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What is DNA?
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| DNA, or deoxyribonucleic acid, is the fundamental building block for an individual's entire genetic makeup. It is a very long molecule that carries genetic information that governs a person's physical characteristics. Each individual inherits half their DNA from their mother and the other half from their father. With the exception of identical twins, no two individuals share the same DNA sequence. DNA is found in the nucleus of every cell in the body except red blood cells and is the same throughout the body. That is, for any individual, the DNA sequence recovered from white blood cells is identical to that found in other tissues, bones or bodily fluids. 
There are many regions of DNA molecules that do not hold any known
genetic information, but that vary enormously from person to person.
These are called non-coding DNA, and are used by forensic scientists to
distinguish between individuals. Because forensic scientists do not
look at the whole of a person's DNA sequence, but rather a set of
characteristics in non-coding DNA, the results are called a DNA
profile. DNA profiles are a very powerful means of determining whether
two samples may or may not have come from the same person. If two DNA
profiles do not match, they must have come from two separate
individuals.
The Structure of DNA
DNA molecules are made up of two strands that wrap around each other in a helix to resemble a twisted ladder. The rungs of the ladder are made up of pairs of smaller molecules (monomers) called bases. There are four different bases present in DNA: adenine (A), cytosine (C), guanine (G), and thymine (T). Each base only binds with a specific complementary base - A with T and G with C. The binding of two complementary bases to form a ladder rung is called a base pair.
The human genome consists of some three billion base pairs along
forty-six DNA molecules contained in twenty-three pairs of chromosomes.
The order of bases along the DNA molecules is known as the DNA
sequence. A gene is a particular area of a DNA molecule where the
sequence of bases carries information required to produce a particular
protein from a particular sequence of amino acids. Proteins provide the
structural units for living organisms. Genes can vary in size, but
average about 3,000 base pairs in length. Human DNA contains an
estimated 30,000 genes. Therefore, genetic or coding DNA makes up less
than 5% of total human DNA. The remainder of the DNA is non-coding DNA.
Some areas of non-coding DNA are consistent across all people, while
other areas vary significantly between individuals. It is this
variability that provides the basis for distinguishing between
individuals through DNA profiling.
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