5 Basic Virology

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PART III Basic Virology


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Basic Concepts ofBasic Concepts ofVirusesViruses


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Viruses : non-cellular organisms

General Characteristics of Viruses :

1. Smallest: 20300 nm, most were too small to beseen with the light microscope ;

2. Simplest: non-cellular entity, contain DNA or RNA

and protein. 3. Obligate intracellular parasites: growing only

within the living cells.

4. Self-replication :Once it has invaded a cell it is ableto direct the host cell machinery to synthesize newintact infectious virus particles.


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Significance ofignificance of Studingtuding

Virology for ife Science

irology for ife Science


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I. Viruses and infectious

diseases

New presented viruses and viral infectionstill harm peoples health

AIDS

HBV infection

Influenza

SARS


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Virus and Cancer

Infections Cause of Cancer


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Size : 20 -- 300 nm( the most virus150 nm );

Shape: The commonest sape ---- small spherical virusOthers ----- filamentous, brick, bullet


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Size and Shape

Virion: The basic infectious particle of avirus is known as the virion.

Simple virion (naked virus):a nucleic acid and capsid

Complex virion (enveloped virus):

nucleic acid , capsid, and envelope


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Structure

Viruses contain: nucleic acid genome (RNA or DNA)

protective protein coat (called thecapsid)

viral envelope( enveloped virus)

Non-structural protein


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(I) Nucleic acid1. Single kind: consist of DNA or RNA

( never both).

1) DNA :

single-stranded DNA (ssDNA) :

double-stranded DNA ( dsDNA ): --- thecommonest type of nucleic acid in viruses ofhuman.


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2) RNA:

double-stranded RNA (dsRNA) : single-stranded RNA (ssRNA):

--- the commonest type of nucleic acidin viruses of human..

ssRNA : is the same as the viral

mRNA , can direct as viral mRNA -ssRNA : as a template transcribecomplementary mRNA


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2. Function:

1). viral genome

: viralnucleic acid conteins all geneticinformation of the virus.

2). possess infectiousness

-- infectious nucleic acid


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infectious nucleic acidinfectious nucleic acid : infectious nucleic acid is purified viral

DNA or RNA(without any protein) that can

carry out the entite viral growth cycle andresult in the production of complete virusparticles. such as the genomes of dsDNAviruses and +ssRNA viruses.

such as: DNA virusdsDNA;

RNA virus (+ssRNA) ;


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(II) . Capsid

protein coat ,1. capsid is composed of distinct

morphologic units---- capsomeres

--- icosahedral symmetry:

--- helical symmetry:

--- complex symmetry:


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Capsomeres


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IcosahedronIcosahedron and symmetryand symmetry


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2. Function:

1) protection --- protect viral nucleicacid from enzymic action.

specific binding sites --- mediate theviral attachment

antigenicity ---- important antigens


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(III). Envelope ():

1. Consists of lipid bilayer and glycoprotein:

lipid --- derived from host cell membrane.

Glycoprotein --- coded by the viral genome.

Spikes() --- virus-encoded glycoproteinprotruding from lipid bilayer, which areimportant for adsorption and entry into thehost cell.


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2. Functions: 1) as the binding sites -- the spikes of

enveloped virus attach to host cell receptors

2) have antigenicity ---- envelopeglycoproteins are also important antigens

3) confers instability on the virus---enveloped viruses are more sensitive to heatand lipid solvents.


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Spikes (

)


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(IV) . Non-structural protein:

Code by virus; it is not the composition ofvirion. It can exist in virion and infected cell.

(1) viral enzymes:

such as RNA-dependent RNA polymerase ortranscriptase .

(2) specific viral protein:

such as : transformation protein of tumorvirus.


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III. Viral Multiplication

Replication: The process ofintracellular viral multiplication,consisting of the synthesis of

PROTEINS; NUCLEIC ACIDS; andtheir assembly into a new

infectious.


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I. Replication cycle

:

adsorption

penetration

uncoating

biosynthesisassembly and release


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(I) Replication cycle

:1. adsorption ():

The proteins on the suface of the virion

(called virion attachment proteins

VAP) attach to specific receptors on the

host cell surface.Such as: HIV(gp120) --- Helper Tlymphocytes(CD4 protein)


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2. penetration

:virus enter the cells.

Entry of Naked viruses:endocytosis

Entry of enveloped virus :

simple fusion of membranes


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3. uncoating

Rapid change from stable structure to

release of genome


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4. Biosynthesis: Geneexpression & Genome Replication

Synthsis of viral proteins

Synthsis of viral nucleic acid


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4.1 dsDNA viruses:

Replicate in the nucleus, use the host cell DNA

dependent RNA polymerase to synthesize theirmRNA


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4.1 dsDNA viruses: replicate in the nucleus use the host cell DNA dependent RNA polymerase to

synthesize their mRNA

(1) early transcription and translation: early mRNA ( use cell RNA polymerase ) early proteins ( non-structural proteins): --- enzyme which is necessary for viral

replication. --- specific viral proteins: regulation protein

()transformation protein.

.


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(2) viral DNA replication : (3) late transcription and translation :

late proteins (structural proteins ): ---- capsid protein and

envelope protein

dsDNA


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dsDNA

+ -

RNA

mRNA

(enzymes)DNA

mRNA

(structure proteins )

dsDNA

DNA

+ - + -


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4.2 RNA viruses ( three kinds) :

(1)+ssRNA viruses:

The +ssRNA itself can direct as viralmRNA .

(+)ssRNA


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(+)ssRNA

mRNA

+ ( RI )

RNA

RNA

-

-

+ssRNA

mRNA ,

+ssRNA

RNA


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(2) -ssRNA viruses:

Use the virion RNA polymerase to

synthesize viral mRNA.

( )ssRNA


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()ssRNA

RNA

-

-+

mRNA

ssRNA

RNA

+ ( RI )


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(3) Retroviruses:

use the virion reverse transcriptase tosynthesize a DNA cope of the viral RNA

genome; use the host cell RNA polymerase to

synthesize the viral mRNA


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(3) Retroviruses:

+ssRNAReverse transcriptase

cDNA

Proviral DNA (dsDNA)

Integrated into host chromosome

Viral mRNA transcription Viral RNA replication

+ssRNA


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RNADNA

dsDNA

DNA(provirus)

DNA

++

mRNA

+ ssRNA


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RNA Virus:

The mRNA is translated into a singlepolypeptide (polyprotein), which iscleaved.

Be cleaved by a virus-coded proteaseinto various proteins.


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5. assembly and release

Assembly: --- Assembly of DNA virus occurs in the

nucleus

--- Assembly of RNA virus occurs in thecytoplasm


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Virus budding


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(II ) Unnormal multiplication:


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(II.) Unnormal multiplication:

3. Interference

The infection of cell by a virus results inthat cell becoming resistant to infection by

other viruses.


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Section

Viral heredity and variation


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Viral genomeViral genome

hereditary substance:

DNA ; or RNA


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EXAMPLES of Mutant

Mutant( )

--- temperature sensitive mutant, ts

--- drug-resistant mutant

Temperature-sensitive mutant (ts):

permissive temperature: 28

~35

non permissive temperature: 36

~40

ts mutant -- attenuated mutant ( vaccine mutant )


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RecombinationRecombination &&ReassortmentReassortment

Recombination:

the exchange of genes between two related

viruses which infected the same cell. ( active virus-active virus; active virus-inactive virus).


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Reassortmentexchange of genetic material between

two segmented viruses which infected the

same cell.

Gene integrationGene integration


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Gene integrationGene integration

Viral genome insert into host cell genome

e.g. tumor viruses


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Interaction of viral genetic ProductsInteraction of viral genetic Products

--------nnongeneticComplementation()genetic production

reactivation between two viruses ( one or both of which

may be defective)

Phenotypic mixing(): If two different

viruses infect a cell, progeny viruses may contain coatcomponents derived from both parents and so they willhave coat properties of both

parents.


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Section 4Effects of Physical and Chemical Agents

on Viruses


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Resistance:

low temperature - 196pH 59

antibioticSensitivity

high temperature: >50

radiation and UV

lipo-solvent : enveloped virus


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Section 5Section 5

Classification of AnimalClassification of AnimalVirusesViruses


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UNCONVENTIONAL AGENTS


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UNCONVENTIONAL AGENTS

Subvirus : refer to the kind of infectiousfactor which is smaller than virus.

1. viroid(

) and virusoid(

)

--To contain RNA only

--To have only been shown to beassociated with plant disease.


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2. prion (

)

prions are infectious particles encoded bygene of normal host cells , they arecomposed solely of protein; they areimplicated as the cause of certain slow virus

disease called transmisslble apongiformencephalopathies(TSEs) in animals andhuman.


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cellular PrP, PrPc

()---- normal

scrapie prion protine,

PrPsc ()---- pathogenic

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5 Basic Virology