Physical Analysis of Chromosome Size Variation

Early characterization of genetic material from a wide range of organisms involved the determination of base composition and genome size. Aside from the intrinsic value of such information, these properties were studied because they could be obtained for the large number of samples where cytogenetic and transmission genetic analysis was onerous or obscure. As it turned out, these general features divulged some of the most fundamental aspects of gene and genome organization and evolution. The base compositional differences among bacteria led to theories about mutational processes that foreshadowed the neutral theory of molecular evolution [1–3] and, among eukaryotes, to the discovery of the isochore structuring within chromosomes [4]. With respect to genome-size variation, the results were equally consequential. Across life forms, there seemed to be little relationship between the amount of genetic material and the degree of organismal complexity (the so-called “C-value paradox”), which has led to inquiries about the amounts, the accumulation, and the function of non-coding DNA in genomes [5–9]. Within bacteria, genomesize would appear to have direct consequences on the biology of an organism: because of the high coding content of bacterial DNA, variation in genome-size implies differences in the absolute number of genes.