1. General Virology

8/12/2019 1. General Virology

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General Virology

What are Viruses?

A PIECE OF BAD NEWS WRAPPED UP IN A PROTEIN

Peter Medawar


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Scientists Contributed to the Early

Understanding of Viruses

Many Scientists Contributed to the EarlyUnderstanding of Viruses

1. Dimitri Ivanowsky studied the tobacco mosaicdisease, TMD, (causing stunted and mottledtobacco leaves) and defined it as filterable virus(virus = poison)

2. Martinus Beijerinck concluded that the virus,TMV, responsible for the TMD was contagiousliving liquid that was inactivated by boiling

Tobacco leaf with TMD

TMV


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Scientists Contributed to the Early

Understanding of Viruses

3. Walter Reed concluded that the agent responsible for yellow fever in

human was filterable (the first association of virus to human disease))

4. Frederick Twort and Felix dHerelle studied bacteriophages (phage =

eat)

5. By the early 1930s, it was assumed viruses were living microorganisms

below the resolving power of available light microscope.

6. In 1935, TMV was crystalized, suggesting viruses might be non living

agents of disease.

7. Alice M. Woodruffand Ernest W. Goodpasture developed a culturetechnique using chicken eggs.


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What Are Viruses?

- Today more than 5,000 viruses have been identified.

- Estimated 400,000 different viruses virologist believe may exist.

- Viruses Are Tiny Infectious Agents.

- Viruses are small, obligate intracellular parasites.

- They lack the machinery for generating energy and large molecules.

- They need a host eukaryote or prokaryote to replicate.


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Virus Diameter

- The poliovirus is one of the smallestvirus about the diameter of cell

ribosome.

- At the upper end of the spectrum is the

smallpox virus, which approximate the

size of the smallest bacteria, Chlamydia


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Are Viruses alive?

- Viruses have some unique features not seen with the livingmicroorganisms

Growth onartificial

media

Divisionby binary

fission

ContainDNA and

RNA

Containprotein

synthesis

machinery

Containmuramic

acid

Sensitiveto

antibiotics

Bacteria often yes yes yes often yes

Viruses never no Either

DNA or

RNA

no* no no


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Viruses Structure?

Virus structure

- Genome

- Protein coat (capsid)

- Capsomeres units

- Spikes

- Envelope (+/-)


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Virus Structure?

1. Viral genome contains either DNA or RNA, but not both.

2. Protein coat (capsid), made up of capsomeres

- The nucleocapsid is the capsid with its enclosed genome.

Viral genome + Protein coat = nucleocapsid

- Some capsid proteins are spikes that help the virus attach to andpenetrate the host cell.

3. Envelope (+/-)

- Naked viruses are composed only of a nucleocapsid

- Enveloped viruses are surrounded by an envelope.

- A virion is a completely mature assembled, infectious virus outside itshost cell


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1. Viral Genome

- DNA or RNA

- Viruses are categorized as DNA or RNAviruses

- The viral genome is folded or coiled allowingthe viruses to maintain their extreme smallsizes

- The nucleic acid of DNA viruses is usuallydouble stranded (ds) linear or circular

molecules.

- An exception is the with the parvovirus whichhave single stranded (ss) DNA.

- The nucleic acid of RNA viruses is usually single stranded (ss) with the exception of

the reoviruses (ds RNA)

- The nucleic acid in some RNA viruses is composed of separate and non identical

segments e.g. influenza virus.


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1. Viral Genome

- ssRNA viruses are diveded into two groups:

1. Sense RNA strand (positive/plus-strand viruses)- The RNA of the genome has the same polarity as the viral mRNA and

thus can function as messanger RNA.

2. Antisense RNA strand (negative/minus-strand viruses)

- The genome has the polarity opposite to that of the mRNA and therefore

cannot be translated into proteins until it has first been transcribed into a

complementary strand


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2. Viral Protein coat (Capsid)

- Virus-coded proteins.

- Responsible for virus antigenicity

- It gives shape or symmetry to the virus

- The organization of the capsomeres yeilds the viral symmetry.

- The number of capsomeres varies but is specific and constant for each viralspecies.

- It provides a protective covering for the viral genome

- The construction of its amino acids resists temperature, pH, and otherenvironmental fluctuations.

- Spikes help attach the virus to the host and facilitate penetration of the cell.


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3. Viral Envelope

- Enveloped viruses are surrounded by a flexible membrane.

- The envelope is composed of lipids and protein, similar to the host cellmembrane.

- This envelope is acquired from the host cell membranes (cell membrane,nuclear membrane, less frequently from the endoplasmic reticulum) during

replication and is unique to each type of virus

- Enveloped viruses may lose their infectivity if the envelope is destroyed withorganic solvents or detergents (why?)

- The envelope is usually is generally a loose-fitting structure over thenucleocapsid.

- Many enveloped viruses contain spikes projecting from the envelope, theyfunction for attachement and host cell penetration


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4. Other Components of Viral Particles

Enzyme components of the virus particle.


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Virus Structure Pattern

- Viruses can have helical, icosahedral, or complex symmetry

1. Helical symmetry e.g. TMV and rabies viruse

- Viruses exist in the form of helix.

2. Icosahedral symmetry e.g. herpes simplex and polioviruses

- Icos = twenty, edros = side)

- The icosahedron has 20 trianglular faces and 12 corners

3. Complex symmetry e.g smallpox virus and some bacteriophages

- The virus has a combination of icosahedral (head) and helical (tail) symmetry


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Virus Nomenclature and Classification

- Nomenclature and Classification of viruses do not Use ConventionalTaxonomic Groups.

- Viral nomenclature has used a variety of conventions

1. After the disease they caused

e.g. measles virus and poxviruses

2. After where they were originally isolated.

e.g. Ebola and Marburg viruses

3. After the researcher who discovered it

e.g. Epstein-Barr virus

4. Morphological factors

e.g. Coronaviruses (corona = crown) have crown like capsid

e.g. Picornaviruses (pico = small, rna=RNA genome)


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The taxonomic system used for classification of viruses is based on the

following morphological and biochemical criteria:

1. Type of viral genome

- DNA, or RNA viruses , single-stranded (ss) or double-stranded (ds)

- RNA viruses are further classified according to (+) or (-) polarity

2. Capsid symmetry: Icosahedral, helical or complex symmetry

3. Presence or absence of envelope: Naked or enveloped viruses.

4. Diameter of the virion, or of the nuleocapsid

5. Body tissues affected by the virus


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Virus Classification

6. Classification system developed by The International Committee on

Taxonomy of Viruses (ICTV):

- 80 families, eah ending with viridae (e.g. Herpesviridae, Coronaviridae)

were recognized.

- Viruses have been categorized into hundreds of genera; each genus

name ends with the suffix virus (e.g. Herpesvirus, Coronavirus).

- Binomial nomenclature have not yet been agreed upon for species.

- Viruses from 24 different families cause disease in human


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Virus Host Specificity

- Viruses Have a Host Range and Tissue Specificity (tropism)

- A host range refers to what organisms the virus can infect.

- Even within a host rang, many viruses only infect certain celltypes or tissues within a multicellular plant or animal.

- This limitation is known as tissue tropism or tissue attraction


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Virus Host Range

- Host range depends on capsid structure and envelope proteins.

- Most viruses have a very narrow host range.e.g. bacteriophage infects specific species of bacterial cells

e.g. smallpox virus infects only human and primates.

- A few viruses may have a broader host range.

e.g. the rabies virus infects humans and most worm-blooded animals

- Viruses do not cross eukaryote / prokaryote boundary


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Factors Affecting Host Range

1. Cell surface receptors

2. Availability of replication machinery

3. Ability to get out of cell and spread4. Host anti-viral response


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Tissue Tropism

e.g. HIV

- The host range for HIV is a human.

- In humans, HIV primarily infects a specific white blood cells called T helper cells.

- The envelope has protein spikes for binding to receptor molecules on these cells

(CD4 receptors).

- The virus does not infect cells in other organs

e.g. Rabies virus

- The host range for rabies virus is worm-blooded animal.

- The virus infects cells in the nervous system because its envelope contains protein recognizingreceptors only on these tissues.

- A virus host range and tissue tropism are linked to infectivity


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Viral Replication Cycle-Virulent phage

Lytic cycle e.g. Bacteriophage T-4 & lambda phage

The Replication of Bacteriophages Is a Five-Step Process

1. Attachment

- Occurs when a phages tail fibers match with a receptor site

on the bacteriums cell wall or sometimes on bacterial flagella

2. Penetration- Occurs when the phage tail releases lysozyme to dissolve a portion of the cell wall

and the DNA is injected into the bacterial cytoplasm.

3. Biosynthesis

- The production of new phage genomes and capsid parts.

4. Maturation

- Aassembly of viral parts into complete virus particles.

5. Release.

- Exit of virions from the bacterium resulted in bacterial cell lysis.


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Viral Replication Cycle-Temperate Phage

lysogenic cycle) e.g. (Lambda) phage

- Temperate phages do not lyse the host, but insert their DNA into the bacterialchromosome as a prophage.

-- The bacterial cell continue to grow and multiply.

-- As bacteria undergo DNA replication and binary fission, the prophage is copied

and vertically transferred to daughter cells as part of bacterial chromosome.

-- As cells divide, each daughter cell is infected with viral genome.

- At certain point, when bacterial cells become stressed, the prophage exised itself

and go through a lytic cycle, lysing the bacterial cells as new phage released.


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Bacteriophage Replication

Lyticcycle

Lysogeniccycle


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Animal Viruses Replication Cycle


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Animal Virus Replication Cycle

- Animal virus replication has similarities to Phage Replication

1. Attachement

- Animal viruses attach to host plasma membrane via spikes on the capsid or

envelope

- Since receptor sites vary from person to person , some people are moresusceptible to a certain virus than others.

2. Entery followed by uncoating of nucleocapsid- Animal viruses are usually taken into the cytoplasm as intact nucleocapsids by

two different mechanisms:

i. Entry of an enveloped viruses e.g. HIV and influenza virus

- The envelope fuses with cell plasma membrane and the nucleocapsid passes intohost cells cytoplasm

- The nucleic acid is released (uncoating) by the effect of host cellular enzymes


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A i l Vi R li ti C l


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3. Virus replication and protein synthesis

A. DNA virus replication

- The DNA supplies the genetic codes for enzyme that synthesizes viral parts from

available building block.

- The DNA genomes of the DNA viruses are synthesized in the nucleus exceptpoxviruses which replicate entirely in the host cytoplasm.

- Some viruses e.g. herpes viruses posses replicase of their own

- The small DNA viruses e.g. polyomavirus, code for polypeptides that modifythe celluylar polymerasese to perform replication of viral DNA.

B. DNA virus-Protein synthesis

- Cellular polymerases transcribed mRNA in the nucleus of the host cell from DNAstrands.

- Exception is the poxviruses, which use their own enzymes to replicate in thecytoplasm.



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ds DNA

ss DNA



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Hepadnaviruses e.g. hepatitis B virus- The genome consists of ssDNA antisense strand and a short sense DNA

strand.- The infected cell transcribes an RNA sense-strand from antisense DNA strand.- The virus Reverse transcriptase synthesizes a complementary antisense DNAand a short sense DNA from the tempelate RNA strand



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C. RNA viruses replication- Eukaryotic cell possess no enzymes for RNA replication, so the virus must supply

the RNA-dependent RNA polymerases.

- These enzymes are thus in any cases are virus-coded proteins or in some casesare components of the virus particle

i. (ss) RNA

- Whether sense or antisense strands, complementary strands of the genome are

produced first then transcribed into daughter strands.

ii. (ds) RNA

- Sense-strand RNA is produced from the genome.

- The strand function at first, as mRNA and later as a matrix for synthesis ofantisense-strand RNA.

Animal Vir s Replication C cle


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iii. Retroviruses

- Posses 2 sense-strands RNA genome

- Sense strand RNA is transcriped by enzyme in the virion (reverse

transcriptase) into complementary DNA.

- The DNA is complemented to make (ds) DNA and integrated in the cellgenome.

- Transcription into sense-strand RNA is the basis for both viral mRNA andthe genomic RNA in the viral progeny



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D. RNA viruses- Protein synthesis

+ strand RNA viruses

- The + strand RNA virus can act as a messenger RNA molecule and

immediately begin supplying the codes for protein synthesis.

ii. - strand RNA viruses e.g. orthomyxoviruses

- They use their RNA as a templete to synthesize a complementary strandof RNA .

- Usually the enzyme RNA polymerase is present in the virus to synthesize

the complementary strand.

- The synthesized RNA then is used as a messenger RNA molecule forprotein synthesis


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ds RNA virus

retrovirus

Viral ReplicationBacteriophage vs Animal Viruses


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Bacteriophage vs Animal Viruses

Bacteriophage Animal virus

Virus receptors

- Tail fibers

Spikes distributed over the capsid e.g.

adenovirus

- Spikes in the envelope e.g. HIV

Attachment site Cell wall, flagellum, pilus - Cell membrane receptors

Penetration Capsid stays outside the cell

Genome injected into cell

cytoplasm

- Intact nucleoside is taken into cytoplasm

by:

Endocytosis (naked virus) or by fusion of

envelope with plasma membrane

Uncoating Absent present

Latency + with certain phage

(prophages)

+ with certain animal viruses (proviruses)

Viral Replication Cycle


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Viral Replication Cycle

4. Viral maturation

- The capsid proteins are produced in the cytoplasm.

- The proteins then migrate to the nucleus and join with nucleic acidmolecules for assembly.

5. Acquisition of an envelope and release- Envelope proteins are synthesized and incorporated into a nuclearenvelope, endomembrane or or plasma membrane.

- The virus pushes through the membrane, forcing a portion of themembrane ahead of it and around it, resulting and envelope.

- This process, called budding, not necessary kill the cell during the virusexit.

- Naked viruses leave the cell when the cell membrane rapture, leading tocell death.


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Why would it be advantageous for phage a phage to carry out a

lysogenic cycle rather than a lytic cycle?

Are viruses alive or non living? Support your answer?

Is it possible to use bacteriophage as therapy to treat bacteria

infections in hum

Documents

1. General Virology