RNA polymerase
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RNA polymerases, often abbreviated to RNAP, are the enzymes in a cell responsible for synthesizing strands of RNA. Because this function is essential to life, all organisms (and quite a few viruses) contain at least one type of RNA polymerase (most organisms contain at least three types). Depending on the RNA polymerase, transcription requires either a DNA or an RNA template; although, in most contexts, "RNA polymerase" refers to the DNA-dependant RNA polymerases that use a DNA template to transcription genes into mRNA ("messenger" RNA). Reverse transcriptases are enzymes (usually associated with retroviruses)that perform the opposite function, using an RNA template to synthesize a DNA product.