Chapter 11: Nucleolus & Ribosomes

Nucleolus & Ribosomes details the ultrastructure of the nucleolus, distinguishing between the granular and fibrillar components that correspond to the transcription of precursor RNA by RNA polymerase I and the subsequent assembly of ribosomal subunits. The text shifts to the ribosome itself, characterizing it as a complex ribonucleoprotein engine composed of unequal small and large subunits. Significant comparisons are drawn between the sedimentation coefficients and molecular composition of prokaryotic (70S) and eukaryotic (80S) ribosomes, as well as the specialized ribosomes found in mitochondria and plastids. A major portion of the chapter is dedicated to the mechanics of translation, dissecting the process into three distinct phases: initiation, where the ribosome assembles on messenger RNA with the help of initiation factors; elongation, which involves the energy-dependent translocation of transfer RNA (tRNA) through the ribosomal A, P, and E sites alongside peptide bond formation catalyzed by peptidyl transferase; and termination, triggered by specific stop codons. The chapter also explores the interactions between antibiotics and the ribosomal machinery, explaining how drugs like streptomycin, tetracycline, and puromycin selectively inhibit bacterial protein synthesis by targeting specific ribosomal domains. Finally, the summary covers the genetic regulation of ribosome biogenesis, including the phenomenon of gene amplification in amphibian oocytes and the clinical application of silver staining nucleolar organizers to diagnose and stage human lymphomas based on cellular growth rates.