A.10 Viruses
Viruses are not classified as living organisms since they require the use of the
host cell’s biological system in order to reproduce. They are infective agents that
are smaller than bacteria. Viruses are essentially packages of chemicals, known
asvirons, that are able to penetrate a host cell’s membrane. However, a particu-
lar virus is only able to invade specific types of host cells. Once inside the host
cell the virus uses the hosts cell’s biological systems to form multiple copies of its
own proteins and nucleic acids. These new proteins and nucleic acids may
directly cause a pathological condition and/or may be assembled into new virons
in the host cell. The new virons are released from the host by either lysis or a
biological process called budding. The former causes the death of the cell, whilst
the latter does not always result in the death of the host cell. However, in both
cases, the virons are liberated into the extracellular fluid and are now able to
infect other host cells.
Viruses contain a central core of either RNA or DNA fully or partially
surrounded by a protein coating called acapsid(Figure A10.1(a) ).Capsomers
are the individual proteins that form the capsid. Enveloped viruses have an
additional external lipoprotein envelope that surrounds the capsid (Figure
A10.1(b) ). Viruses are broadly divided intoRNA viruses, RNA retroviruses
andDNA virusesaccording to their mode of action and structure. In RNA
viruses, viral RNA replication occurs almost entirely in the cytoplasm. The viral
mRNA used to produce viral proteins either forms part of the RNA carried by
the virus or is synthesized by an enzyme already present in the viron. However,
RNA retroviruses synthesize viral DNA using their RNA as a template and
viral enzyme systems known asreverse transcriptases. This viral DNA is incorp-
orated into the host genome to form a so calledprovirus. Transcription of the
provirus forms viral mRNA, which is used to produce viral proteins. Most
DNA viruses form viral mRNA by transcription of the viral DNA using the
host cell’s polymerases. Some of the viral proteins produced using this viral
mRNA are enzymes that catalyse the production of more viral DNA. Antiviral
drugs act by either inhibiting viral nucleic acid synthesis, inhibiting attachment
to and penetration of the host cell or inhibiting viral protein synthesis.
APPENDIX 10 VIRUSES 257