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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