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Transcription - Transcription RNA polymerase the enzyme...

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Transcription: RNA polymerase- the enzyme that catalyzes RNA synthesis, does not need a primer; rather, it can initiate transcription de novo Transcription is less accurate than replication 1 mistake occurs in every 10,000 nucleotides added Eukaryotic cells have three RNA polymerases: o RNA polymerase I, II, III RNA pol II is the enzyme we focus on when dealing with eukaryotic transcription in the second half of this chapter because it is the most studied of these enzymes. It is also the polymerase responsible for transcribing most genes- indeed essentially all protein-encoding genes. RNA pol I and Pol III are each involved in transcribing specialized, RNA-encoding genes. Specifically, POL I transcribes the large RNA precursor gene, whereas Pol III transcribes tRNA genes some small nuclear RNA genes, and the 5S rRNA gene. Bacteria have a single RNA polymerase: o The bacterial RNA polymerase core enzyme alone is capable of synthesizing RNA and comprises two copies of the *a* subunit and one of each of the B, B’, and *w* subunits. This enzyme is closely related to the eukaryotic polymerases. Specifically, the two large subunits, B and B’ are homologous to the two large subunits found in RNA Pol II. To transcribe a gene, RNA polymerase proceeds through a series of well-defined steps grouped into three phases: initiation, elongation, and termination. Initiation: o A promoter is the DNA sequence that initially binds the RNA polymerase (together with any initiation factors required). Once formed the promoter- polymerase complex undergoes structural changes required for initiation to proceed. As in replication initiation, the DNA around the point where transcription will start unwinds: the base pairs a disrupted, producing a “transcription bubble” of single-stranded DNA.
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