STM 326
VIROLOGY
LECTURE 1
DEFINITION
Virology is a part of microbiology or pathology that deals with the study of viruses and virus-like agents (Viroids and Prions), including their taxonomy. disease producing properties, cultivation and genetics VIROLOGY also is studying how a bundle of protein called interteron, which are produced during a viral infection of a cell, triggers an immune response. these
proteins Somehow inhibit the replication of the Virus in the cell.
Virology also is looking at how viruses may cause some types of cancer and how Some viruses cause the body's immune response to Occasionally malfunction an develop autoimmune disorders.
WHAT ARE VIRUSES
Viruses are small, acellular agents that are unable to multiply outside a host cell (intracelular, obligate parasitism). The assembled virus (virion) is tormed to include
only one type of nucleic acid (RNA or DNA) and, in the simplest viruses, a protective protein coat. The nucleic acid contains tne genetic information necessary to program
the synthetic machinery of the host cell tor viral replication.
The protein coat serves two main functions:
it protects the nucleic acid Trom extracellular environmental insults such as nucleases
it permits attachment or the Viron to the membrane ot the host cell, the negative charge of which would repel a naked nucleic acid.
Once the viral genome has penetrated and thereby infected the host cell, virus replication mainly depends on most cell machinery for energy and synthetic requirements.
does have traditionally been viewed in a rather negative context as agents responsible for disease that must be controled or eliminated. However, viruses also have certain beneficial properties that can be exploited for useful purposes (for example in gene therapy or vaccinology)
Viruses have an interesting role too in genetics, because of their ability to carry extra genetic material into host cells. This ability has been studied by virologist through transferring material specitying a particular enzyme into the nucleus of host cells that lack the ability to synthesize that enzyme. his method has particular interest for those studying hereditary enzyme-deficiency diseases such as diabetes, because these gene transfers may help to cure such diseases.
The various virion components are synthesized separately within the cell and then assembled to form progeny particles. This assembly type of replication is unique to Viruses and distinguishes them from all other small, obligate, intracellular parasites.
These other acellular infectious agents include:
Viroids
Prions
Virusoids
Viroids are the smallest infectious pathogens known. They are composed solely of a short strand of circular, single-stranded RNA that has no protein coating (capsid).
They are majorly plant pathogens and like viruses, can reproduce only within a host cell. Examples include Potato spindle tuber viroid (PSTV).
Prions are small infectious particle composed of abnormally folded protein that causes progressive neurodegenerative conditions. These mis-folded proteins do not multiply in the host organism that they infect, but they affect the brain structure by acting as a template, inducing proteins with normal folding to convert to the abnormal prion form. Examples are Bovine spongiform encephalopathy (BSE or mad cow disease) seen in cattle and Ivestock and Creutzfeldt-jakob disease (CJD) seen in humans.
Virusoids are cirular single-stranded RNAS dependent on plant viruses for replication and encapsidation. The f virusolds consist of several hundred enome of virusoid nucleotides and does not code for for any proteins. Since they depend on helper viruses, they are classified as satelites. Example is the hepatitis D virus which is capable of causing pathology when in the presence o e nepatitis B virus
The basic structure of viruses may permit them to be simultaneously adaptable and selective. Many viral genomes are so adaptable that once they have penetrated the cell membrane under experimental conditions, viral replication can occur in almost any cell. On the other hand, intact viruses are so selective that most Virions can infect only a limited range of cell types. This selectivity exists largely because
penetration of the nucleic acid usually requires a specific reaction for the coat to attach to the host cell membrane and some specific intracellular components.
Viruses can infect all forms of life (bacteria, plants, protozoa, fungi, insects, fish, reptiles, birds, and mammals); Viruses are difficult targets for chemotherapy because they replicate only within host cells, mainly utilizing many of the host cell's biosynthetic processes. The similarity of host-directed and virus-directed processes makes it difficult to find antiviral agents specific enough to exert a greater effect on viral replication in infected cells than on functions in uninfected host cells. It is becoming increasingly apparent, however, that each virus may have a few specific steps of replication that may be used as targets for highly selective, carefully aimed
chemotherapeutic agents. Therefore, proper use of such drugs requires a thorough knowledge of the suitable targets, based on a correct diagnosis and a precise understanding of the replicative mechanisms for the offending virus. Knowledge of the pathogenetic mechanisms by which virus enters, spreads within, and exits from the body also is critical for correct diagnosis and treatment of disease and for prevention of spread in the environment. Effective treatment with antibody- containing immunoglobulin requires knowing when virus is susceptible to antibody (for example, during viremic spread) and when virus reaches target organs where
antibody is less effective. Many successful vaccines have been based on knowledge
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