Chapter 14: Gene Expression: From Gene to Protein

Gene expression represents the molecular process by which genetic information stored in DNA is converted into functional proteins, establishing the fundamental connection between genotype and phenotype. This chapter traces the historical development of understanding this process, beginning with Archibald Garrod's early observations and advancing through the groundbreaking experiments of Beadle and Tatum on Neurospora, which demonstrated that individual genes encode specific enzymes. The concept evolved from the one gene–one enzyme hypothesis to the more inclusive one gene–one polypeptide hypothesis, recognizing that proteins vary in their roles and composition. Gene expression operates through two coordinated processes: transcription and translation. During transcription, RNA polymerase synthesizes messenger RNA from a DNA template, with the process differing significantly between prokaryotes and eukaryotes. In eukaryotic cells, messenger RNA undergoes extensive processing including the addition of protective structures at both the five-prime and three-prime ends, as well as splicing where non-coding sequences are removed and coding sequences are joined together. This splicing mechanism is particularly important because it allows a single gene to generate multiple distinct proteins through alternative combinations of exons. The genetic code, comprised of three-nucleotide sequences called codons, translates messenger RNA information into amino acid sequences, with the code being remarkably consistent across living organisms. Translation requires transfer RNA molecules that recognize codons through complementary anticodon sequences while transporting their associated amino acids to the ribosome, a complex molecular machine composed of ribosomal RNA and proteins. The ribosome coordinates the sequential binding and release of transfer molecules while catalyzing the formation of peptide bonds between amino acids. Following synthesis, newly formed polypeptide chains fold into three-dimensional structures and may undergo chemical modifications or be directed to specific cellular compartments through targeting sequences. The chapter concludes by examining mutations, the source of genetic variation underlying evolution, categorizing them by type and consequence, and redefining a gene as a DNA sequence that encodes either a functional protein or an RNA molecule with catalytic properties.