How an Organism Works

Abstract This chapter’s Narrative considers how organisms regulate the expression of their genes, and hence the levels of their encoded proteins, via complex feedback systems. These programs play out in time in response to internal and external cues and are governed by transcription factors called activators and repressors that bind to promoter sequences in DNA. In multicellular organisms, such patterns of gene expression also play out in space, with some genes expressed in some cells/tissues and not others, creating embryonic and adult forms. The chapter’s Reflections consider how the complexity and improbability of being a human self, of having a life, invites profound gratitude for and celebration of one’s existence.