Too Many Mutants with Multiple Mutations

ABSTRACT

It has recently become clear that the classical notion of the random nature of mutation does not hold for the distribution of mutations among genes: most collections of mutants contain more isolates with two or more mutations than predicted by the mutant frequency on the assumption of a random distribution of mutations. Excesses of multiples are seen in a wide range of organisms, including riboviruses, DNA viruses, prokaryotes, yeasts, and higher eukaryotic cell lines and tissues. In addition, such excesses are produced by DNA polymerases in vitro. These “multiples” appear to be generated by transient, localized hypermutation rather than by heritable mutator mutations. The components of multiples are sometimes scattered at random and sometimes display an excess of smaller distances between mutations. As yet, almost nothing is known about the mechanisms that generate multiples, but such mutations have the capacity to accelerate those evolutionary pathways that require multiple mutations where the individual mutations are neutral or deleterious. Examples that impinge on human health may include carcinogenesis and the adaptation of microbial pathogens as they move between individual hosts.     

Too Many) Mathematical Models of Circadian Clocks (?

Many mathematical models derived from the principles obtained from empirical observations in chronobiology have been proposed and explored. They cover several organisms and phenomena, and utilize quite different formal approaches. These models can be divided into the ones that intend to describe pacemaker core function, such the Goodwin-oscillator family, non-genetic approaches or purely mathematical (i.e., without clear biochemical correlations) models, and the ones that represent events depending on pacemaker activity, i.e., photoperiodic phenomena. We aim to illustrate the diversity of mathematical and methodological approaches to describe circadian systems and related matters.