In considering how viruses multiply or replicate themselves it is
best to avoid any comparison with the growth of an organism. From
a survey of what is known of replication in the viruses of plants and
the higher animals a general picture of biosynthesis emerges, an
assembly rather than a multiplication. Luria puts it like this—4 virus
multiplication belongs on the level of the replication of sub-
cellular elements’, or according to Pirie 4it is the exploitation
and diversion of the pre-existing synthetic capacities of the host
cell’.
In the replication of many viruses, especially the smaller ones,
a dual process is involved in which the protein and nucleic acid are
formed separately and then polymerized into the infectious particle.
The new advances in the techniques of electron microscopy and
X-ray diffraction studies (described in chapter 4) have given us a greater
knowledge of the ultrastructure of virus particles. Using this, it is
possible, to a certain extent, to visualize the process of its construction
or replication.
It was shown by Markham and Smith that the purified virus of
turnip yellow mosaic would separate, after centrifuging, into two
portions called the ‘top and bottom components’. On the electron
microscope the two kinds of particles looked similar, but the top
component was of lower density and contained no nucleic acid.
Moreover, infectivity was confined to the bottom component which
did contain the nucleic acid. This was the first definite proof that the
infectivity of a plant virus was centred in the nucleic acid of the virus
particle and that the nucleic acid was inside the protein coat. Since
then a number of other viruses has been shown to have top and
bottom components, and empty shells similar to the ‘ top component ’
have also been observed in several animal viruses.
Tobacco mosaic virus
A great step forward was made by Gierer and Schramm and
Fraenkel-Conrat when they showed independently in 1956 that the
RNA of tobacco mosaic virus (TMV) could initiate infection in the
absence of the protein part of the virus particle. It has since been
,,,about 60 per cent of the infectivity of normal TMV. The whole
process has been carried out in vitro by Takahashi, whereby infectious
Bacterial viruses
,,,. 1). Anderson first
showed how the phage-particle infects the cell. Contrary to what
might be expected, the virus attaches itself tail first to the bacterial
cell wall which it then penetrates, allowing the DNA contents of the
head to pass down into the bacterium. ...
Kellenberger and his co-workers have shown that the breakdown of
the bacterial nucleus is accompanied by the formation of marginal
vacuoles containing DNA, from which the pool of phage DNA
develops some minutes later. The phage DNA starts to increase
Fowl-plague virus
This belongs to the myxovirus group and seems morphologically
similar to influenza A virus; the multiplication cycle has been
studied by Werner Schafer from whose work this account is taken.
... The cycle of replication is thought to be somewhat as follows
