DNA is a long threadlike molecule with a uniform diameter of 2 nm, although its length varies greatly from the smallest to the largest chromosomes. Most human cells have 46 molecules of DNA totaling 2 m in length. This makes the average DNA molecule about 43 mm (almost 2 in.) long. To put this in perspective, imagine that an average DNA molecule was scaled up to the diameter of a telephone pole (about 20 cm, or 8 in.). At this diameter, a pole proportionate to DNA would rise about 4,400 km (2,700 mi.) into space— far higher than the orbits of space shuttles (320-390 km) and the Hubble Space Telescope (600 km).
At the molecular level, DNA and other nucleic acids are polymers of nucleotides (NEW-clee-oh-tides). A nucleotide consists of a sugar, a phosphate group, and a single- or double-ringed nitrogenous
base (fig. 4.1a). Two of the bases in DNA—cytosine (C) and thymine (T)—have a single carbon-nitrogen ring and are classified as pyrimidines. The other two bases—adenine (A) and guanine (G)—have double rings and are classified as purines (fig. 4.1b). The bases of RNA are somewhat different, as explained later, but still fall into the purine and pyrimidine classes.
The structure of DNA, commonly described as a double helix, resembles a spiral staircase (fig. 4.2). Each sidepiece is a backbone composed of phosphate groups alternating with the sugar deoxyribose. The steplike connections between the backbones are pairs of nitrogenous bases. The bases face the inside of the helix and hold the two backbones together with hydrogen bonds. Across from a purine on one backbone, there is a pyrimidine on the other. The pairing of each small, single-ringed pyrimidine with a large, double-ringed purine gives the DNA molecule its uniform 2 nm width.
A given purine cannot arbitrarily bind to just any pyrimidine. Adenine and thymine form two hydrogen bonds with each other, and guanine and cytosine form three, as shown in figure 4.2b. Therefore, where there is an A on one backbone, there is normally a T across from it, and every C is normally paired with a G. A-T and C-G are called the base pairs. The fact that one strand governs the base sequence of the other is called the law of complementary base pairing. It enables us to predict the base sequence of one strand if we know the sequence of the complementary strand.
The essential function of DNA is to carry instructions, called genes, for the synthesis of proteins.
Humans are estimated to have 25,000 to 35,000 genes. These constitute only about 2% of the DNA. The other 98% is noncoding DNA, which plays various roles in chromosome structure and regulation of gene activity, and some of which may have no function at all. Some of the noncoding DNA is sometimes thought of as “junk DNA” to suggest that it might be merely harmless debris accumulated by mutation over eons of evolutionary time.