Molecular models for the circadian clock: I. The chronon concept

Abstract

According to the chronon concept of temporal control proposed here, circadian rhythms in eukaryotic cells and organisms result from the recycling of a sequential machine that regulates the transcription of template RNA from DNA. The eucell is regarded as an event-generator (a clock) whose circadian escapement consists of a sequential transcription component (the chronon) and a chronon recycling (CR) component. The chronon is a very long polycistronic complex of DNA whose transcription rate is limited by some functions of eukaryotic organization that are relatively temperature independent. Each eucell contains hundreds of chronons on each of its nuclear chromosomes, and many sets of extranuclear chronons (free of any attachment to euchromosomes) in its different cell organelles. Upon a given chronon, RNA transcription proceeds unidirectionally from the initiator cistron, C t , to the terminator cistron, C t , Once the products of translation of the message of C t have been formed, the point of no return is passed and the escapement mechanism operates on its CR component. The CR includes posttranscriptional events of translation, end-product formation and polymer assembly, and pretranscriptional events that cause an initiator substance to accumulate; when the initiator arrives at its target cistron C t , the system proceeds to recycle into its next circadian phase. The relationship of chronons to other macromolecular categories and events, as well as to cytological, mitotic and circadian phenomena are discussed.