General Characteristics Of Viruses

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Viruses are an

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uniques intracellular pathogen

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Life cycle is marked by:

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acute, chronic and latent phases
(the latent phase could be dramatically marked by reversion to virulence

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Are viruses filterable through bacteria filters

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yes

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virus genome

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simple, they lack energy synthesizing organelles, enzyme systems, and protein synthesizing machinery

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How is a virus made

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they are reproduced by assembling subunits into infectious particles, not by dividing as other microbes do.

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The infectious particle of a virus is the

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virion

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What is a virion?

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this is essentially a mass of DNA or RNA, but never both.

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What surrounds the virion

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Protein coat

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Virus life cycle

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consists of extracellular and intracellular (biphasic) phases

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What is the extracellular virus particle

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virion and is the infectious agent

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What does the virus do intracellularly

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cause disease by killing or altering cell it invades.

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The virion has at least two components

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DNA or RNA
Capsid

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DNA or RNA is the

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genome

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Capsid is

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the outer symmetrical protein shell

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What is a capsid composed of

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individual polypeptides called protomers, which is coded by the virus

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During assembly of capsid assembly what happens

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one or more protomers aggregate into clusters called capsomers

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Capsomers

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are characteristic of a virus and important in its identification

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what is a nucleocapsid

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it is the nucleic acid and capsid together

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A virion is composed of three main parts

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the phospholipid bilayer envelope, a protein coat called a capsid, and its genetic material.

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What is the function of a capsid

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1. protects nucleic acid from environmental damage
2. facilitates attachment to susceptible host cells
3. confers structural symmetry on the virion
4. confers antigenicity

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What is a naked viruses

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only nucleic acid core and capsid, no envelope

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If you have an envelope you are

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a nucleocapsid virus

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What does an envelope contain

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many copies of few polypeptide chains which is determined by the viral genes

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What are these polypeptide chains called

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peplomers

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how to these peplomers get into the envelope

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they are inserted into the viral envelope as determined by the virus. they protrude like spikes from the envelope.

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What are the protein spikes in the envelope

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they are proteins that are glycosylated by cellular enzymes (called glycoprotein spikes)

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What else to you find in a viral envelope

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lipids and carbohydrate

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How do the lipids and carbohydrates come to be on the envelope

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they are determined by the host cell unlike the viral envelope proteins which come from the virus

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What is the function of the spikes in the envelope

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for attachment to the host cells and could be antigenic and enzymatic in function

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What are the two DNA viruses that have an envelope

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Herpes
hepatitis B

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Why does herpes and hepatitis B require an envelope

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without it the virus can not enter and infect the host cell

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What shapes are the nucleocapsid capsid take on

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icosahedral or helical

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what does an icosahedral virus look like

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individual spheres represent capsomers. they are arranged in 20 equilateral triangular planes with 12 vertices, thus forming icosahedron

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What is another name for an icosahedral viruses

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cubic viruses or spherical viruses

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Important feature of icosahedral virus is

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rigidity

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All DNA containing viruses are

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icosahedral (except pox viruses which are complex)

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RNA viruses can be

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icosahedral or helical

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another name for helical viruses are

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coiled viruses

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What does a helical virus look like

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the capsid is composed of identical polymers arranged in a helix, with the nucleic acid coiled between the turn of the helix

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How is the diameter of a helix virus determined

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by the size and shape of the protomers

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How is the lengh of a helix virus determined

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depends on RNA molecules enclosed

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Important feature of a helix virus is

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extremely flexible
This allows for more variety of shapes

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What makes pox virus different

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it does not fit into either icosahedral or helical category. It is classified as complex or binal viruses

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Pox virus

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nucleocapsid is surrouned by lipid containing layer; however the lipid is not derived from host membrane and therefore, is a true envelope

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Classification of viruses

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is done on the basis of: nature of viral genome, structure of genome, capsid symmetry, envelope, replication

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Major criteria for viral classification

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1. the nature of nucleic acids, DNA or RNA
2. the structure of nucleic acid
3. The symmetry of nucleocapsid (icosahedral or helical)
4. the presence of cell derived envelope
5. the number of capsomers or the diameter of the helix

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All DNA containing viruses except Pox virus have

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cubic (icosahedral) nucleocapsids

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The true envelope DNA viruses,

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herpes viruses and hepatitis B virus are the only classes of medical significance.

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RNA containing viruses can be

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cubic (icosahedral), helical or unknown

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RNA- containing viruses with a helical or unknown capsid are

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always enveloped

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RNA- containing viruses with a cubic capsid are

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either enveloped or naked

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DNA- containing viruses with cubic (icosahedral) symmetry and naked nucleocapsid are

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Parvoviridae, Papovaviridae, adenoviridae

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DNA- containing viruses with envelopes or complex coats

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herpesiviridae, iridoviridae, poxviridae

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RNA- containing viruses with cubic (icosahedral) capsid symmetry

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picornaviridae, caliciviridae, reoviridae, togaviridae

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RNA- containing viruses with helical symmetry

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orthomyxoviridae, paramyxoviridae, rhabdoviridae, coronaviridae, bunyaviridae

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RNA- containing viruses with asymmetrical or unknown architecture

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retroviridae, arenaviridae