is that expected for rapidly growing cells, and that explains why it contains several copies of its genome, not just one.

It is important to realize that the number of ribosomes in bacteria like E. coli is tightly regulated so that if all the ribosomes present in a cell constantly make protein at the maximum possible rate (10-20 amino acids incorporated per ribosome per second), the protein mass in the cell will have just doubled by the time cell division begins. The number of copies of each of the enzymes involved in intermediary metabolism is similarly regulated to balance supply and demand. Hence the macromolecular composition of these cells depends on their overall growth rate, which in turn is coupled to the mix of small organic molecules available in the environment. Finally, it should be noted that the water content of these cells, 70% by weight, is typical of actively metabolizing cytoplasm.

A cell a tenth the volume of E. coli is easy to envision. It might have 2 copies of a genome that is about the same size, which would imply an interior DNA concentration.⁵ times that of E. coli. This means that there would not quite be room in such a cell for the 3,000 ribosomes and 5 × 10⁵ protein molecules it would require, and still maintain a water content of 70%. However, if the organism's metabolism were adjusted so that its generation time was a bit longer than that of E. coli, all would be well. There is a limit to how far this kind of balancing of generation times and macromolecular compositions will take you. The reason is that while the number of copies of each kind of macro-molecule a cell requires scales approximately with its volume, the amount of genome it takes to encode them does not. The consequences become obvious when we consider cells whose volumes are 1% that of E. coli. A cell that size has a mass of only ˜10¹⁰ daltons. If it must do all the things E. coli can, it needs a genome the same size, i.e., one that weighs 3 × 10⁹ daltons. If it also has to be 70% by water weight, there is no room for anything else.

It is instructive to examine Table 2, which compares the DNA concentrations in three DNA-containing objects, the interior volumes of which stand approximately in the ratio of 10,000:100:1: E. coli, Mycoplasma genitalium , a small mammalian parasite, and the head structure of bacteriophage T4, which is a virus. Even though the genome of M. genitalium is significantly smaller than that of E. coli, as one anticipates it must be, it isn't two orders of magnitude smaller, and consequently the DNA concentration in M. genitalium is much higher than in E. coli. The DNA concentration in the T4 head, which is higher still, approaches that of pure, hydrated DNA.

It is important to realize that the DNA in T4 heads is metabolically inert, and is intended to be so.