transcription in prokaryotes

Meaning: Bacteria use one RNA polymerase. The core enzyme plus a sigma factor forms the holoenzyme. Sigma recognizes the promoter, typically sequences near −10 and −35 upstream of the start site, and is released after initiation. The polymerase unwinds the DNA, reads the template strand 3'→5', and adds ribonucleotides to the RNA 5'→3'. It needs no primer. Termination is either rho-dependent (the rho protein, a helicase, pulls the RNA off the polymerase) or rho-independent (a GC-rich hairpin forms in the RNA, followed by a run of U's, and the polymerase stalls and releases).

On the MCAT: Expect comparisons with eukaryotes. Bacteria have one polymerase instead of three, no nucleus, and no 5' cap, poly-A tail or splicing. Genes are often grouped in operons (lac, trp) and give one polycistronic mRNA. Because there is no nuclear envelope, ribosomes can begin translating the mRNA before transcription ends. Passages may ask you to predict the effect of a promoter mutation (less transcription), a missing sigma factor (the polymerase cannot initiate at the promoter), or a repressor or inducer on an operon.

Common confusion: The RNA is complementary and antiparallel to the template strand. Its sequence matches the coding (nontemplate) strand, with U in place of T. Also, the promoter is bound by the polymerase but is not itself transcribed.