(Obtained from www.anselm.edu/.../genbio/geneticsnot.html)
(The diagram above shows that the defendant had the victim’s blood on his clothes)
Web Description: A method of comparing the genetic similarities or differences between individuals. This technology is often used as a forensic tool to identify the source of blood and tissue samples found at crime scenes. (http://www.theworkforcepartnership.com)
Science behind DNA
DNA- Deoxyribonucleic Acid is a long chain molecule made up of many nucleotides. A Nucleotide contains 3 molecules held together by condensation reactions, these molecules are: Phosphate group, Deoxyribose and an Organic base. www.biology.arizona.edu/.../06t.html
These nucleotides are linked between then the sugar group of one nucleotide and the phosphate group of another. The base containing nitrogen is the only part of the nucleotide which changes. They are interchangeable by the 4 bases, Adenine, Cytosine, Guanine and Thymine. These bases form the double helix backbone of a DNA strand. http://www.scq.ubc.ca/a-monks-flourishing-garden-the-basics-of-molecular-biology-explained/ The sequence of these bases is what determines our characteristics. Each individual has 99.9% of the same DNA; it is only identical twins that have exactly the same pattern. It is because of this 0.1% we can identify which person a piece of DNA has come from. This is what DNA fingerprinting works from, it screens for certain DNA sequences which are found in some persons but not others.
What Is DNA Fingerprinting?
DNA fingerprinting is a procedure which can distinguish the difference in DNA from certain persons without having to observe the complete 3 billion bases in the whole genome. It is called fingerprint as it is improbable that any two people will have the same fingerprint.
How Is It Done?
The way in which the cells are obtained depends on what is being tested. For example if it is for a paternity test blood will be drawn from the veins. Also DNA can be obtained from saliva which is taken using a cotton swab wiped against the inside of the cheek.
Firstly the DNA is removed from the cells using the detergents on the cell membrane. Cell membranes are coated in detergents and this dissolves the lipids of the cell membrane exposing the protein and nucleic acids. Restriction enzymes are added and each recognize a precise sequence and cut it away. Restriction enzymes will cut the DNA at a specific sequence. Restriction enzymes are naturally in bacteria where they are used to cut up attacking viral DNA, the enzymes hold the value that they will only cut the DNA at certain base sequences. For example the enzyme EcoRI comes from the bacteria Escherichia coli strain RY13 and the DNA is split wherever the base sequence is GAATTC. This then produces different lengths of sequences which then need to be ordered according to their lengths. Agarose Gel Electrophoresis is used to do this. DNA is a negatively charged particle so it will shift in the direction of a positive electrode in an applied electric field. A charged particle is a particle which is either positive or negative. The fragments travel in this electric field in accordance with their size. The gel slows down the larger particles but the smaller particles move quicker therefore the sequences are arranged in order. www.biotech.iastate.edu/.../bio6.html
Southern Blotting is then used to transfer the fragments to a nitrocellulose filter. The filter is placed over the gel and the DNA fragments are then absorbed to. The paper acts as a wick to extract an alkaline buffer solution all the way through the gel. The strands then break up revealing the base sequences.
Chemical probes are then added and they contain the complementary bases for the bases which...