VIRAL MORPHOLOGY & REPLICATION

ANTIVIRAL AGENTS

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(CLASSIFICATION AND MECHANISM OF ACTION)

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Lecture objective

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• To familiarize you with the structural components of virus, which can act as antigens during the infection process.

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• The unique nature of viral nucleic acid and its role in the infection process.

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• The morphological types of virus in order that this information can be used in making a diagnosis.

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• To develop an understanding of the virus replication cycle in order to appreciate how the physician can interrupt this cycle.

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What is virus

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• Viruses are obligate intracellular parasites that either contain double or single stranded DNA or RNA enclosed in a protein coat called capsid.

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• Some viruses possess a lipid envelope that contain antigenic glycoprotein.

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• Most viruses contain or encode enzymes essential for viral replication inside the host cell.

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• Viruses usurp the metabolic machinery of the host cell.

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Structural components

Nucleic acid

• DNA
• RNA

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Capsid

• This accounts for most of the virion mass.
• It is a complex and highly organized entity which gives form to the virus.
• Subunits called protomeres aggregate to form capsomeres which in turn aggregate to form the capsid.

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Envelope

• This structure is composed of lipid, protein and carbohydrate which lies to the outside of the capsid.

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• It contains a mosaic of antigens from the host and the virus.
• A naked virus is one without an envelope.

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Spikes

• These are glycoprotein projections which have enzymatic, absorption and hemagglutinating activity.
• They arise from the envelope and are highly antigenic.

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Classification

DNA viruses

• DNA viruses enter the host cell nucleus.
• It is there the viral DNA is transcribed into messenger RNA(mRNA) by host cell polymerase.

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• mRNA is then translated into virus specific proteins.

E.g. of DNA viruses

• Poxviruses (Smallpox)
• Herpesviruses (Chicken pox, shingles, genital viruses)
• Adenoviruses (Conjunctivitis, sore throat)
• Hepadnaviruses (Hepatitis B virus- HBV)
• Papillomaviruses (warts).

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RNA viruses

• For RNA viruses, replication in the host cell relies either on enzymes in the virion (the whole infective viral particle) to synthesize its mRNA or on the viral RNA serving as its own mRNA.

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• The mRNA is translated into various viral proteins, including RNA polymerase which directs the synthesis of more viral mRNA and genomic RNA.
• Most RNA viruses complete their replication in the cytoplasm.
• But influenza viruses are transcribed in the host cell nucleus.

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E. g of RNA viruses

• Rubella virus (German measles)
• Rhabdoviruses (Rabies)
• Picornaviruses (poliomyelitis, hepatitis A, meningitis, colds)
• Arenaviruses (meningitis, lassa fever)
• Flaviviruses (West Nile meningoencephalitis, yellow fever, hepatitis C)

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• Orthomyxoviruses (Influenza)
• Paramyxoviruses (measles, mumps)
• Coronaviruses (Colds, severe acute respiratory syndrome – SARS )

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The Retroviruses

• The retroviruses are RNA viruses that contain a reverse transcriptase that makes a DNA copy of the viral RNA template.
• The DNA copy integrates into the host genome at which point it is referred to as a provirus.
• The provirus is transcribed into both genomic RNA and mRNA for translation into viral proteins.

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The retroviruses cause diseases such as :

• Acquired immunodeficiency syndrome(AIDS)
• T-cell leukemias(Human T-cell lymphotropic viruses-1, HTLV-1).

Note

The polymerase of hepadnaviruses possesses reverse transcriptase activity.

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The morphology

• The morphology of a virus is determined by the arrangement of the protomeres.

Icosahedral

• When protomeres aggregate into units of five or six (capsomeres) and then condense to form a geometric figure having 20 equal triangular faces and 12 apices.
• (Adenoviruses, Herpes viruses )

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Helical

• When protomeres aggregate to form a capped tube.
• (Corona viruses, Paramyxoviruses, Orthomyxoviruses)

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Complex

• Any other arrangement of protomeres results in a complex morphology.
• (Rhabdoviruses)

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11�THE REPLICATION CYCLE

1.Adsorption.

• Viruses can enter cells via phagocytosis, viropexis or adsorption.
• Adsorption is the most common process and the most highly specific process.
• It requires the interaction of a unique protein on the surface of the virus with a highly specific receptor site on the surface of the cell.

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2.Penetration

• Viruses fuse their membrane with the membrane of the host cell.
• This involves local digestion of the viral and host cell membranes and hence the release of the nucleocapsid into the cytoplasm.

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3.Uncoating

• During this stage, the capsid is digested away from the nucleic acid.
• This always occurs in the cytoplasm of the host cell.

Note

• The period of the replication cycle between the end of the uncoating stage and maturation of new viral particles is term the eclipse.

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• Thus during the eclipse stage, no complete viral particles can be viewed within the cell.

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4.Replication of nucleic acid

• The replication of viral nucleic acid is a complex and variable process.
• The specific process depend on the nucleic acid type.

DNA virus replication

• With the exception of the poxviruses, all DNA viruses replicate in the nucleus.
• The DNA strands/strand would be transcribed into specific mRNA, which in turn is translated to synthesize virus specific proteins such as enzymes necessary for biosynthesis of virus DNA.

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• Messenger RNA transcribed during the later phase of infection migrates to the cytoplasm and is translated into nucleic acid or genetic material.

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• Proteins for virus capsids are synthesized and are transported to the nucleus to be incorporated into the complete virion which are later released after cell lysis.

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RNA virus replication

• With the exception of the orthomyxoviruses and retroviruses, all RNA viruses replicate in the cytoplasm of the host cell.

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• The exact process varies with the species of virus.

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• The single-stranded RNA that is released after uncoating will act as either
1. The mRNA to synthesize viral-coded proteins.
2. A template to synthesize mRNA.
3. A template to synthesize double-stranded RNA, which is then used as a template to synthesize mRNA.

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4. A template to synthesize double-stranded DNA, which is then utilized as a template to synthesize mRNA.
1. This latter process occurs only with the retroviruses (oncomaviruses) .

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5.Maturation and release

• Maturation consists of two main processes:
1. The assembly of the capsid.
2. Its association with the nucleic acid.
• After they are assembled into mature viruses,virions may become concentrated in large numbers at the site of maturation, forming inclusion bodies.

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Release process

• Virions are released in different ways, which depend on the virus and cell type but mainly through autolysis or extruded without lysis.

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• RNA-containing viruses are released rapidly after maturation and there is little intracellular accumulation hence do not form inclusion bodies.

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• DNA-containing viruses mature in the nucleus and are not rapidly released hence form inclusion bodies.

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18�Summary.

1. Viruses contain either DNA or RNA as their genetic material, but not both. This nucleic acid usually has unique chemical and/or physical features which makes it distinguishable from human nucleic acid.

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• Viral nucleic acid is enclosed in a capsid made up of protein subunits called protomeres.

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• Some species of viruses have a membrane, the envelope surrounding the capsid, other species do not have an envelope i.e. they are naked. Enveloped viruses have glycoprotein spikes arising from their envelope. These spikes have enzymatic, absorptive and/or antigenic activity.

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• The morphology of a virus is determined by the arrangement of protomeres.

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19�summary

5. All viruses undergo a replication cycle in their human host cell consisting of adsorption, penetration, uncoating, nucleic acid replication, maturation and release stages.

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• During the viral replication cycle, an accumulation of mature viruses, incomplete viruses and viral parts occurs within the cell. This accumulation is the inclusion body. The size, shape, location and chemical properties of the inclusion body are used by the pathologist to diagnose viral infectious disease.

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• A virally infected cell generally presents three signals that it is infected :

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a) The first is the production of double -stranded RNA

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• The second is the expression of viral protein on the surface of the plasma membrane, thus causing activities of cytotoxic T-cells, natural killer cells and sometimes indication of antibody synthesis.

20�summary

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c) The third is the formation of an inclusion body either within the cytoplasm or the nucleus or very rarely within both the cytoplasm and the nucleus.

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8. All DNA- containing viruses replicate in the host cell nucleus.

The exceptions to the rule are the poxviruses.

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9. All RNA-containing viruses replicate in the host cell cytoplasm.

The exceptions to the rule are the retroviruses and the orthomyxoviruses.

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10. There are closely related terms like :

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• Virusoids --- which are nucleic acid that needs helper viruses for their activation in living cells.

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• Viriods ----- are naked nucleic acid that do not have protein envelope.

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• Prions ----- are the infectious particle that can integrate with the host cell and change their normal regulatory function.