In addition to proteins, a number of other types of macromolecules have crucial roles in living systems. One well-known example is deoxyribonucleic acid or DNA, which carries the hereditary information in a majority of living organisms. Here we see the structure of a short piece of double-stranded DNA. Each strand is a linear chain comprising 12 nucleotides, each of which in turn has three parts: a phosphate group (in orange), a five-membered sugar ring (ribose) and a flat, aromatic base.
The two strands wind around each other, forming the world-famous double helix, which resembles a winding staircase. The phosphate groups are on the outside of the helix, while the bases from the opposing strands join together in the middle of the structure, forming the "steps" in the staircase.
We can make things a bit clearer by giving the two strands
Each rung in the DNA ladder is formed by two bases (one from each strand), which together form a base pair. The formation of a base pair involves two or three hydrogen bonds between nitrogen and oxygen atoms of the opposing bases. (). These interactions contibute to the overall stability of the double helix.
Four different bases occur in standard DNA molecules. Two of these consist of a single six-memebered aromatic ring, and are called cytidine (abbreviated as C) and thymidine (T). The other two consist of fused five- and six-membered aromatic rings, and are called adenine (A) and guanine (G). Each base pair combines one "small" and one "large" base, giving rise to either an A/T or a G/C base combination:
In addition to the classical ball-and-stick view ( ) there are also more schematic representations for DNA molecules: .
Another aspect of the structure can be seen more clearly in a "space-filling" representation, in which each atom is shown as a small sphere that gives a better view of the space it occupies: Here we see the major and minor grooves that run along the outer surface of the double helix.