Virology

Viruses Of Plants And Animals

Viruses of plant and animal differ greatly in shape. They do not have some characteristic morphology as shown by bacteriophages. Here we would discuss how viruses of plants and animals were discovered and how their discovery is the beginning of new age of microbiology.

Viruses And Vaccination

Virus is not new in medical terminology. Some virus disease have been known since centuries. Indeed, the first infectious disease for which a practical and effective method of prevention was developed was a virus disease. In 1796 Jenner first vaccinated  an 8 year old boy against smallpox.

The first American known to have vaccinated was Daniel Oliver Waterhouse. The 5 year old Daniel and two servants were vaccinated July 8, 1800. They were later inoculated with smallpox virus and found fully protected. Such daring and dramatic demonstration proved the value of vaccination and brought it in the attention of officials and the public.

One of Pasteur’s associates, Charles Chamberland, had meanwhile learned that porcelain filter would prevent passage of bacteria but allow passage of causative agent of rabies. Because in those days the term virus was least used to some toxic substances those can cause disease, it was only natural to describe those unseen filter-passing agents of disease as filterable viruses.

Tobacco Mosaic Virus And Filterability

In 1892, Dmitrii Ivanowski discovered that the causative agent of Tobacco mosaic disease was filterable. It was shown by his experiment in which he placed some bacteria free filters and transfer of disease causing agent to healthy Tobacco leaves. His work was supported by Beijerinck, 1898.

These experiments marked the beginning of a new phase of microbiology, the study of infectious agent invisible even through the most powerful microscope of that time. Although viruses cannot be grown on the non-living culture media, viruses are capable of causing disease in plants and animals.

The discovery in 1935 that the tobacco mosaic virus (TMV) can be crystallized. Was the most important contribution towards virology. Since the finding of TMV, it is discussed that viruses are living or non living. Because crystals of TMV can cause disease in tobacco plants and can replicate themselves, is the property of living. But on the other hand, they cannot be grown on artificial media and show non living properties outside living cells. 

The question that viruses are living or non-living has not been ever settled to everyone’s satisfaction.

Structure And Composition

Like viruses of bacteria (bacteriophage) plant and animal viruses have a central core of nucleic acid surrounded by a capsid, which is made up of capsomeres. Most plant and animal viruses have a characteristic symmetry: (1) icosahedral in case of spherical viruses, (2) helical in case of the rod shaped viruses, (3) complex in case of a miscellaneous group.

In some animal viruses the nucleocapsid (nucleic acid and capsid) is covered by an outer membrane like structure called envelope. Which is made up of lipoprotein. Virions that have envelope are sensitive to lipid solvent such as ether and chloroform. Their capacity to infect can be neutralized by these solvent. Nonenveloped viruses are called naked virions. Such viruses are not affected by lipid solvents.

Nucleic Acid

The viral genome, containing all the genetic information, is composed of nucleic acid. Like bacteriophages, animal and plant viruses contain either DNA or RNA but never both as the genetic material in the same virion. Further, the genome of higher organisms consist of dsDNA. But the genome of a virus can consist of DNA or RNA that is either double stranded or single stranded. All four type of genome have been found in bacterial, animal, and plant viruses.

Virions contain only a single copy of the nucleic acid i.e. they are haploid. The exception is the retroviruses; they have diploid virions because they contain two identical ssRNA genomes.

Single stranded viral RNA molecules which act as mRNA directly in the host cells have been designated as positive, or plus (+) strand. Viruses with negative, or minus (-) strands RNA, must first replicate their RNA (using RNA transcriptase carried within the virion) to form a complementary strand which then act as a mRNA. RNA tumor viruses have two equal positive strands RNA molecules.

Reference: Microbiology Pelczar

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