DNA is nature’s finest fingerprint and can be used to identify individuals, to identify how individuals of a species are related to other individuals, and to identify how species are related to each other.
WHAT IS DNA?
DNA is found in every living thing, and even some of the borderline biological entities such as viruses, which are intermediate between living and non-living things. It is a biological blueprint or set of instructions that determines what kind of animal or plant its owner will become. Most cells contain many DNA molecules packed in a packaged form called chromosomes. Each DNA molecule has sections called genes that are used to produce particular proteins. The complete set of DNA in all the chromosomes is known as the genome.
DNA is the abbreviation of DeoxyriboNucleic Acid, which was first observed in the 1800s, but its structure was not determined until 1953 by James Watson, Francis Crick, Rosalind Franklin and Maurice Wilkins. They found DNA has a double helix structure consisting of two strands, each made up of nucleotides.
DNA is the basic hereditary material that is present in the nucleus of almost every cell in every plant and animal. (There is also a different kind in the mitochondria, which are basically the power plants of most cells.) In humans and many other red-blooded animals, identical DNA molecules are found in every cell except the red blood cells (which have no nucleus).
Each strand of DNA is built up of many nucleotides, each consisting of a nucleic acid (base) plus a phosphate group and a sugar molecule. There are only four types of bases – adenine (A), cytosine (C), guanine (G), and thymine (T) – and the same four are found in the DNA of all the plants and animals on the planet. The order of these bases in the strands of DNA is like a code, and it is this code that can be used in identification. The bases on the two strands pair up: A with T and C with G, and so a short piece of the double strands might read like this:
A A C T G C A T C G G T
T T G A C G T A G C C A
A single chromosome contains around 100 million base pairs.
HOW DNA TESTING IS DONE
DNA testing is also known as DNA profiling, DNA fingerprinting, DNA typing, and genetic fingerprinting.
In this process, DNA is extracted from the tissue and then the sections of interest are usually amplified. To do this, the DNA sample is heated to 95 degrees Celsius to separate the two strands, and then primers are added, which attach to opposite ends of the two strands. An enzyme called a polymerase is then added, and it builds new copies of each of the two strands, starting at the primers. In this way millions of copies can be made of the section of DNA to be looked at. The sequences of the base pairs can then be determined.
HOW DNA IS USED IN HUMAN IDENTIFICATION
DNA evidence is used to identify bodies, to identify criminals, to confirm paternity, and for many other reasons. All kinds of human tissue can be used in the identification, including skin cells, hair, blood, semen, and bone.
Human DNA consists of about 3 billion bases, but about 99.9% of these occur in the same order in all people regardless of their country of origin or racial background. In humans it is the 0.1% difference that is used in identification, and this small amount of DNA is unique to every individual except identical twins, who have identical DNA in every cell. The three million base differences means that it is virtually certain no one has ever had exactly the same DNA as any other human being (except identical twins).
Forensic scientists use 13 DNA regions that vary from person to person to create a DNA profile or fingerprint for an individual. The profile is basically the code shown earlier in this article. The chance of any other individual having an identical DNA profile for the 13 regions is almost immeasurably small.
Mitochondrial DNA can also be used in human identification, since the mitochondrial DNA of a mother is identical to that in all her offspring. (Sperm do not have mitochondria, so they contribute only nuclear DNA.) The Y chromosome is also used to identify relationships between males, as this is identical in fathers and their sons.
HOW DNA IS USED IN IDENTIFICATION IN OTHER SPECIES
DNA testing is now adding to and even at times superseding the traditional methods of taxonomy by which species are identified and classified. The testing methods are the same as in human DNA tests, and it looks at the few variable regions of the DNA. Samples from one species can be compared to samples from the same sections of DNA in another species to determine how closely they are related.
Unknown samples can be compared to samples of known origin to determine what the unknown sample is.
DNA is at the cutting edge of technologies we have for biological identifications, whether for identification of human victims or criminals, or for paternity, or identification and classification of other species. It is hard to imagine now a time when the powerful tools of DNA testing and sequencing were not available.
MAJOR SOURCES/FURTHER READING:
Biological Science, by William T. Keeton
Biochemistry, by Albert L. Lehninger
Genetics home reference: http://ghr.nlm.nih.gov/handbook/basics/dna
National human genome research institute: http://www.genome.gov/25520880
National Center for Biotechnology Information: http://www.ncbi.nlm.nih.gov/About/primer/genetics_genome.html
DNA forensics: http://www.ornl.gov/sci/techresources/Human_Genome/elsi/forensics.shtml
Molecular cell biology: http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mcb.chapter.2119