Living things organize themselves in a variety of ways. The simplest, called prokaryotes, have only a single cell, and it doesn't have much structure. Unicellular eukaryotes also have only a single cell, but it's more complex. Multicellular eukaryotes -- all the living things that can be seen with the naked eye -- are the most complex. Not only do they have many cells, but their cells do different things: as they grow and develop, their cells differentiate and read different Instructions from their Books. Viruses aren't living things, but infect living things in order to reproduce. Retro-viruses add their tiny Instruction Books to the Instruction Books of cells they infect.

There are many different classes of workers. Three common types are: transporters, enzymes, and structural proteins. Transporters move things around, enzymes chemically change things, and structural proteins hold living things together.

Workers are often organized to work more efficiently. Groups of workers stick together to form a complex which carries out a set of closely-related jobs. Workers are also made into pathways to carry out jobs with many steps.

The final results of workers' efforts are an organism's traits. Some traits are produced by a single worker, others are produced by many workers acting together. While it's possible to figure out what traits a living thing will have if you know what workers it makes, it's often hard to figure out what workers and Instructions it has by the traits it displays.

Life's Instruction Books are made of DNA, and consist of two complementary pairs of letters: A-T and G-C. These four letters are used to write all the Book's Instructions. The complementarity of the bases allows organisms to repair many mutations. Eukaryotes' Books have a number of curious properties: they aren't all filled with Instructions, the Instructions themselves are broken into pieces, and the Books are divided into chromosomes. Eukaryotes also keep two copies of their Instruction Books.

Workers are made of links called amino acids, and the properties of those links determine what a worker does. Most of the links fall into a few groups with similar properties. Within a group, links can be conservatively substituted for each other. In enzymes, some of the links make up the worker's active site, which is the part of the enzyme that changes molecules. Active sites and the links around them are grouped into domains. Every enzyme has at least one domain. Domains determine the worker's specificity.

Workers are made to carry out Instructions by special workers that read the Instruction Book and follow the genetic code.

Eukaryotes' two copies of their Instruction Books interact to produce their traits. The basic unit of a trait is an allele.