6.1 Flashcards
What is DNA profiling also known as
Genetic fingerprinting
How does DNA profiling work
The genomes are split into exons and introns and the introns are used to form the STRs
Each short tandem repeat is removed from the genome
The number of times an STR has been repeated, is use to make comparisons between samples
What are exons
They are regions of DNA that are expressed (genes) and therefore are coding
What are introns
They are regions of DNA that are found in-between genes and therefore, are non coding and are not expressed (they are used in genetic fingerprinting)
What are short tandem repeats
They are made up of many short DNA sequences of 2 to 50 base pairs that are repeated within introns
How are STRs used in DNA profiling
STRs appear at the same loci in all individuals, however the number of times they are repeated between individuals differs, allowing comparisons to be made
How are STRs shown on genetic fingerprint samples
The more an STR has been repeated, the heavier it is the thicker the band and the less it travels down agarose gel
How are STRs removed from genomes
They are removed using restriction endonucleases which are enzymes found in bacteria. Each specific enzyme cuts the DNA at a certain point.
If there is an overhang of bases, it produces a sticky end
If there is no overhang, it is known as a blunt end
What are polymerase chain reactions used for
They are used to amplify (make more copies) of DNA when not enough DNA is available for fragments to be isolated using restriction endonucleases. It ensures there are sufficient amount for gel electrophoresis
How does a polymerase chain reaction take place
The DNA sample, DNA polymerase, DNA primers and free nucleotides are added to the reaction test tube
The sample is heated to 95 degrees
The sample is cooled to 55 degrees
The temperature is then raised to 70 degrees which is the optimum temperature for DNA polymerase to work
The replication is repeated around 20 to 30 times
During a polymerase chain reaction, why is the sample heated to 95 degrees
This breaks the hydrogen bonds between the strands
Usually the enzyme would denature at this temperature, but DNA polymerase is thermally stable
During a polymerase chain reaction, why is the sample cooled to 55 degrees
This allows the primers to bind to DNA
What is the role of the primers in a polymerase chain reaction
They act as scaffolding to allow the enzyme to bind and begin building a complementary strand
During a polymerase chain reaction, why doesn’t DNA polymerase denature when the sample is heated to 95 degrees
DNA polymerase is thermally stable
Explain the process of gel electrophoresis
The agarose gel provides a stable medium which the DNA fragments can move through
The gel is submerged in a buffer solution and connected to electrodes (produces a p.d.)
The - charged DNA fragments migrate through the gel according to their size and overall charge
Smaller fragments with a smaller number of STRs travel faster and therefore end up closer to the + electrode
A reference sample with fragments of a known length may be added as a reference or ‘marker’
Once the fragments have been separated into invisible bands, the sample is transferred to a nylon or nitrocellulose membrane
The DNA double strand splits and stick to the membrane
The membrane is placed in a bag with a DNA probe. The single stranded probe binds to fragments with complementary sequences
If the probe is radioactive, x-ray film is used to detect the fragments
If the probe is fluorescent, it is viewed using UV light
How is the time of death initially estimated
The core body temp is measured within the first 24 hrs either through the rectum or an abdominal stab
The rate of decreasing temp should follow a sigmoidal curve
Environmental conditions such as clothing, ambient temp, humidity, size/position of person however can affect the rate of decreasing temp
What is forensic entomology
It uses insect presence on or near the body to estimate time of death
Samples are collected and data about the species and size of insects is recorded
What is insect succession and how is it used to estimate time if death
Insects colonise cadavers in a predictable sequence known as insect succession
As each insect arrives in the body, it changes the conditions, making it attractive to other species
These insects come in waves until the body becomes a skeleton
The length of each wave depends on environmental conditions which surround the body
What is rigor mortis and how does it happen
It happens after death when the muscle cells become starved of oxygen and oxygen-dependent reactions,us has aerobic respiration, stop.
When these reactions stop, respiration becomes anaerobic, producing lactic acid. The production of this acid causes cell pH to drop, becoming more acidic and inhibiting enzymes which in turn, inhibits anaerobic respiration.
ATP is no longer produced and as a result, bonds between the muscle proteins become fixed
What is autolysis
It is part of decomposition when the body’s own enzymes from the gastro-intestinal tract and lysosomes breakdown cells.
What is the first sign of decomposition
Putrefaction - it results in greenish discolouration of the skin in the abdomen which spreads across the body turning the body a darker reddish/green and the purple/black
The colour is due to the production of sulphaemoglobin
When are post mortems performed and what is investigated
They are performed when death is sudden or unexpected, or the cause of death is unknown
Blood and tissue samples may be taken and tested for infection
An incision is made down the front of the body and organs are removed for investigation
Contents of the stomach are often analysed
Define pathogen
A disease causing micro-organism such as bacteria or a virus
Name three conventional ways of identifying a body
Wallets/identification cards
Dental records
Clothing
