SHAPES OF VIRUSES
A virus can have one of two structures. These are:- Helical virus. Ahelical virusis rod- or thread-shaped. The virus that causes
rabies is a helical virus. - Icosahedral virus. An icosahedral virusis spherically shaped. Viruses that
cause poliomyelitis and herpes simplex are icosahedral viruses.
HOW VIRUSES REPLICATE
The easiest way to understand how viruses replicate is to study the life cycles of
viruses called bacteriophages. Bacteriophages replicate by either a lytic cycle or
a lysogenic cycle. The difference in these two cycles is that the cell dies at the
end of the lytic cycle and remains alive in the lysogenic cycle.
The first two scientists to observe bacteriophages were Frederic Twort of
England and Felix d’Herelle of France in the early 1900s. The name bacterio-
phage is credited to d’Herelle and means “eaters of bacteria.”
Lytic Cycle
The most studied bacteriophage is the T-even bacteriophage. The virions of T-
even bacteriophages are big, complex, and do not contain envelopes. The T-even
bacteriophages are composed of a head-and-tail structure and contain genomes
of double-stranded DNA. The lytic cycleof replication begins with the collision
of the bacteriophage and bacteria, called attachment.The tail of the bacterio-
phage attaches to a receptor site on the bacterial cell wall. After attachment, the
bacteriophage uses its tail like a hypodermic needle to inject its DNA (nucleic
acid) into the bacterium. This is called penetration.The bacteriophage uses an
enzyme called lysozyme in its tail to break down the bacterial cell wall, enabling
it to inject its DNA into the cell. The head or capsid of the bacteriophage remains
on the outside of the cell wall. After the DNA is injected into the host’s bacter-
ial cell’s cytoplasm, biosynthesisoccurs. Here the T-even bacteriophage uses the
host bacterium’s nucleotides and some enzymes to make copies of the bacterio-
phage’s DNA. This DNA is transcribed to mRNA, which directs the synthesis of
viral enzymes and capsid proteins. Several of these viral enzymes catalyse reac-
tions that make copies of bacteriophage DNA. The bacteriophage DNA will then
direct the synthesis of viral components by the host cell.
Next maturationoccurs. Here the T-even bacteriophage DNA and capsids are
put together in order to make virions.
CHAPTER 12 Viruses, Viroids, and Prions^187