Chapter 19: Gene Expression II: Protein Synthesis & Sorting

Gene Expression II: Protein Synthesis & Sorting complex molecular operation relies on a diverse set of participants, including ribosomes—massive ribonucleoprotein machines with specialized A, P, and E binding sites—and transfer RNA (tRNA) molecules, which act as adaptors by matching anticodons to mRNA codons. A critical feature of this system is the wobble hypothesis, which allows for flexibility in the third base of a codon, enabling cells to function with fewer tRNA varieties than there are codons. The chapter details the three main stages of synthesis: initiation, involving the assembly of the translation machinery at a start codon (often AUG); elongation, a cycle of tRNA binding, peptide bond formation (catalyzed by the 23S rRNA ribozyme), and translocation; and termination, where release factors recognize stop codons. Beyond synthesis, the text addresses the consequences of genetic mutations, such as missense, nonsense, and frameshift errors, and how cells employ quality control mechanisms like nonsense-mediated decay to destroy faulty transcripts. Following translation, proteins undergo posttranslational modifications, including chemical alterations and folding assisted by molecular chaperones like Hsp70 and BiP. Finally, the chapter examines protein targeting and sorting, distinguishing between cotranslation import into the endoplasmic reticulum (ER) via signal recognition particles (SRP) and the posttranslational import of proteins into organelles like mitochondria and chloroplasts using specific transit sequences and membrane translocators.