Chapter 17: Gene Expression: From Gene to Protein

Gene expression represents the fundamental molecular process through which cells convert the hereditary information stored in DNA into functional proteins that drive cellular activities and determine organismal characteristics. This chapter examines the complete pathway of information transfer, beginning with transcription, wherein RNA polymerase binds to promoter sequences and synthesizes messenger RNA complementary to the DNA template strand through initiation, elongation, and termination phases guided by transcription factors and regulatory proteins. In eukaryotic cells, the nascent RNA transcript undergoes extensive processing modifications including the addition of a protective 5' cap structure at the terminus, attachment of a polyadenyl tail to enhance stability and translation efficiency, and removal of introns through splicing mechanisms that join exons to produce mature mRNA ready for protein synthesis. The genetic code operates as a universal triplet system in which consecutive three-nucleotide codons specify particular amino acids or termination signals, establishing the correspondence between nucleic acid sequence and protein composition. Translation occurs at ribosomes, where transfer RNA molecules deliver amino acids specified by their anticodon sequences, which base pair with corresponding mRNA codons during a cyclical process involving ribosomal initiation complexes, elongation cycles that sequentially add amino acids to the growing polypeptide chain, and termination upon recognition of stop codons. Significant differences distinguish prokaryotic from eukaryotic gene expression systems, particularly regarding the temporal coupling of transcription and translation in bacteria versus the compartmentalization and extensive RNA modification required in eukaryotes. The chapter also addresses mutations as alterations in DNA sequence, including point mutations affecting single nucleotides and frameshift mutations that shift the reading frame and dramatically alter downstream amino acid sequences, demonstrating how nucleotide changes propagate through the expression pathway to modify protein structure and function. By synthesizing these mechanisms into a cohesive framework, the chapter illustrates the central dogma of molecular biology and explains how genetic information directs protein production to establish the biochemical foundation of life.