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Figure 2–89. Reconstitution of viral particles (TMV) from isolated protein subunits and RNA-molecules (After Fraenkel-Konrat, 1962).
In some viruses (phage φX 174) the nucleotide composition of DNA
differs from the normal one. The rule A + G = T + C is violated and the
“normal” formation of A—T and G—C base pairs is impossible. T-even
phages contain 5-hydroxymethylcytosine (HMC) instead of cytosine. In this
respect there are many other differences considered in detail in the
literature on virology and molecular biology. In unfolded state, after an
osmotic shock (sudden diluting of the phage suspension in a concentrated
salt solution with distilled water) the molecule of DNA has “enormous”
dimensions, about 50 nm in length, exceeding several hundred times the
length of the head containing it before (Fig. 2–90).
Now, let us return to the question posed: are the viruses cells? First
of all the answer to this question should be looked for in elucidation of their
origin and evolution. There exist three main hypotheses about the origin of
viruses: 1) from primitive precellular forms of life; 2) product of a
“regressive” evolution of bacteria; 3) came off and acquired independence
cellular components.
Most unacceptable is the first hypothesis that viruses are precellular
forms of life. More recognized is the second hypothesis about the origin of