DNA, also known as deoxyribonucleic acid, is a thin, chainlike molecule found in every living cell on earth. It directs the formation, growth, and reproduction of cells and organisms. Short sections of DNA called genes determine heredity—that is, the passing on of characteristics—in living things. DNA is found mainly within a cell’s nucleus, in threadlike structures called chromosomes. DNA even occurs in bacterial cells, which do not have a nucleus, and in some viruses.
All DNA consists of thousands of smaller chemical units called nucleotides. Nucleotides are chemically bonded to one another to form thin, chainlike molecules known as polynucleotides. Each nucleotide contains a compound called a phosphate, a sugar called deoxyribose, and a compound called a base. The phosphate and sugar are the same in all DNA nucleotides, but the bases vary. There are four DNA bases: (1) adenine, (2) guanine, (3) thymine, and (4) cytosine. The exact amount of each nucleotide and the order in which they are arranged are unique for every kind of living thing.
The DNA molecule consists of two polynucleotide chains arranged in a double helix (spiral). This double helix resembles a twisted rope ladder. Polynucleotide chains of phosphates and sugars form the sides of the ladder. Each rung represents two matching bases, called base pairs. Weak chemical bonds between specific base pairs hold the chains together.
DNA bases are paired in a specific way. For example, an adenine base on one polynucleotide chain always bonds with a thymine base on the opposing chain. Similarly, guanine on one chain always bonds with cytosine on the other. Because the sequence of bases in one chain is determined by the sequence of bases in the other, scientists say the two chains are complementary.
Before a cell divides, it duplicates its DNA. The two chains of polynucleotides separate lengthwise, splitting the bonds between the base pairs. The separated chains, each resembling half a ladder split down the middle, serve as templates (patterns) for two new DNA molecules. A cell’s nucleus contains additional nucleotides. The bases of these nucleotides bond with their corresponding bases on one of the templates. This process is repeated thousands of times to form two new DNA molecules identical to the original. Thus, when the cell divides, each of the resulting daughter cells receives identical DNA molecules.
