The cells in which viruses can penetrate, develop and reproduce are
called permissive, and the cells where it is impossible — non-permissive.
Also, there exist cells, which are permissive for some viruses and non-
permissive — for others. The reasons for that are also unclear.
The interaction of viruses with the cells is a complex biological
process. It is realized in several successive phases: attaching the virus to
the cell surface; penetrating of virus particle or its components into the cell;
disintegration of the virus particle; synthesis of the viral components in the
cell; formation of mature viral particles; leaving the cell by mature viral
particles (virions). The term virion is used as a synonym of mature viral
particle.
Coming into contact of the viral particles with the cell surface is
attended by their adsorption, which is a result of their interaction with
mucoproteins located on the cell-wall surface. While it is known that the
phages are attached to bacterial cells by the tails and their spikes, which
prepare the channel for their differential penetration, in plant and animal
cells this question is still unclear. It is supposed that viral particles
penetrate intact in them by using mechanisms resembling pinocytosis. This
mode of penetration is called viropexis.
A great part of the investigations on viruses and phages are devoted to
their biochemistry and reconstitution. In this respect a thorough review is
the monograph of Tikhonenko (1966). The initial impression and the formed
opinion that phages are simply organized living particles composed of
strictly distinguished and easily separated protein molecules and nucleic
acid (DNA or RNA) stimulated some investigators to try to reconstitute them
practically (Fig. 2–89). At present, according to the prevailing concept their
structural and functional organization is much more complex and resists
such manipulations.
The genome of viruses is characterized by an exceptional diversity of
types of structural and functional organization. It can be one linear molecule
of DNA or RNA (rhabdo- or paramyxoviruses) or fragmented (orthomyxo-
viruses, buniaviruses, arenaviruses). For example influenza virus consists of
eight RNA molecules, each of them carrying the information for a synthesis of
one virus-specific protein, and in some cases their number is reduced to
three long segments. Besides, genome RNA (TMV, potato X-virus, cucumber
mosaic, poliomyelitis, influenza, encephalomyocarditis, plague in birds, f2,
etc.) or DNA (herpes, papilloma, adenovirus type 2 and 5,
pseudohydrophobia, measles in animals, SV40, φ X 174, f1, etc.) can be as
single chain, as well as double chain.