RNA polymerase

In prokaryotic cells, all RNA classes are synthesized by a single polymerase. In eukaryotic cells there are 3 distinct classes of RNA polymerase, RNA polymerase (pol) I, II and III. Each polymerase is responsible for the synthesis of a different class of RNA.

RNA polymerase I (Pol I) is dedicated to synthesis of pre-rRNA. RNA polymerase II (Pol II) initiates transcription at DNA sequences corresponding to the 5 Cap of mRNAs and transcribes pre-mRNA. RNA polymerase III (Pol III) transcribes tRNA genes, 5S-rRNA genes, and genes encoding several other small RNAs.

In order to begin transcription, RNA polymerase requires a number of general transcription factors (called TFIIA, TFIIB, and so on). The general transcription factors have been highly conserved in evolution.

All RNA polymerases are dependent upon a DNA template from which to synthesize RNA by addition of complementary bases to an elongating backbone. In RNA, U is substituted for T. The transcribed RNA is complementary to the template strand of the DNA duplex, and is a replica of base-sequences in the non-template, coding strand. The non-template strand is called the coding strand because its sequences are identical to those of the mRNA (with U substituted for T).

p53 - A 3D animation showing the molecule p53 binds to DNA and initiates the transcription of mRNA : diagram - pre-mRNA processing : diagram - intron excision in mRNA precursors : life cycle of an mRNA ~ click on Quicktime Q : alternative splicing - click on fig 1 for animation :
NCBI Molecular Cell Biology - Transcription Initiation Complex : Bacterial Transcription Initiation (NCBI MCB) : SUMMARY transcription initiation (NCBI MCB)