Biology has captivated the imagination of researchers with diverse backgrounds as never before. For example, physicists are now exploring the origin and consequence of noise in gene expression, which appears to be important in epigenetic phenomena. Engineers are looking at biological systems from a design perspective. No doubt, conventional biologists will continue to provide insights by combining conventional approaches with high-throughput ones. These diverse efforts have resulted in the disintegration of biology into sub-disciplines. This is unavoidable because biology is inherently complex. No matter which branch of biology one studies, if the ultimate goal is to understand biology as a unitary subject, we then need to integrate these seemingly disparate aspects into a coherent whole. This is what I have attempted to do in this book. Galactose-metabolizing enzymes are expressed in yeast upon exposure to gal- tose but not to glucose. This observation, first made more than a century ago, was to later become a paradigm par excellence with wide-ranging implications. The problem to be tackled here was how yeast (or any organism) adapts to changing environmental conditions. This fundamental problem continues to keep us occupied even today. Come to think of it, for survival, organisms ought to adapt. Therefore, it is not surprising that adaptation transcends every conceivable biological process that goes on in a living system.

Introduction. An overview. Yeast is a eukaryotic model organism. A cell as a biochemical entity.- Adaptation to Environment. Growth and multiplication. Enzyme adaptation. Induction to Leloir enzymes.- Genetic Dissection of Galactose Metabolism. Genetic analysis of GAL regulon. Genetic mapping of GAL genes.- Genetic Analysis GAL Genetic Switch. Negative control by the repressor. Operator repressor model of GAL regulon. Genetic interactions. Conditional lethal mutations. Revised model of GAL genetic switch. Signal transduction in GAL regulon.- Molecular Genetics of GAL Regulon. Cloning: a perspective. Genomic organization of GAL cluster. Isolation of GAL4: the transcriptional activator. Isolation of GAL80: the repressor. Isolation of GAL3: the signal transducer.- Signal Transduction Revisited. Revised model of signal transduction. Genetic dissection of signal transduction.- Versatile Galactose Genetic Switch. Transcription activation. Glucose repression. Fine regulation of GAL genetic switch.- Paradigmatic Role of Galactose Switch. GAL regulon and genomics. GAL regulon and systems biology. Galactose metabolism and evolution. GAL Switch as a tool. Lessons learned.