DNA fingerprinting
The origins of DNA fingerprinting go back to the early 1980s, when Dr. Alec Jeffreys was searching for sites in human DNA that differed from one individual to the next. Such variation between people is minimal; our similarities vastly outweigh our differences. Over 99% of the human genome- a sequence of over 3 billion genetic letters- is common to everyone. The tiny variations that do exist are what make people unique, define their appearance, predispose them to disease, and gives geneticists a clue into their DNA picture.
On September 15th, 1984, Dr. Jeffreys and his team had analyzed DNA from a human family, a baboon, a cow, a mouse, and even a tobacco plant. They found that the resulting patterns, consisting of 15 to 20 highly variable bands, were specific to individuals (only identical twins share the same pattern). And when they looked at the human family group, they could see that the parents had different patterns, while their offspring had a composite of both. Dr. Jeffreys dubbed his discovery DNA fingerprinting.
How DNA fingerprinting works
- The DNA is chemically extracted from blood, skin, hair roots, saliva, or semen. Chemical scissors called restriction enzymes, which cut DNA strands wherever a specific genetic sequence occurs, are used to chop up the sample in predetermined places. This isolates special regions of DNA that vary in length from one person to another, producing many fragments of different lengths.
- The fragments are sorted using a process called electrophoresis, which involves drawing them through a porous slab of gel using an electric field. The smaller the fragments, the more easily they pass through the pores in the gel.
- The sorted fragments of DNA are "unzipped" to produce single strands of DNA and then "blotted" onto a nylon membrane. But they are invisible. So a radioactive probe is applied, consisting of radioactive material attached to short strands of DNA which bind selectively to DNA fragments. Using X-ray film, which is sensitive to radioactivity, the fragments become visible as dark bands and produce a fingerprint. In modern DNA profiles, software translates the variation in lengths of different DNA fragments into both peaks and numbers.
DNA profiling
Every person carries about 3 billion DNA bases in each cell and the DNA in each cell is virtually identical. Subtle changes in the composition of these DNA bases between people give us our individuality. Approximately 1 in every 1000 bases of DNA varies between people. These differences are not randomly distributed throughout the DNA but appear in clusters. DNA profiling utilizes one type of cluster, known as single tandem repeats (STRs).
Two decades after its discovery, forensic DNA analysis has become an invaluable crime-fighting tool, and many countries have compiled large DNA databases to give them a DNA picture of suspects. Apart from the role DNA profiling plays in the conviction of criminals or the absolution of suspects, DNA technology is being used increasingly by family law courts for resolving paternity disputes. In 2001, American labs alone performed more than 300,000 paternity tests. Paternity testing is based on the principle that each DNA profile is composed of a series of bands, two per STR used. For each STR, one of the STR bands is inherited from the biological mother, one from the biological father. Therefore, the DNA profile of any individual is a composite of half of each of the parent's DNA profile.
Identification of bodies and biological remains damaged beyond recognition is another forensic application of DNA profiling. Using the reverse of the principles applied to paternity testing, DNA samples are taken from close blood relatives of the deceased and DNA profiles obtained from the bodies or body parts are matched for a DNA picture.
As you can see, DNA testing is crucial in many aspects of life, including the building blocks of life itself. A DNA picture is worth a thousand words, or is it chromosomes?
By Greg Hitchcock