Let me start off this discussion with what DNA is all about, because you won’t understand DNA testing unless you know what DNA is in the first place.
DNA is short for deoxyribonucleic acid, a substance which is present in sizeable amount in the nucleus of all nucleated cells of the body.
A person’s DNA molecules are organized in the nucleus into structures called chromosomes, of which there are 23 pairs in humans. Each chromosome contains numerous genes — a segment of the DNA strand of variable length where the code for the production of a specific protein resides. Humans have between 30,000 to 40,000 genes. The total amount of DNA present in all the 23 pairs of chromosomes comprises the human genome.
The human genome contains the person’s genetic code — the set of instructions that governs the production of the numerous proteins that the cells of the body need for their metabolic growth and development and reproductive activities. In effect, the genetic code determines the physical and intellectual characteristics, physiological responses and functions, and to a certain extent, the behavior of a person. It is written in code with DNA molecules serving as the alphabet. A copy of the genetic code is carried by all the cells of the body (except for the red blood cells which have no nucleus).
Structurally, a DNA molecule is like a twisted ladder. The two sides of the ladder are made up of sugar and phosphate molecules, and they are connected by rungs of nitrogen-containing bases. The human genome contains three billion nitrogen-containing bases in its DNA molecules, but there are only four kinds of bases: Adenine (A), thymine (T), cytosine (C) and guanine (G). The code for the numerous proteins that are written in the chromosomes differ not in the kind of bases that are present in them but in the particular order of the bases (called DNA sequence) along the double stranded sugar-phosphate backbone.
A person’s DNA sequence is uniquely his/hers. The complicated process that occurs when the egg cell (of the mother) and the sperm cell (of the father) divide before, and unite during fertilization, ensures this. The probability that two living persons (except for identical twins) have identical DNA sequence is practically zero.
It is not yet possible to completely map out a person’s DNA sequence, but it is now possible by a procedure called DNA testing to obtain segments of a person’s DNA sequence from some chromosomes and compare these with those of another person or with those of a particular population. This procedure has proved very useful for identification purposes. It is a more accurate method of identification than fingerprinting, blood typing or any other identification procedure that is presently being employed.
To undertake DNA testing, one only needs some nucleated cells from any body tissue such as a single root of hair, saliva, urine or semen as sample.
In forensic medicine, DNA testing has been very helpful in exonerating crime suspects. In investigating crimes, segments of the DNA molecule of a suspect are matched with those found in the crime scene and then the results are statistically analyzed. The test cannot categorically say that the sample came from the suspect, but it can tell with certainty if the sample did not come from the suspect. Hence, it is more useful in acquitting than in convicting suspects.
In the same light, it is now possible to compare segments of the DNA molecule of one person to those of a population and identify genetic markers that are shared uniquely by specific populations or family groups. Statistical analysis of this comparison can resolve questions regarding a person’s ancestry.
In addition to accuracy, DNA testing has other advantages over other identification procedures. DNA is resistant to degradation. DNA testing can still be done even if the sample has been treated with detergents, acids, bases, oil-based products, salt, and even if it has been contaminated by bacteria. Likewise, DNA degrades very slowly hence, DNA testing can still be performed on samples even if they are several years old or on samples taken from corpses that have been buried for years.
