Forensic techniques

Question 1

Describe the stages of the carbon cycle

Answer 1

1. photosynthesizing plants remove CO2 from atmosphere
2. eating passes carbon compounds along the food chain
3. respiration in plants and animals returns CO2 to atmosphere
4. organisms die and decompose. Saprotrophs break down dead materials and release CO2 via respiration.
5. combustion of materials releases CO2

Question 2

What is a saprotroph?

Answer 2

An organism that feeds by extracellular digestion, e.g. fungi

Question 3

Describe extracellular digestion by saprotrophs

Answer 3

-release enzymes which catalyze the breakdown of dead plant and animal material into simpler organic matter.
-absorb the products of digestion

Question 4

Give some examples of organisms that play an important role in decay

Which two types of organisms

Answer 4

Detritivores - feed on dead organic matter
Saprotrophs - feed by extracellular digestion

Question 5

How can extent of decomposition help determine how long a body has been dead?

Answer 5

Bodies in similar environmental conditions show regular patterns of decay
fresh-bloated-decaying-dry

Question 6

How can DNA fragments be amplified in vitro?

Answer 6

Polymerase chain reaction (PCR)

Question 7

What is PCR (polymerase chain reaction)?

Answer 7

A biochemical technique used to amplify sections of DNA. It has many uses including forensics, where it is used to produce more DNA samples for genetic fingerprinting.

Question 8

What is Taq DNA polymerase?

Answer 8

A thermally stable enzyme that synthesizes a double-stranded molecule of DNA from a single template strand using complementary nucleotides.

Question 9

What does the creation mixture in the first stage of PCR contain?

Answer 9

-DNA fragment to be amplified
-Complementary primers to bind to start of fragment
-Free nucleotides to attach to exposed bases
-DNA polymerase to join nucleotides on new strand

Question 10

Summarize the process of using PCR

Answer 10

1. Heat to 95°C to break H bonds between DNA strands
2. Cool to 54°C to allow primers to bind
3. Heat to 70°C to activate DNA polymerase and allow free nucleotides to anneal
4. New DNA acts as template for next cycle

Question 11

How can DNA probes be used to locate specific alleles?

Answer 11

The probe is designed so that its sequence is complementary to the allele you want to find. They are labelled, amplified using PCR, then added to a sample of single stranded DNA. The probe will bind itself if the allele is present.

Question 12

What is gel electrophoresis?

Answer 12

A technique that separates nucleic acid fragments or proteins by size using electric current.

Question 13

How does gel electrophoresis work?

Answer 13

● DNA fragments of varying lengths are placed at one end
of a slab of agarose gel

● Electric current applied. DNA fragments move towards the
positive end of the gel as DNA has a negative charge

● Shorter fragments travel further. The pattern of bands
created is unique to every individual

Question 14

What is DNA profiling?

Answer 14

Comparing DNA sequences by cutting them into fragments and comparing the fragments with each other for genetic identification or determining genetic relationships.

Question 15

What can DNA profiling be used for?

Answer 15

Identifying individual organisms and determining genetic relationships.

Question 16

What are STRs?

Answer 16

Short tandem repeats. Sections of repeated nucleotides within introns that produce variation in individuals.

Question 17

How does DNA profiling work?

Answer 17

1) DNA is extracted and purified from the sample
2) Amplification using PCR (primers are used to
attach tags to STRs)
3) The DNA is then cut into small fragments using
restriction endonucleases
4) The DNA fragments are then separated using gel
electrophoresis and the banding pattern is
analysed

Question 18

Name 5 factors that can help determine time of death.

Answer 18

-seral stage of succession
-level of decomposition
-forensic entomology (types of insects in body)
-extent of muscle contraction
-body t°

Question 19

How can serial stage of succession help determine how long a body has been dead?

Answer 19

Predictable sequence of ecological succession. Different organisms colonies the corpse at each stage. Use a succession database.

Question 20

What is forensic entomology?

Answer 20

Determines age of insects on a corpse using known life cycles. Measured in accumulated day degrees, which represents time. This and knowledge of seral stages can accurately determine post mortem interval.

Question 21

How can body t° help determine how long a body has been dead?

Answer 21

Metabolic reactions stop, so body t° decreases at a predictable rate. Only applicable up to 24 hours after death since body reaches same t° as surroundings.