DNA, short for deoxyribonucleic acid, is often called the “blueprint of life.” That’s because almost every living organism on this planet, whether bacterium, plant, fungus, or animal contains some form of the molecule. The information contained in the DNA’s coding segments, called genes, determines whether the organism turns out to be a fish or a dandelion, a person or a chimp. It also creates the characteristics that make one individual similar to, but still different from, another. No two people’s DNA – even identical twins’ – is exactly alike (Ruff, 2002).
DNA is double-stranded. Two strands are linked together by chemical bonds, forming a very stable structure. A good way to picture this is to imagine a ladder – in DNA, the strands (composed of a sugar, a base, and a phosphate group) are the side supports, and the chemical bonds are the rungs. DNA is formed into a double-helix. A DNA strand can contain millions of bases. A chain that long would be extremely difficult to store in a minuscule cell. To solve this problem, the DNA twists itself around its long axis to make it a smaller package.
DNA, though, is just a blueprint; by itself it does not create or maintain an organism. Rather, through a complex series of events, sequences of bases along the framework that code for something (known as genes) are “translated” into amino acids and then into proteins that the body’s cells can use to obtain and store energy, transport materials, communicate, and reproduce, among other things.
But while genes make up a certain percentage of a person’s DNA, they do not account for all the base pairs; in fact, perhaps as much as 90 percent of the DNA is non-coding - what is sometimes called “junk DNA” (Ruff, 2002). The key to the profiling, or fingerprinting, process lies within these non-coding regions, many of which contain stretches of short, identically repeated sequences of nucleotides known as variable number tandem repeats (VNTRs). The number of VNTRs at specific places (called loci) on each chromosome varies from person to person and can be used to pinpoint a sample’s source.
One of DNA fingerprinting’s most widespread applications is in criminal investigations. It used to be that police had to rely on fingerprints left at the scene to determine a person’s innocence or guilt. However, people who commit crimes often know enough to wear gloves or to wipe away their prints, leaving no evidence behind. And even when police do find a print, they can only compare it to ones they have on file; if the person has never been fingerprinted, they can’t make a match. With DNA fingerprinting, though, hair, blood, semen or saliva found at the scene can be used to identify the culprit, even in the absence of prints.
Investigators use DNA evidence to solve crimes in one of two ways (Justice.gov, nd). If the suspect has been identified, police can take a sample of that person’s DNA and compare it to the evidence gathered from the crime scene. If there is no suspect, the collected evidence can be compared with any of a number of DNA databases, such as CODIS, which keeps a record of previously tested offenders – if there is a match, the police can arrest the suspect. This process also allows officials to link one crime scene to another through DNA evidence.
Recognizing its power and potential, in 1988 the U.S. judicial system accepted the use of DNA fingerprinting as evidence, although it is still somewhat controversial. This use applies not only to current court cases, but also to inmates who were convicted before the technique existed or when it was still too new to be admissible. DNA is an extremely stable molecule; it can last for years in previously collected samples, such as blood or semen stains on clothing. The evidence, therefore, is still valid. In about one-third of the reviewed cases, the individuals were found not guilty of the crime for which they had originally been convicted and imprisoned (Ruff, 2002).
DNA is also much more effective than actual fingerprints when it comes to identifying bodies, since after someone dies the flesh quickly decays. Bones, teeth and hair, however, last much longer, and they all contain DNA. As a result, it has grown in importance in establishing forensic evidence, and more branches of the judicial system are relying on DNA profiling to solve a wide array of criminal offenses.
References
(nd). Advancing Justice Through DNA Technology. Justice.gov. Retrieved from
Ruff, M. (2002). DNA Fingerprinting. San Antonio, TX: Your Information Center.
