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| DNA Property of species Horizontal view |
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DNA, or deoxyribonucleic acid, is the hereditary material in humans
and almost all other organisms. Nearly every cell in a person’s body has
the same DNA. Most DNA is located in the cell nucleus (where it is
called nuclear DNA), but a small amount of DNA can also be found in the
mitochondria (where it is called mitochondrial DNA or mtDNA).
The information in DNA is stored as a code made up of four chemical
bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human
DNA consists of about 3 billion bases, and more than 99 percent of those
bases are the same in all people. The order, or sequence, of these
bases determines the information available for building and maintaining
an organism, similar to the way in which letters of the alphabet appear
in a certain order to form words and sentences.
DNA bases pair up with each other, A with T and C with G, to form
units called base pairs. Each base is also attached to a sugar molecule
and a phosphate molecule. Together, a base, sugar, and phosphate are
called a nucleotide. Nucleotides are arranged in two long strands that
form a spiral called a double helix. The structure of the double helix
is somewhat like a ladder, with the base pairs forming the ladder’s
rungs and the sugar and phosphate molecules forming the vertical
sidepieces of the ladder.
An important property of DNA is that it can replicate, or make copies
of itself. Each strand of DNA in the double helix can serve as a
pattern for duplicating the sequence of bases. This is critical when
cells divide because each new cell needs to have an exact copy of the
DNA present in the old cell.
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