VIRAL MULTIPLICATION
The nucleic acid in a virion contains only a few of the genes needed for the synthesis of new viruses.
These include genes for the virion's structural components, such as the capsid proteins, and some of the
enzymes used in the viral life cycle. Enzymes needed for protein synthesis, ribosomes, tRNA, and energy
production are supplied by the host cell and are used for synthesizing viral proteins, including viral
enzymes.
Thus, for a virus to multiply, it must invade a host cell and take over the host's metabolic
machinery.
A single virion can give rise to several or even thousands of similar viruses in a single host cell.
In a few viral infections, cells survive and continue to produce viruses indefinitely.
The multiplication of viruses can be demonstrated with a one-step growth curve.
The data are obtained by infecting every cell in a culture and then testing the culture medium and
cells for virions and viral proteins and nucleic acids.
Multiplication of Bacteriophages
The basic mechanism of viral multiplication is similar for all viruses.
Bacteriophages can multiply by two alternative mechanisms: the lytic cycle or the lysogenic cycle.
- Lytic cycle ends with the lysis and death of the host cell, whereas the host cells remain alive in
the,lysogenic,cycle.
The T-even bacteriophages (T2, T4, and T6) have been studied most extensively, we will describe
the multiplication of T-even bacteriophages in their host, E. coli, as an example of the lytic cycle.
T-Even Bacteriophages: The Lytic Cycle
The virions of T-even bacteriophages are large, complex, and non-enveloped, with a
characteristic head-and-tail structure.
The length of DNA contained in these bacteriophages is only about 6% of that contained in E.
coli, yet the phage has enough DNA for over 100 genes.
The multiplication cycle of these phages, like that of all viruses, occurs in five distinct stages:
attachment, penetration, biosynthesis, maturation, and release.
