All DNA consists of two long chains of subunits, twisted around each other to form a double helix. The subunits of each strand consist of nucleotides, each of which contains any one of four chemical constituents, attached to a phosphorylated molecule of the 5-carbon sugar deoxyribose. The four nucleotides in DNA are adenine (A), thymine (T), guanine (G) and cytosine (C). The specific sequence of nucleotides in the long chain of DNA identifies it as, for example, human, canine or a particular plant species. However, individuals within each group also have their own unique DNA sequences at other locations. At a chromosomal location where individual people differ, one person may have the sequence GATCGT and another GAACGT. These differences in nucleotide sequences allow individuals within a species to be identified.
A vast amount of genetic variation exists in human populations and, except for identical twins, all people are genetically different. Forensic typing is based upon this genetic variation. Analysis of genetic variation requires that such differences are traceable. We refer to such traceable features as markers. Millions of people share the same blood groups, so this is not very helpful for identification purposes. However, the discovery of DNA markers made it possible to effectively distinguish between people. From a genetic perspective, what differs between people is the sequence of the four nucleotides on the DNA molecule. Interestingly, only five percent of our DNA is made up of genes that code for all the proteins our body needs for growth and to function. Very little variation between people exists in these genes or coding regions. However, some regions in the other ninety-five percent of our DNA, that do not code for any proteins, are highly variable and may be used to distinguish people from one another. As the purpose of many of these non-coding chromosomal regions is unknown, they are loosely referred to as ���junk DNA’.
The DNA markers that are used for forensic purposes are found in a number of different chromosomal locations or loci within the non-coding regions of the DNA molecule. The chromosomal locations chosen for forensic DNA analysis are termed short tandem repeats (STRs) as, at each locus, a pattern of two or more nucleotides is repeated in what has been termed a ‘genetic stutter’. At each locus, there are two forms (alleles) of the repeated sequence; one is maternally inherited, one paternally. The number of repeats of these DNA sequences varies considerably amongst individuals and thus allows scientists to differentiate between people. A person’s DNA profile is simply a list of the number of repeats of a given sequence at every paired chromosomal location under analysis. It is important to note that the number of variations at any one STR locus is limited and thus numerous loci are considered in any forensic analysis. The more STR regions that are tested simultaneously, the lower the probability of any two individuals sharing a profile. To ensure that no two people tested will have the same DNA profile, between nine and thirteen locations on different chromosomes are tested simultaneously.
The process of obtaining a DNA profile begins with forensic experts taking a biological sample such as blood, semen, skin, saliva or hair from a person, crime scene or body. The genetic material, or DNA, is isolated from the sample and quantified. This is then referred to as the DNA sample.
Selected fragments containing the forensic DNA markers or STRs are then replicated, using a process known as PCR (polymerase chain reaction), which can be described as a form of molecular photocopying. After being placed in a special gel, the fragments are separated according to their length, using an electric current, a process called electrophoresis. A laser then lights up florescent tags on the fragments, so that the fragment length of each STR marker can be measured. The fragment length is determined by the number of repeats of a given sequence at every chromosomal location under analysis. The resulting patterns, which resemble supermarket barcodes, are photographed and examined and converted into a digital profile. The fragment length of each STR marker is recorded as a series of numbers. This sequence of numbers is termed the ‘DNA profile’. The resultant DNA profile is, in other words, the electronic representation of the physical DNA sample.
The process of obtaining a profile is illustrated below. The illustration refers to one STR marker (specific region on a chromosome) on chromosome 11. This STR consists of repeat units of the nucleotide sequence ATCT. On one DNA strand, there are seven repeats of the nucleotide sequence and on the other strand, there are five repeats. The DNA profile is recorded, and, if required, stored on a database as a sequence of numbers, in this case seven and five. In South Africa, ten marker regions are simultaneously analysed and thus a sequence of up to 20 numbers will make up the DNA profile.
The process of obtaining a profile is illustrated in the figure below