Genetic information in humans is stored as deoxyribonucleic acid (DNA), which is used as a template to synthesize proteins. But proteins can't be synthesized from DNA alone: Ribonucleic acid (RNA) is needed. RNA comes in several varieties, and RNA polymerases are in charge of making these varieties.
A genetic code is found in the sequence of DNA bases. As James Watson and Francis Crick showed in 1962, DNA is a double helix made up of a long sequence of four types of bases that complement each other in pairs: adenine (A) complements thymine (T), and guanine (G) complements cytosine (C). The genetic code can therefore be read as a long sequence of one-letter words; knowing one of the strands of the double helix implies knowing the other one (i.e., opposite T is A). In DNA, the bases are deoxyribonucleotides (hence deoxyribonucleic).
Transcription, the first step in transforming DNA into proteins, is mediated by an enzyme named RNA polymerase. It consists of reading a sequence of DNA and transforming it, base by base, into a complementary sequence of RNA. The main difference between DNA and RNA is the chemical composition of the bases: RNA is made up of ribonucleotides (hence ribonucleic acid) and is single-stranded. In addition, uracil (U) is substituted for thymine (T) in RNA.
For each DNA base, RNA polymerase adds to its RNA molecule a complementary RNA base. As is usually the case in genetics, the DNA sequence is critical. This sequence determines the type of RNA that's being synthesized, as well as which of the existing RNA polymerases (I, II or III) will do it.
There are three basic types of RNA: messenger, transfer and ribosomal. Messenger RNA carries the information needed to make a protein; transfer RNA transports the amino acids that will form the protein; and ribosomal RNA is an integral part of a structure called a ribosome, which forms the scaffold in which the protein will be synthesized.
The enzyme RNA polymerase I is in charge of synthesizing the majority of the ribosomal RNA. Because ribosomal RNA is the most abundant RNA in a cell, RNA polymerase I is responsible for most RNA synthesis.
One way to differentiate between the three RNA polymerases is by their sensitivity to amanitin, a toxin produced by the fungus Amanita phalloides. RNA polymerase is insensitive to it while RNA polymerase II is very sensitive and RNA polymerase III is somewhat sensitive. This toxin is responsible for over a hundred deaths annually.