(q.v.). The table above compares properties of some of the viruses mentioned in the dictionary. Viruses have been isolated which parasitize organisms that belong to the Eukaryotes, the Bacteria, and the Archaea.
Certain double-stranded DNA viruses from each group have major coat proteins called double-barrel trimers that are architecturally similar. This means that these viruses all evolved from a common ancestor that existed before cellular life evolved on earth.
In the intervening 3 billion years mutations have erased all similarities in nucleotide sequences in the genomes of these viruses.
Viral nu- cleic acid molecules are linear (1), circular (c), or linear, but in two or more segments (ls). Virus acro- nyms: ALV (avian leukosis virus), EBV (Epstein- Barr virus), HAdv-2 (human adenovirus-2), HCMV (human cytomegalovirus), HHV (human herpes vi- rus), HIV (human immunodeficiency virus), MMTV (mouse mammary tumor virus), MoMLV (Moloney murine leukemia virus), RSV (Rous sarcoma virus), SARS (severe acute respiratory syndrome), SV40 (simian virus 40), TMV (tobacco mosaic virus), VAR (variola virus), VV (vaccinia virus).
See Appen- dix A, Prokaryotes; Appendix C, 1971, Baltimore; 2004, Rice et al.; Appendix E, Individual Databases; Appendix F; bacteriophages, Baltimore classification of viruses, enveloped viruses, herpes virus, oncogenic viruses, peplomers, plus (+) and minus (−) viral strands. virusoid molecules that, like viroids (q.v.), are single-stranded, circular RNAs. Unlike viroids, they are encapsulated within the virion of several plant vi- ruses.
The velvet tobacco mosaic virus is an example. virus receptors sites on the cell membrane to which viruses attach. Such sites contain neuraminic acid (q.v.). viscoelastic molecular weight determination a method using a viscometer that allows the determi- nation of the molecular weights of the largest mole- cules present in a solution.
The technique is very useful in determining the molecular weights of very long DNA molecules, since a fraction of these are fragmented during the isolation procedure. See Ap- pendix C, 1973, Kavenoff and Zimm. Visconti-Delbruck hypothesis according to this proposal, bacteriophages multiply upon entering a host, and the replicating units so formed mate re- peatedly. Mating occurs in pairs and is at random with respect to the pairing partner.
During any given mating cycle, a segment of genetic material from one parent can exchange with that from a second parent phage, yielding recombinant units.
See Appendix C, 1953, Visconti and Delbru¨ck. visibles referring to phenotypically observable mutants, as opposed to lethals, which are scored by the absence of an expected class of individuals in a cross designed to detect induced mutants. visual pigments molecules that participate in re- actions that occur after the absorption of a photon