Dna

Instructions that provide almost all of the information necessary for a living organism to grow and function are in the nucleus of every cell. These instructions tell the cell what role it will play in your body. The instructions are in the form of a molecule called deoxyribonucleic acid, or DNA . DNA is the chemical responsible for preserving, copying and transmitting information within cells and from generation to generation. In humans, the DNA molecule consists of two ribbon-like strands that wrap around each other, resembling a twisted ladder. This is often described as a double helix. DNA is contained in tightly coiled packets called chromosomes , found in the nucleus of every cell. Chromosomes consist of the double helix of DNA wrapped around proteins. The twisted ladder is made up of repeating units called nucleotides , each of which is a single building block...

dna— DNA fingerprinting or DNA profiling, any of several similar techniques for analyzing and comparing DNA from separate sources, used especially in law enforcement to identify suspects from hair, blood, semen, or other biological materials found at the scene of a violent crime. It depends on the fact that no two people, save identical twins, have exactly the same DNA sequence, and that although only limited segments of a person's DNA are scrutinized in the procedure, those segments will be statistically unique

Methods
A common procedure for DNA fingerprinting is restriction fragment length polymorphism (RFLP). In this method, DNA is extracted from a sample and cut into segments using special restriction enzymes. RFLP focuses on segments that contain sequences of repeated DNA bases, which vary widely from person to person. The segments are separated using a laboratory technique called electrophoresis, which sorts the fragments by length. The segments are radioactively tagged to produce a visual pattern known as an autoradiograph, or "DNA fingerprint," on X-ray film. A newer method known as short tandem repeats (STR) analyzes DNA segments for the number of repeats at 13 specific DNA sites. The chance of misidentification in this procedure is one in several billion. Yet another process, polymerase chain reaction, is used to produce multiple copies of segments from a very limited amount of DNA (as little as 50 molecules), enabling a DNA fingerprint to be made from a single hair. Once a sufficient sample has been produced, the pattern of the alleles (see genetics) from a limited number of genes is compared with the pattern from the reference sample. A nonmatch is conclusive, but the technique provides less certainty when a match occurs