DNA is an essential component of every living cell as well as some viruses and bacteriophages. Distinction is made between cellular and viral (phage) DNAs.

Cellular DNAS. The DNAs of prokaryotic and eukaryotic cells differ in both molecular weight and location in the cell. In simply structured prokaryotic cells with a single membrane (bacteria , cyanobacteria and some other organisms) and a single chromosome, virtually all of DNA is present in the form of just one macromolecule with a molecular weight exceeding 2 ^. 10⁹. In bacteria, the DNA molecule (accounting for about 1 wt % of the cell) occupies the nuclear zone and does not seem to be bound to proteins. The cytoplasm of bacteria has been found to contain extrachromosomal DNA molecules. Such molecules are known as plasmids (episomes). The molecular weight of plasmid DNA ranges from 1 ^. 10⁶ to 75 ^. 10⁶. Eukaryotic cells (cells of higher organisms, fungi, protozoa, and most algae) contain a nucleus surrounded by the membrane and have a few or many chromosomes. In such cells, the DNA is almost entirely concentrated in the nucleus and distributed among the chromosomes in which it is bound to proteins (histones and other nuclear proteins). The molecular weight of such DNAs is twice that of the corresponding prokaryotic DNAs. In addition to nuclear DNA, eukaryotic cells contain a small amount of cytoplasmic DNA. This category includes satellite DNA as well as mitochondrial and chloroplast DNAs. Their molecular weights are much lower, as compared to chromosomal DNAs (ranging from one to tens of millions). Analogues of plasmid DNAs in bacterial cells have been found among cytoplasmic DNAs in eukaryotes.

Just as DNA, RNA is an essential component of all living cells and also many viruses and bacteriophages. A distinction is made between cellular and viral (phage) RNAs respectively.

Cellular RNAs. Three main types of cellular RNAs are known: messenger (mRNA), ribosomal (rRNA) and transfer (tRNA). Table 5-1 lists some characteristics of each type of RNA as well as their percentage of the total cellular RNA in E. coli.

The RNA content of most cells exceeds that of DNA many times. In bacterial cells, almost all of RNA is found in the cytoplasm. More than 10 per cent of all RNAs in animal cells (e.g., liver) is found in the nucleus (mRNA), about 15 per cent are in mitochondria (tRNA and tRNA), and more than 50 per cent are in ribosomes (rRNA). Each ribosome is made up of three rRNA molecules with sedimentation constants of 23S, 16S and 5S for E. coli. Transfer RNAs differ but insignificantly in molecular weight. The greatest molecular weight differences are observed in mRNAs, which has to do with their function as carriers of genetic information.

Viral RNAs. RNA is a constituent of most plant viruses. Among the most thoroughly studied is the tobacco mosaic virus which contains RNA having a molecular weight of 2 ^. 10⁶. Many bacteriophages, such as MS2 and R17, also contain RNA (with a molecular weight in the neighborhood of a million). Also known are RNA-containing animal viruses, such a s polio virus. All of the above viruses contain a single-stranded RNA molecule per viral particle.

Table 5-1.