Chapter 21: Transcription and RNA Processing

Transcription begins at promoter regions where RNA polymerase binds and unwinds the DNA double helix to form a transcription bubble. In prokaryotes, the sigma subunit of RNA polymerase recognizes consensus sequences at the -10 and -35 positions, initiates transcription, and then dissociates to allow core enzyme progression through the elongation phase. Transcription terminates at specific DNA sequences through either rho-independent mechanisms involving RNA hairpin formation or rho-dependent mechanisms requiring the rho protein and ATP hydrolysis. Eukaryotic transcription is substantially more complex, employing three specialized RNA polymerases and requiring general transcription factors including the TATA-binding protein to position the enzyme at promoter sites. Chromatin structure and histone modifications regulate accessibility to eukaryotic genes. Gene expression is controlled through transcriptional regulation by repressor proteins that block transcription and activator proteins that enhance it, exemplified by the lac operon model in bacteria where the lac repressor provides negative control and the cAMP-regulatory protein provides positive regulation. RNA processing in eukaryotes includes five prime capping with methylguanosine, three prime polyadenylation for stability, and splicing, in which the spliceosome removes introns and ligates exons through transesterification reactions. The spliceosome comprises small nuclear RNAs and associated proteins that catalyze two sequential phosphodiester bond rearrangements, releasing introns as lariat structures. Transfer RNA and ribosomal RNA undergo ribonuclease cleavage and base modification. These processes collectively ensure that genetic information encoded in DNA is accurately transcribed and processed into stable, functional RNA molecules essential for protein synthesis and cellular regulation.