FORENSIC APPLICATIONS OF DNA PROFILING

•	Outline the concept and principles of DNA profiling.
•	Outline the nature and purpose of paternity testing.
•	Describe the importance of DNA profiling to crime scene investigation.
•	Provide illustrative case studies.

Basis of DNA profiling

1.	DNA is the short form for deoxyribonucleic acid and is found in every nucleated cell of the human body.

2.	DNA is the seat of the genome and is responsible for the transcription and translation of all genes into proteins which makes up the human body.

3.	DNA is a continuous chain 3 billion base pairs long:

a.	Sections of the bases form genes which codes for the human body.

b.	There are several non-coding sections of DNA which are genetic loci used for DNA profiling.

4.	DNA profiling is a method of identification that compares fragments of DNA. With the exception of identical siblings, the DNA of each person is unique.

Every strand of DNA has pieces that contain genetic information which informs an organism's development (exons) and pieces that, apparently, supply no relevant genetic information at all (introns). Although the introns may seem useless, it has been found that they contain repeated sequences of base pairs. These sequences, called Variable Number Tandem Repeats (VNTRs), can contain anywhere from twenty to one hundred base pairs. 

Every human being has some VNTRs. To determine if a person has a particular VNTR, a Southern Blot is performed, and then the Southern Blot is probed, through a hybridization reaction, with a radioactive version of the VNTR in question. The pattern which results from this process is what is often referred to as a DNA fingerprint. 

Constructing a DNA fingerprint

1.	Extraction of the DNA from a sample such as hair, blood, saliva or a tissue sample. 

2.	Isolating the DNA in question from the rest of the cellular material can be done chemically, by using a detergent to wash the extra material from the DNA. It also can be done mechanically, by applying a large amount of pressure in order to "squeeze out" the DNA. 

3.	The sample is then segmented or cut up using restriction enzymes. 

4.	The DNA is then poured into a gel, such as agarose, or onto a sheet that has been coated with this special gel. 

5.	Gel electrophoresis:

a.	An electrical charge is applied to the gel, with the positive charge at the bottom and the negative charge at the top. 

b.	This process is called electrophoresis Because DNA has a slightly negative charge, all the DNA will be attracted towards the bottom of the gel. 

c.	However, the smaller pieces will be able to move more quickly and thus farther towards the bottom than the larger pieces. 

d.	The different-sized pieces of DNA will therefore be separated by size, with the smaller pieces towards the bottom and the larger pieces towards the top. 

6.	The DNA must be denatured, so that all of the DNA is rendered single-stranded. This can be done either by heating or chemically treating the DNA in the gel 

7.	The DNA must then be blotted. The gel with the size-fractionated DNA is applied to a sheet of nitrocellulose paper, and then baked to permanently attach the DNA to the sheet.

8.	Restriction Fragment Length Polymorphism

a.	The locus of interest is “highlighted” through the use of radioactive reagents and identified by hybridization.

b.	Each person has two bands at each locus.

c.	The two bands constitute the DNA profile of the person at that locus.

d.	The profile is usually developed on a X-ray film and forms a permanent hard copy of the profile. 

Processing DNA profiles for Crime cases

1.	In crime cases, the quantity and quality of DNA obtained is usually low and degraded.

2.	As DNA degrades, the RFLP locus are broken into shorter sections.

3.	Polymerase chain reaction or amplification develops DNA profile at shorter sections of DNA which are less likely to be affected by degradation. 

4.	The process of amplification involves copying these areas many times with the result that the quantity of targeted DNA is greatly increased.

5.	The amplified product is run in a gel and is separated by sizes.

6.	The products can be visualized by silver staining or laser-sensitive dyes, and dried to give a permanent copy of the results.

7.	These increase the sensitivity of the system 10-fold.

8.	The reagents can be labeled with different colored dyes specific for different locus and this allowed for simultaneous detection of different locus. 

Paternity testing

1.	At any locus, the DNA profile has two bands, one band which is inherited from the mother and the other band from the father.

2.	If the profile of the child is compared to the profile of the mother, the maternal allele can be identified.

3.	The other allele of the child must originate from the father.

4.	This principle is used in paternity testing to establish whether a man is the biological father of the child.

Crime scene investigation

1.	In crime cases, DNA profiling compares the bloodstain found at the crime scene with the blood sample of the suspect.

2.	Both bands of the profile must match as unlike paternity analysis where only one band of the child’s profile matches the patient’s profile.

3.	The DNA profile of from different parts of the body is identical as each person started from one cell.

4.	This forms the basis of forensic comparison where the blood sample from a rape suspect is matched against the semen left at the crime scene.

5.	When a body is burnt beyond recognition and the identity of the body cannot be established by other means (dental records) the identity of the body can be confirmed if the blood samples of the parents are available for DNA profiling. 

The first legal case involving the use of DNA fingerprinting was in 1984, in England and established a Ghanaian immigrant's relationship to her son. Since then the technique has become almost routine in forensic cases and is particularly suited to identification in rape cases.