoresis we can see how many different DNA fragments are present in a sample.
• We can then compare the size or position or thickness of the bands
• If they are closely related there will be more similarities.