The nucleic acids, DNA and RNA, are composed of repeating subunits called nucleotides. Each nucleotide is in turn made of three structures:
A nucleotide = a phosphate + a 5-carbon sugar + a cyclic nitrogenous base
DNA
In the case of DNA, the 5-carbon sugar found in nucleotides is called 2'-deoxyribose. (The name means that compared to ribose, this sugar is lacking a hydroxyl group at the 2' position, as shown in the diagram above.) There are four bases: adenine, thymine, cytosine, and guanine (abbreviated A, T, C, G). These bases fall into two groups, based on molecular structure: adenine and guanine are purines, while thymine and cytosine are pyrimidines.
DNA is a polynucleotide, meaning it is a polymer consisting of nucleotides joined together. Nucleotides are joined by a phosphodiester bond, in which the phosphate (which is attached to the 5' carbon of deoxyribose) of one nucleotide becomes connected to the hydroxyl (OH) group (attached to the 3' carbon) of the other nucleotide. This creates a chain of alternating phosphate and sugar groups, with the bases sticking out the side. The nucleotides are all connected by a 5' phosphate joined to a 3' hydroxyl. Any chain of nucleotides will therefore have a free (unattached) 5' phosphate at one end of the chain, and a free 3' hydroxyl at the other end. These are referred to as the 5' end and the 3' end of the DNA strand, respectively.
DNA is usually found as a double-stranded molecule. This means that two chains are joined together in a ladder configuration. The strands are joined by hydrogen bonding between the bases on the nucleotides. These hydrogen bonds are called base pairs. Hydrogen bonds are very weak bonds, but enough of them acting together are strong enough to hold two strands of DNA together. Chromosomes have hundreds of millions of such base pairs, which are more than enough to hold the strands together.
Base pairing must be specific in order to work. Only certain combinations of bases will hydrogen bond with each other. For example, adenine will only base pair with thymine, and cytosine will only base pair with guanine. Therefore, in order for two strands of DNA to base pair, the appropriate bases must be present in corresponding positions on the two strands. Two such strands are said to be complementary.
The two strands of DNA run in opposite directions when base paired (the 5' end of one strand is next to the 3' end of the other strand). The two strands are therefore said to be antiparallel. When double stranded, DNA forms a double helix, which is like a spiral staircase in which both rails spiral around a central point.
