Genetic information is encoded in deoxyribonucleic acid (DNA) molecules. Therefore, DNA is an essential component of independently living organisms. Genes are the DNA segments that carry genetic information (1).

Some DNA sequences do not code for genes and have structural roles (for example, in the structure of chromosomes), or are involved in regulating the use of the genetic information; for example, repressor sites are DNA sequences that allow binding of a repressor, which stops the process of gene expression.

DNA consists of two long polymers (called strands) that run in opposite directions and form the regular geometry of the double helix. The monomers of DNA are called nucleotides. Nucleotides have three components: a base, a sugar (deoxyribose) and a phosphate residue. The four bases are adenine (A), cytosine (C), guanine (G) and thymine (T). The sugar and phosphate create a backbone down either side of the double helix. The bases interact via hydrogen bonds with complementary bases on the other DNA strand in the helix. 

It is the sequence of these four bases that encode genetic information. The interaction between two bases on opposite strands via hydrogen bonds is called base pairing. As shown in figure 3, adenine forms a base pair with thymine, and guanine forms a base pair with cytosine. These are the most common base pairing patterns but alternative patterns also are possible.

The chemical structure of DNA
Figure 3 Chemical structure of DNA; two polymers composed of phosphate-deoxyribose backbones and four bases : A, C, G, T linked via two  (A-T) or three (G-C) hydrogen bonds; the two strands run in opposite directions (image from wikipedia).

The majority of DNA in a cell is present in the so-called B-DNA structure. However, it can also adopt other 3D structures (Figure 4). Z-DNA, found in DNA bound to certain proteins, is a rarer structure. In Z-DNA, the bases have been chemically modified by methylation and the strands turn in a left-handed helix, the opposite direction from that of the B form. Z-DNA formation is an important mechanism in modulating chromatin structure (2).The A-DNA structure, which has a wider right-handed helix, occurs only in dehydrated samples of DNA, such as those used in X-ray crystallography.

Structural differences between common DNA forms
Figure 4 Structural differences between common DNA forms: A-, B- and Z- (from left to right) (image from wikipedia).