UC San Diego

The defining feature of living organisms is their capacity to reproduce and pass on the genetic information so that their progeny can flourish. For bacteria, reproduction is a feat by itself – Escherichia coli cells cultured in optimal conditions grow rapidly and divide about every 20 minutes. In other words, the cell has to replicate all cellular contents, and be ready to divide evenly into two daughter cells within this 20 minutes. Biosynthesis of new cellular materials, e.g. proteins, nucleic acis, lipids and other metabolites accumulate and roughly doubles after every generation. Notably, the deoxyribonucleic acid (DNA) encodes genetic informationand needs to be duplicated in order to faithfully pass on this information to the progeny. This process of DNA replication in the cell needs to dynamically adapt to fluctuation in growth condition and cellular physiology. Such coordination is controlled at the first step of replcation – the initiation of replication. In this thesis, I presented the development of methods for measuring DNA replication duration (replication period), the quantitative relationship between DNA replication and cell size as well as the mechanism of replication initiation. DNA replication measurement laid the foundation of studying the quantitative relationship between cell size and DNA replication. A general growth law was proposed to describe cell size regulation in light of three physiological variables including biosynthesis rate, cell cycle progression and replicaiton initiation. Of the three variables, the mass when cell initiates replication (initiation mass) remains invariant despite a wide spectrum of antibiotics or growth limitation challenge. This invariant initiation mass called into question about the mechanism of initiation to achieve such constancy. We proposed a simple threshold model to explain how cells can maintain a invariant initiation mass by regulating the expression of initiation regulators (initiators). The initiaiton mass is inversely proportional to the initiator levels, which is held constant. Experimental evidence was provided to test our model prediction.