Forensics

Question 1

what are factors to estimate TOD

Answer 1

Extent of decomposition
Stage of succession
Forensic entomology
Body temperature of the deceased
The degree of muscle contraction

Question 2

what is extent of decomposition and what should you look out for

Answer 2

established visually by looking at the appearance of a body that is decomposing

Decomposers break down cells and tissues over the course of a few days - skin will often appear greenish in colour

next stage of decomposition involves the breakdown of tissues and organs by micro-organisms over the course of a few days or weeks - process produces gases, such as methane, which will lead to bloating

A few weeks after death the remains of the soft tissues will turn to liquid which becomes visible as it leaves the body

Question 3

what will the rate of decomposition be affected by

Answer 3

factors such as temperature and availability of oxygen

Question 4

what is stage of succession

Answer 4

change in the types of organisms found in the dead body over time

Question 5

what stages of succession would occur on body above ground

Answer 5

Bacteria will be found in and on the dead body immediately after TOD

As tissue decomposition sets in it creates ideal conditions for flies to lay eggs

When tissue dries out over time flies will leave the body as they prefer a moisture-rich environment

Question 6

what is forensic entomology and what factors can affect this

Answer 6

study of insect colonies at different times after death

Drugs that may be present in the body
Humidity of the surroundings
Oxygen availability
Temperature

Question 6

how is body temperature a factor

Answer 6

Respiration and other metabolic processes produce heat in living organisms

Once a person dies metabolic reactions will eventually come to an end, process of cooling is known as algor mortis

Body temperature decreases by 1.5-2.0 °C per hour,

Question 7

what is degree of muscle contraction

Answer 7

muscle in body contract 4-6 hours after TOD leading to stiffening known as rigor mortis as a result of changes to proteins in muscle cells after death

Since no more oxygen reaches the muscle cells after death they will start to respire anaerobically, producing lactic acid lactic acid decreases the pH in the muscle cells, denaturing the enzymes that produce ATP Without ATP the myosin heads cannot be released from the actin filaments, locking the muscles in a contracted state

begin in smaller muscles and end in larger

Question 8

what can DNA profiles be used to determine

Answer 8

genetic relationships between different organisms

Question 9

how can DNA profile be created

Answer 9

Isolating a sample of DNA e.g. from saliva, skin, hair, or blood
Producing more copies of the DNA fragments in the sample using the polymerase chain reaction (PCR)
Carrying out gel electrophoresis on the DNA produced by PCR
Analysing the resulting pattern of DNA fragments

Question 10

what is PCR

Answer 10

The polymerase chain reaction - method of DNA replication / DNA amplification

It is used to produce large quantities of specific fragments of DNA or RNA from very small quantities

Question 11

where is pcr carried out

Answer 11

PCR machine, or thermal cycler, which automatically provides the optimal temperature for each stage and controls the length of time spent at each stage

Question 12

what does each PCR reaction require

Answer 12

DNA or RNA to be amplified
DNA polymerase
Primers
Free nucleotides
Buffer solution

Question 13

3 stages of PCR

Answer 13

Denaturation
The double-stranded DNA is heated to 95 °C which breaks the hydrogen bonds that hold the two DNA strands together

Annealing
The temperature is decreased to 50-60 °C so that primers can anneal to the ends of the single strands of DNA

Elongation / Extension
The temperature is increased to 72 °C, as this is the optimum temperature for Taq polymerase / DNA polymerase to form phosphodiester bonds and form complimentary strand of DNA

Question 14

what is the effect of each PCR

Answer 14

doubles the amount of DNA

Question 15

what can you do after PCR is completed

Answer 15

DNA is treated with restriction enzymes and a fluorescent tag can be added; both in preparation for gel electrophoresis

Question 16

what do restriction enzymes do to DNA

Answer 16

break the DNA up into fragments of different length

Question 17

what do fluorescent tags do

Answer 17

enable the DNA fragments to be seen under UV light

Question 18

briefly explain separation of DNA by gel electrophoresis

Answer 18

DNA fragments are inserted into a well at the end of a piece of agar gel, before a current is passed through the gel

Molecules move through the agar due to the difference in charge across the gel

Question 19

method for gel electrophoresis

Answer 19

agarose gel plate in a tank and cut wells into the gel at one end
Submerge the gel in a tank containing electrolyte solution
Pipette the DNA samples into the wells using a micropipette
Connect the negative electrode to the end of the plate with the wells and connect the positive anode at the far end
Probes are then added, after which an X-ray image is taken or UV-light is shone onto the paper producing a pattern of bands which can be compared to the control. or standard, fragments of DNA

Question 20

how will DNA fragments move when connected to electrodes

Answer 20

towards the anode due to the attraction between the negatively charged phosphates of DNA and the anode
The smaller mass / shorter pieces of DNA fragments will move faster and therefore further from the wells than the larger fragments

Question 21

what are probes

Answer 21

single-stranded DNA sequences that are complementary to the regions of interest

A radioactive label which causes the probes to emit radiation that makes the X-ray film go dark, creating a pattern of dark bands

A fluorescent dye which fluoresces when exposed to UV light, creating a pattern of coloured bands

Question 22

when were fragments of DNA produced

Answer 22

after PCR by cutting the DNA samples into pieces using restriction enzymes
Restriction enzymes cut DNA at specific locations in the DNA, so will always cut in between sections of repeated bases known as variable number tandem repeats (VNTRs)
Different people have different numbers of repeats in their VNTR regions, so the fragments will differ in length depending on whether there are few or many repeats

Question 23

explain the role of DNA primers in the production of the amplified 345bp sequence

Answer 23

primers have a specific base sequence
(1)

bind to complementary bases (at either end) of the DNA be amplified
(1)

therefore, provide a site for the DNA polymerase to bind (1)

Question 24

describe how DNA profiling can be carried out to show difference in snake species

Answer 24

DNA obtained from the two types of grass snake (1)
{ PCR / restriction enzymes } used to produce fragments of DNA
(1)
gel electrophoresis used to analvse the DNA samples
(1)
(gel electrophoresis ) used to separate the fragments of DNA (1)
more differences in the pattern of bands produced would indicate that the snakes are different species

Question 25

describe changes that occur inside body after first week of death

Answer 25

(body) temperature falls (1)
rigor mortis / stiffening of the muscles (1)
autolysis / break down of cells by enzymes in the body (1)
putrefaction / discolouration / bloating

Question 26

devise an experiment using electrophoresis to compare amplified DNA of two types of human

Answer 26

restriction (enzymes / endonucleases } used to cut DNA (frommodern humans and Neanderthals) into fragments (1)

DNA samples are loaded onto (agarose) gel
(1)

(electric current is passed through / potential difference isapplied across)
the gel (1)

markers are added to visualise the bands (1)

the position of the bands produced can be compared (1)