Introduction to Viruses

Introduction to virusesStructure and classification

References: Main textbook: Medical Microbiology, Jawetz, Melnick, 26th ed.,2013
1st lecture of Medical Virology for 3rd year student s
(College of Medicine- University of ThiQar)

Presented by Dr. Mohammed J. M. Shallal

• Viruses can infect all forms of life (bacteria, plants, protozoa, fungi, insects, fish, reptiles, birds, and mammals).
• Viral diseases are considered as one important reason of mortality and permanent disability, especially among infants and children. Antibiotics are effectively control most bacterial infections, while the viral infections pose a relatively greater and less controlled threat to human health.
• Viruses: are the smallest infectious agents (20-300nm) in size, composed of nucleic acid surrounding by protein shell which is in some type of viruses surrounding by lipid envelope.
• Viruses are small, sub cellular that are unable to multiply outside a living host cell. Containing only one kind of nucleic acid (DNA or RNA) as their genome
Medical virology is the science that deals with the study of medically important viruses



Nucleic acid is enclosed in a protective protein shell which may be surrounded by lipid containing membrane.
Viruses replicate only in living cells (obligatory).


By 2000, the International Committee on Taxonomy of Viruses had organized more than 4000 animal and plant viruses into 56 families, 9 subfamilies, and 233 genera.
However, there are only 21 families of viruses are capable of causing human infections .

Properties of viruses?

They are inert in the extracellular environment. Viruses do not posses all the enzyme systems necessary for the synthesis of new viral materials, therefore they are dependent on the parasitized cell for survival and multiplication.
They are parasite at genetic level.
Viral nucleic acid containing information causing programming the infected host cell to synthesize number of virus specific macromolecule for the production of virus progeny.
The coat protein assemble together to form a capsid which stabilize the virus against the extracellular environment (such as nucleases) and facilitate attachment and perform penetration of the virus to the susceptible host cell.
The virus is obligatory intracellular microorganism , so the virus does not live outside the cell.

Viruses differ from other microorganisms by structure, biology and replication

Property
Virus
Bacteria
Size
20-300nm
1000nm
Genome(nucleic acid(
DNA or RNA, but not both
DNA and RNA
Cell wall
Envelope present in some viruses
Have cell wall
Ribosome
No ribosome
Have ribosome
Multiplication by binary fission
(-)
(+)
Sensitivity to antibiotics
• (-)
(+)
Growth in culture media
Grow only in living host cells
Grow in culture media


Virus particle = virion

There are some terms related to viruses:

Capsid: The protein shell, or coat that encloses the nucleic acid genome: The capsid has specific receptors that are responsible for binding to specific type of cells and specific number of identical subunits called Capsomeres

Capsomeres: Morphologic units seen in the electron microscope on the surface of icosahedral virus particles. Capsomeres represent clusters of polypeptides, but the morphologic units do not necessarily correspond to the chemically defined structural units.

•

Defective virus: A virus particle that is functionally deficient in some aspect of replication.

Envelope: A lipid-containing membrane that surrounds some virus particles. It is acquired during viral maturation by a budding process through a cellular membrane. Virus-encoded glycoproteins are exposed on the surface of the envelope. These projections are called peplomers.

Nucleocapsid: The protein-nucleic acid complex representing the packaged form of the viral genome.




Virion: The complete virus particle. In some instances (eg, papilloma viruses, picorna viruses), the virion is identical with the nucleocapsid. In more complex virions (herpesviruses, orthomyxoviruses), includes the nucleocapsid plus a surrounding envelope. Virion, serves to transfer the viral nucleic acid from one cell to another.

Picornaviruses

Orthomyxoviruses

Basis of viral Classification:
Viruses are classified on the basis of morphology, chemical composition, and mode of replication.
The viruses that infect humans are currently grouped into 21 families, reflecting only a small part of the spectrum of the multitude of different viruses whose host ranges extend from vertebrates to protozoa and from plants and fungi to bacteria.
The following criteria have been used as a basis for the classification of viruses:
A- Based on chemical and physical criteria:
1- Morphology: Viruses are grouped on the basis of size and shape, chemical composition and structure of the genome (symmetry), and mode of replication.

Icosahedral symmetry (cubic(: composed of 12 vertices and 20 equal triangular sides, with approximate outline of sphere, e.g Herpes virus and Adenovirus.

Helical symmetry: the capsomeres are arranged like steps in a spiral strain case or hollow, rod shaped, the helix rigid or flexible, e.g influenza and parainfluenza viruses.


Complex viruses: e.g Poxvirus, in which there are many layers around the capsid.

Types of symmetry of virus particles

2- Virus genome properties, including

Type of nucleic acid (DNA or RNA).
Size of genome in kilobases (kb) or base pairs (bp).
Strandedness (single or double).
Whether linear or circular, sense (positive, negative).
Segments (number, size).
Nucleotide sequence.
G + C content.
3- Physicochemical properties of the virion, including:
Molecular mass,
pH stability,
Thermal stability,
Susceptibility to physical and chemical agents, especially Ether and other detergents.


In some RNA viruses they are positive polarity and in other are negative polarity.............
1- Positive polarity means that the RNA sequence is as the same as the sequence of mRNA, so do not need transcription.
2- Negative polarity means that the base sequence is different from the sequence of mRNA.

Gene Segments (number, size).

Nucleotide sequence.
G + C content.
3- Physicochemical properties of the virion, including:
Molecular mass,
pH stability,
Thermal stability,
Susceptibility to physical and chemical agents, especially Ether and other detergents.

(4) Virus protein properties, including: number, size, and functional activities of structural and nonstructural proteins, amino acid sequence, modifications (glycosylation, phosphorylation), and special functional activities (transcriptase, reverse transcriptase, neuraminidase and fusion acticity).
(5) Genome organization and replication, including:
gene order,
Strategy of replication (patterns of transcription, translation).
Cellular sites (accumulation of proteins, virion assembly, virion release).
(6) Antigenic properties.
(7) Biologic properties, including: Natural host range, Mode of transmission, Vector relationships, Pathogenicity , Tissue tropisms and pathology.


B- Classification according to diseases they produce:
Generalized diseases: in which virus is spread throughout the body via blood stream and in which multiple organs are affected Skin rashes may occur, these include Measles, rubella, chicken pox, yellow fever and enteroviruses.
Diseases primarily affected specific organs:
a- Diseases of CNS, such as poliomyelitis, rabies, aseptic meningitis and herpes simplex.
b- Diseases of liver, such as hepatitis type A,B,C,D,E, yellow fever and rubella virus.
d- Diseases of skin or mucous membranes, such as herpes simplex, molluscum contagiosum, warts and herpes zoster.
e- Diseases of Eye, such asadenovirus, herpes keratoconjunctivitis and epidemic haemorragic conjunctivitis.

f- Diseases of the gastrointestinal tract, such as rotavirus and enteric adenviruses.

g- Sexually transmitted diseases, such as herpes, hepatitis B virus, papilloma viruses, reteroviruses (HIV) and cytomegalovirus.
C- Classification of viruses based on nucleic acid genome
DNA viruses: divided into:
A- Enveloped DNA viruses like: Hepadnaviruses (Hepadnaviridae)Herpesviruses and (Herpesvridae).
B-Non enveloped DNAviruses like: Parvoviridae, Papillomaviridae ,Polyomaviridae (Papovaviridae) and Adenoviridae .
Poxviridae :Complex DNA viruses –C
RNA viruses divided into :

A- Enveloped RNA viruses such as-Non ,Picornaviruses ,Astroviruses Caliciviruses and Reoviruses.

B- Enveloped RNA viruses such as Togaviruses, Arenaviruses ,Flaviviruses,Reteroviruses , Orthomyxoviruses, Bunyaviruses ,Rhabdoviruses, Paramyxoviruses and Filoviruses

RNA viruses

RNA viruses

Evolutionary Origin of Viruses
Three hypotheses of viral origin can be summarized as follows:
(1) Pre-cellular origin hypothesis: viruses originated before cells
(2) Escape host gene hypothesis: Viruses may be derived from DNA or RNA nucleic acid components of host cells that became able to replicate autonomously and evolve independently. Fragments of cellular genomes became infectious
(3) Regressive evolution hypothesis: cells or proto-cells evolved into virions . Viruses may be degenerate forms of intracellular parasites. There is no evidence that viruses evolved from bacteria, though other obligately intracellular organisms, eg, rickettsiae and chlamydiae, presumably did so.
Non of the above hypotheses explain the origin of virus..but the Pre-Cellular theory is most popular.


Universal System of Virus Taxonomy

A system has been established in which viruses are separated into major groupings called families on the basis of virion morphology, genome structure, and strategies of replication. Virus family names have the suffix -viridae.
And each family is divided to subfamily –virinae within each subfamily, subdivisions called genera are usually based on physicochemical or serologic differences.

Criteria used to define genera vary from family to family. Genus names carry the suffix –virus.

Nomenclature of viruses

In the early days of virology, viruses were named according to common pathogenic properties: organ tropism and/or modes of transmission, and often also after their discoverers. From the early 1950s until the mid-1960s, it was popular to compose virus names by using (abbreviations derived from a few or initial letters.

The name “Picornaviridae” is derived from pico (small) and RNA; the name “Reoviridae” is derived from respiratory, enteric, and orphan viruses because the agents were found in both respiratory and enteric specimens and were not related to other classified viruses; “Papovaviridae” is from papilloma, polyoma, and vacuolating agent; “Retrovirus” is from reverse transcriptase; “Hepadnaviridae” is from the replication of the virus in hepatocytes and their DNA genomes, as seen in hepatitis B virus.

Adenoviridae (adeno, “gland”; refers to the adenoid tissue from which the viruses were first isolated); Astroviridae (astron means star); Arenaviridae (arena “sand”) describes the sandy appearance of the virion. Bunyaviridae (from Bunyamwera, the place in Africa where the type strain was isolated); Calicivirus (calix, “cup” or “goblet” from the cup-shaped depressions on the viral surfaces); Coronaviridae (corona, “crown”) describes the appearance of the peplomers protruding from the viral surface; Filoviridae (from the Latin filum, “thread” or “filament”) describes the morphology of these viruses. Herpesviridae (herpes, “creeping”) describes the nature of the lesions; Orthomyxoviridae (ortho, “true,” plus myxo “mucus,” a substance for which the viruses have an affinity; Paramyxoviridae derived from para, “closely resembling” and myxo; Parvoviridae (parvus means, “small”); Poxviridae (pock means, “pustule”); Rhabdoviridae (rhabdo, “rod” describes the shape of the viruses and Togaviridae (toga, “cloak”) refers to the tight viral envelope.

Several viruses of medical importance still remain unclassified. Some are difficult or impossible to propagate in standard laboratory host systems and thus cannot be obtained in sufficient quantity to permit more precise characterization. Hepatitis E virus, the Norwalk virus and similar agents that cause nonbacterial gastroenteritis in humans are now assigned to the calicivirus family.






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