As proposed originally in 1953 by Watson and Crick, deoxyribonucleic acid (DNA) consists of two long chains of nucleotides. The two nucleotide chains twist around one another to form a double helix, which resembles a spiral staircase. The two chains of nucleotides are held to one another by weak hydrogen bonds between bases of the chains.
A nucleotide in the DNA chain consists of three parts: a nitrogenous base, a phosphate group, and a molecule of deoxyribose. The nitrogenous bases of each nucleotide chain are of two major types: purines and pyrimidines. Purines have two fused rings of carbon and nitrogen atoms, while pyrimidines have only one ring. The two purine bases in DNA are adenine (A) and guanine (G). The pyrimidine bases in DNA are cytosine (C) and thymine (T). Purine and pyrimidine bases are found in both strands of the double helix.
The phosphate group of DNA is derived from a molecule of phosphoric acid and connects the deoxyribose molecules to one another in the nucleotide chain. Deoxyribose is a five-carbon carbohydrate. The purine and pyrimidine bases are attached to the deoxyribose molecules and stand opposite one another on the two nucleotide chains. Adenine always stands opposite and binds to thymine. Guanine always stands opposite and binds to cytosine. Adenine and thymine are said to be complementary, as are guanine and cytosine. This is known as the principle of complementary base pairing.
DNA replication. Before a cell enters the process of binary fission or mitosis, the DNA replicates itself to ensure that the daughter cells can function independently. In the process of DNA replication, specialized enzymes pull apart, or “unzip,” the DNA double helix.
As the two strands separate, the purine and pyrimidine bases on each strand are exposed. The exposed bases then attract their complementary bases and induce the complementary bases to stand opposite. Deoxyribose molecules and phosphate groups are brought into the environment, and the enzyme DNA polymerase unites all the nucleotide components to one another and forms a long strand of nucleotides. Thus, the old strand of DNA directs the synthesis of a new strand of DNA through complementary base pairing.
After the synthesis has occurred, one old strand of DNA unites with a new strand to reform a double helix. This process is called semiconservative replication because one of the old strands is conserved in the new DNA double helix.