Forensics Flashcards
what are factors to estimate TOD
Extent of decomposition
Stage of succession
Forensic entomology
Body temperature of the deceased
The degree of muscle contraction
what is extent of decomposition and what should you look out for
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
what will the rate of decomposition be affected by
factors such as temperature and availability of oxygen
what is stage of succession
change in the types of organisms found in the dead body over time
what stages of succession would occur on body above ground
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
what is forensic entomology and what factors can affect this
study of insect colonies at different times after death
Drugs that may be present in the body
Humidity of the surroundings
Oxygen availability
Temperature
how is body temperature a factor
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,
what is degree of muscle contraction
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
what can DNA profiles be used to determine
genetic relationships between different organisms
how can DNA profile be created
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
what is PCR
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
where is pcr carried out
PCR machine, or thermal cycler, which automatically provides the optimal temperature for each stage and controls the length of time spent at each stage
what does each PCR reaction require
DNA or RNA to be amplified
DNA polymerase
Primers
Free nucleotides
Buffer solution
3 stages of PCR
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
what is the effect of each PCR
doubles the amount of DNA
what can you do after PCR is completed
DNA is treated with restriction enzymes and a fluorescent tag can be added; both in preparation for gel electrophoresis
what do restriction enzymes do to DNA
break the DNA up into fragments of different length
what do fluorescent tags do
enable the DNA fragments to be seen under UV light
briefly explain separation of DNA by gel electrophoresis
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
method for gel electrophoresis
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
how will DNA fragments move when connected to electrodes
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
what are probes
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
when were fragments of DNA produced
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
explain the role of DNA primers in the production of the amplified 345bp sequence
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)
describe how DNA profiling can be carried out to show difference in snake species
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
describe changes that occur inside body after first week of death
(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
devise an experiment using electrophoresis to compare amplified DNA of two types of human
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)
