RNA polymerase
RNA polymerase is an enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

 that produces RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

. In cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

s, RNAP is needed for constructing RNA chains from DNA gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

s as templates, a process called transcription
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...

. RNA polymerase enzymes are essential to life and are found in all organisms and many virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

es. In chemical terms, RNAP is a nucleotidyl transferase that polymerizes
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...

A ribonucleotide or ribotide is a nucleotide in which a purine or pyrimidine base is linked to a ribose molecule and exactly one phosphate group. In living organisms the most common bases for ribonucleotides are adenine , guanine , cytosine , or uracil ....

s at the 3' end of an RNA transcript.
RNAP was discovered independently by Sam Weiss, Audrey Stevens, and Jerard Hurwitz in 1960.