The DNA

DNA — deoxyribonucleic acid — is the molecule that carries hereditary information in most living organisms. It is a long polymer made up of repeating units called deoxyribonucleotides, and the actual length of a DNA molecule is usually described not in physical units but by the number of nucleotides (for a single strand) or base pairs (for the familiar double-stranded form) it contains.

This length varies enormously across organisms, and comparing a few real examples gives a sense of scale:

Notice the jump in scale: a bacteriophage's genome is measured in thousands of nucleotides, a bacterium's in millions of base pairs, and the human genome in billions of base pairs. This single number — how many nucleotides long a genome is — is one of the most basic facts genome-sequencing projects report about any organism (see the Human Genome Project later in this chapter). …