Special CAP Lecture: "Efficiency in the Cell: How Cells Make Proteins Rapidly While Working to a Budget"
Dr. Paul Higgs
Physics Department, McMaster University
Tuesday, April 3, 2012
HP 4351

A biophysicist might view a cell as a factory that makes macromolecules. Like all factories, a cell requires energy to work, and energy comes at a cost. Cells that make efficient use of their resources grow and divide faster. Processes that are costly to the cell will be under natural selection, and should evolve towards increased efficiency. One of the most important processes in the cell is translation, the process by which proteins are synthesized by ribosomes. A ribosome is a molecular machine that decodes the genetic information from an mRNA sequence and constructs a protein. Ribosomes bind to one end of a mRNA and move forward, adding one amino acid to the protein for each codon (three nucleotides). One way for cells to be efficient is to make proteins that are cheap; hence, cells tend to make more frequent use of amino acids that are cheaper to synthesize. Another way to be efficient is to speed up the translation process, so that proteins can be made faster with a limited number of ribosomes. The time taken by the ribosome per codon depends on the codon used. Sequence analysis of many bacterial genomes shows that faster codons are indeed preferred. There is also a cost to a cell of making the wrong protein sequence. Thus, selection can act on the accuracy of translation as well as the speed. If more than one ribosome is bound to a single mRNA, these follow one another like trains along the same track. Regions of slow codons can lead to ribosomal traffic jams. We will discuss the application of particle hopping models from statistical physics to the problem of ribosome dynamics and traffic jams. This talk will be an example of how those with a background in physics can build quantitative models that help in understanding important questions in biology and molecular evolution.