Chapter X The Determination of the Molecular Weight of DNA Per Bacterial Nucleoid

Abstract

It is necessary to know the molecular weight of DNA per bacterial nucleoid because the information gives an insight into the genetic potential of the organism. Dividing the molecular weight of the bacterial DNA by the molecular weight of an average bound nucleotide pair as the sodium salt gives the total number of nucleotide pairs. As an average of some 1500 nucleotide pairs makes up one cistron, the maximum total number of cistrons/nucleoid can be estimated. Part of the bacterial DNA may not be expressed phenotypically, because of, for example, unused genes or nonsense DNA, although this unexpressed part appears to be rather small. A few data on the molecular weight of bacterial DNA per nucleoid are available. They show that the values can vary at least 10-fold. The amount of DNA is determined chemically in a certain volume of bacterial suspension, containing a known number of bacterial cells. In separate preparations, cells are stained to reveal nucleoids, and the average number of nucleoids/cell is determined. The molecular weight of DNA per nucleoid can then be easily calculated.