by microbiology Doctor dr (doctor-dr)(doctor_dr)
Unit 2: Viruses - Structure, Biochemical Nature, Viral Disease.
VIRUSES
STRUCTURE, BIOCHEMICAL NATURE, VIRAL DISEASES
Before the discovery of disease-causing microorganisms, any disease-causing material was referred to as a virus, which is derived from the Latin word virus, which means dangerous substance. As a result, this word was frequently employed to denote pathogenic bacteria.
HISTORY OF DISCOVERY OF VIRUSES
During their studies on mosaic disease of tobacco plants, L. Pasteur in 1884 and A. Mayer in 1886 demonstrated the existence of viruses as a large group of acellular disease-causing agents composed of nucleic acid encapsulated in a protein coat, about 50 years before the development of the electron microscope. Under controlled laboratory circumstances, they demonstrated that the illness may be transferred from one host to another. However, their tests were unable to determine the nature of viruses since none of these viruses could be produced in pure culture using the methods available at the time.
Before the discovery of disease-causing microorganisms, any disease-causing material was referred to as a virus, which is derived from the Latin word virus, which means dangerous substance. As a result, this word was frequently employed to denote pathogenic bacteria.
In the year 1898, M. Viruses, according to Beijernick, have the ability to replicate, which is a feature shared by all living organisms. He also discovered that these disease-causing filterable viruses proliferate in the cells of the host. F, Loeffler, and P were born in the same year. Foot-and-mouth illness in cattle is also caused by filterable viruses, according to Frosch.
F. Twort and F. d'Herelle later found that certain viruses may infect bacteria on their own. These viruses were dubbed bacteriophages, or bacteria eaters, or just phages. Viruses infect and cause disease in all three major biological groupings, animals, plants, and microbes, according to this research.
W. Stanley demonstrated the structure of the plant virus TMV in 1935. He crystallised the virus and then demonstrated that it is mostly protein. Other researchers then discovered that these crystals contain a modest but consistent quantity of RNA. Other viruses have been shown to have DNA in addition to protein in chemical tests, but no virus has been proven to carry both DNA and RNA. Some viruses also include lipid and tiny quantities of carbohydrates attached to their protein components, in addition to nucleic acid. Poxviruses are the biggest and most complicated viruses, consisting of nucleic acid and numerous internal compartments separated by membranes.
CHARACTERISTICS OF VIRUSES
The chief characteristics of viruses are:
i. They are pathogens that cause illness.
ii. They're pretty tiny, so they're undetectable under a light microscope and can get past filters that keep most germs out.
iii. They are non-living outside the host cells and do not proliferate in conditions meant to promote bacterial development.
iv. They are composed of nucleic acid DNA or RNA core surrounded and protected by a protein coat called capsid.
v. The DNA or RNA function as viral genome. Those that contain RNA contain an unusual biological property of having genetic information permanently encoded in RNA.
TYPES OF VIRUSES
Generally the viruses can be divided into the following types:
i. Animal Viruses: These cause diseases in man and other animals. They usually consist of DNA core protected by capsid.
ii. Plant Viruses: These cause diseases in plants. A plant virus consists of RNA core surrounded by protective protein coat. For example tobacco mosaic virus (TMV).
iii. Retroviruses: This is a special group of viruses. They have RNA as their genetic material. They cause serious disease such as AIDS in humans.
iv. Geminiviruses: These are plant viruses that contain single-stranded DNA as genetic material.
MORPHOLOGY OF VIRUSES
Size and Shape of Viruses
Size and shape are characteristic properties of each type of virus.
Size: They come in a wide range of sizes and shapes. Virions are the tiniest, most basic, and structurally complete infectious viruses. These are available in sizes ranging from 20 to 350 nanometers. The majority of viruses are between 10 and 300 nanometers in size. Viruses that cause poliomyelitis, yellow fever, and foot-and-mouth disease are smaller, with a diameter of around 25 nm, but smallpox viruses are bigger, with a diameter of approximately 250 nm. Bacteriophages come in a variety of forms, but they always have a polyhedral head and tail that they use to infect the host cell. Filamentous bacteriophages exist. Because most viruses are smaller than 150 nm, they can only be seen using an electron microscope.
Shape: The viruses come in a wide range of forms. They can be rod-like (TMV), elongated like an insulated electrical wire, spherical (mumps virus, herpes virus, influenza virus), tadpole-shaped (bacteriophage), or bullet-shaped (bacteriophage) (rabies virus).
Generalized Structure and Biochemical Nature of a Virus
The viruses are non-cellular and have a straightforward structure. Each virus is made out of a DNA or RNA strand that forms a score, which is encircled and protected by a protein coat known as a capsid. Nucleocapsid is the name given to a structure made up of nucleic acid and capsid. A virion is an infective particle that has been fully formed. Most animal viruses, such as herpes and influenza, have an extra lipoprotein envelope generated from the host cell's surface membrane. Plant viruses and phages, on the other hand, are completely naked. Virus coats (capsids) are frequently made up of capsomeres, which are identical repeating units. The capsomeres produce extremely symmetrical structures that may be crystallised and used in X-ray crystallography and electron microscopy to understand their structure.
Nucleic acids make up the viral genome. DNA is found in bacteriophages and animal viruses, whereas RNA is found in most plant viruses. DNA and RNA are never found together in the same virion. Double-stranded or single-stranded DNA or RNA are also possible. The percentage of nucleic acid in a virion ranges from around 1% (influenza virus) to 50% (herpes virus) (bacteriophage). Small viruses (parvoviruses) have three or four genes, while big viruses (therpesviruses and poxviruses) have hundreds. Haploid virions are those that have only one copy of the nucleic acid. Retroviruses are an anomaly, as they have two identical single-stranded RNA strands. They are known as diploids.
Capsids or Virus Coats
Proteins are used to make the capsid. Basic proteins, such as histones; histone-like proteins (adenoviruses); and tiny peptides and polyamines (bacteriophages) are examples of these. Nucleic acids bind to some basic proteins. Furthermore, several viruses have been discovered to include one or more enzymes involved in nucleic acid replication. RNA polymerase is the most prevalent enzyme. Viruses include lipids and carbhydrates as well. Phospholipids, glycolipids, neutral fats, fatty acids, and cholesterol are among them. Phospholipid is the most abundant lipid in the viral envelope. In influenza virus and myxoviruses, carbohydrates are discovered to be linked to proteins.
