Forensic Science - DNA

DNA Profiling

DNA profiling is extensively used in forensic science to identify individuals , criminals in particular , to samples of human tissue or fluids found at crime scenes . All humans will have a majority of their DNA material in common , and DNA profiling is what is used to separate the portions of DNA that is unique to an individual

Electrophoresis

Electrophoresis , in general , is the migration of a charged particle under the influence of an electric field . In the context of DNA forensics , electrophoresis is the process of separating and sorting DNA

br fragments , by passing an electric current through a block of gel (usually polyacrylamide , which has a high resolving power ) containing said DNA fragments at one end , thus creating a DNA pro

DNA strands are broken into these fragments by introduction of a restriction enzyme , which makes one cut on each of the two phosphate backbones of the DNA double helix , on portions of the helix that contain a recognition sequence ' a particular nucleotide sequence that the restriction enzyme reacts to (Restriction Enzyme , 2006

The polyacrylamide gel , which is the most commonly used in actual practice , is a cross-linked polymer of acrylamide , a potent neurotoxin (polyacrylamide itself is non toxic . It is a network of polymer chains similar to a (compressed ) sponge , through which the DNA fragments will migrate (Polyacrylamide , 2002

Electrophoresis depends on this property of the gel to separate the DNA fragments into groups - the smaller fragments will migrate faster , while the larger fragments will catch ' on the network of polymer chains and will thus migrate slower . The fragments thus become grouped according to size , and a DNA pro is obtained (Polyacrylamide , 2002 . This technique is important because the grouping and location of these bands on the gel is unique to the individual (from which the DNA came from and can be used to identify an individual

After the electrophoresis is completed , a dye such as methylene blue is used to visualize the bands (Gel Electrophoresis , 2006

PCR

In some cases , electrophoresis may not be immediately feasible because of a very limited DNA sample in such cases polymerase chain reaction (PCR ) will be useful . PCR is a molecular biological technique that can duplicate specific regions of DNA with accuracy , usually within a few hours . This is useful in cases where only a tiny sample of DNA was obtained and there is a need to develop a DNA pro

To use PCR to duplicate DNA , the DNA sequences at both ends of a strand need to be known . The DNA is duplicated in a thermocycler in the presence of the Thermus aquaticus (Taq ) polymerase and sequence-specific primers of DNA (Slish

The process starts with a gene or segment of DNA , which is denatured (its strands separated ) at 94 ?C . The temperature is then lowered to 45-55 ?C , at which the primers , complementary to the freed ends of the DNA strands , anneal , or attach themselves to their complementary sequence on the DNA strands , serving as catalysts for duplication of the original DNA double helix . Once annealed , DNA polymerase extends the primers at 72 ?C , copying the sequence of the strand . This results in doubling of the amount of DNA per cycle , which takes about two minutes each (Polymerase Chain Reaction , 2006

The thermocycler repeatedly raises and lowers the temperature , which causes the DNA molecules to copy themselves . Within a short time thousands of copies of the target sequence are produced (Rabinow , 1998

Difficulties in PCR

Some difficulties of PCR are : the reaction is very sensitive to divalvent cations and nucleotides proper primer design is of utmost importance for effect amplification - the primers need to be very specific possible reactivity with non-target DNA sequences primers must not be able to anneal to themselves or each other the size of DNA molecules to be amplified is limited polymerase errors - the Taq polymerase can make mismatches when incorporating new bases into a strand and even very small contaminations of unwanted DNA can ruin the results (Slish

References

Gel Electrophoresis . 2006 , Wikipedia , Wikimedia Foundation . Available at : http /en .wikipedia .org /wiki /Gel_electrophoresis

Polyacrylamide Gel Electrophoresis . Chemsoc . Available at http /www .chemsoc .org /ExemplarChem /entries /2002 /proteomics /page .htm

Polymerase Chain Reaction . 2006 . Wikipedia , Wikimedia Foundation Available at : http /en .wikipedia .org /wiki /Polymerase_chain_reaction

Rabinow ,

. 1998 . What is PCR ? Berkley Digital Library Sunsite Available at : http /sunsite .berkeley .edu /PCR /whatisPCR .html

Restriction Enzyme . 2006 , Wikipedia , Wikimedia Foundation . Available at http /en .wikipedia .org /wiki /Restriction_enzyme

Slish , D . The Polymerase Chain Reaction . Plattsburg State University Available at

http /faculty .plattsburgh .edu /donald .slish /PCR .html ...