The Number of Genes in the Mammalian Genome and the Need for Master Regulatory Genes

Abstract

The genome size (the amount of DNA contained in the haploid set of chromosomes) is fairly uniform among divergent mammalian species. It consists characteristically of about 3 x 10⁹ DNA base pairs, thus the mammalian genome is roughly 1000 times the size of the Escherichia coli genome. Any notion of proportionality between the degree of the organism’s complexity and the genome size, however, should quickly be dispelled by the realization that a sluggish lungfish, Lepidosiren paradoxa, is endowed with a genome 35 times greater than that of mammals (Ohno and Atkin, 1966). In fact, such unyieldingly enormous genomes characterize crossopterygian lungfish as well as tailed amphibians, e.g., salamanders and newts. Among flowering plants too, one finds, a peculiar situation with regard to the genome size. For example, four species belonging to the genus Lathyrus are apparently endowed with the identical karyotype as far as the chromosome number and morphology are concerned (2n = 14). Yet there is a maximum difference of threefold in their genome sizes (Rees, 1972). Although life on this Earth owes its existence to the capacity of DNA to semiconservatively replicate itself, based on the inherent complementarity that exists between two pairs of bases, it would appear that a part in which functional messages are encoded constitutes a third or less of the eukaryotic genomic DNA.