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8/12/2019 33 Anintroductiontotheviruses 110907232325 Phpapp02

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An Introduction to the Viruses

Topic 36 and Topic 37


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Size of viruses


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The Viral Structures and Shape


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Capsids

All viruses have capsids- protein coats that

enclose & protect their nucleic acid

Each capsid is constructed from identical

subunits called capsomers made of protein

2 types:

helical

iscosahedral


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Shape of Viruses, determined by the arrangement

of proteins (capsomers) in the capsid


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Helical


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Icosahedral

20-sided with 12 corners

Vary in the number of

capsomers Each capsomer may be

made of 1 or several

proteins

Some are enveloped


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Additional Structures in some Viruses,

envelope and surface proteins


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Function of the envelope


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Complex Viruses-1

Two special groups of viruses termed Complex

viruses. More intricate in structure than the helical,

icosahedral, naked or enveloped

These are :

Poxviruses: very large viruses that contain a DNA core

but lack a regular capsid, in its place are

several layers of lipoprotein and coarse

surface fibrils


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Complex Viruses-2

Bacteriophage :another complex virus

Polyhedral head

Helical tail

Fibers for attachment to host cell


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Naming viruses

No taxa above Family (no kingdom,phylum, etc)

19 families of animal viruses

Family name ends in -viridae ,

Herpesviridae

Genus name ends in -virus, Simplexvirus

Herpes simplex virus I (HSV-I)


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FamilyHerpesviridae

GenusVaricellovirus

Common namechickenpox virus

Disease - chickenpox

EXAMPLE of VIRAL TAXONOMY


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Virus taxonomy based on viral

characteristics

In contrast to other discipline where taxonomy is

well defined, viral taxonomy is much less

structured and depends on a variety of

characteristics including :

Nucleic acid type : DNA or RNA

Nucleic acid strandedness: single stranded (ss) or

double stranded ( ds)

Presence or absence of an envelope

Capsid size or shape : icosahedral or helical

Host specificity, e.g. plant, animal, bacteria


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Example of the Taxonomic characteristics , used

to describe a virus


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VIRAL REPLICATION


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Phage Replication


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6 steps in phage replication

1. adsorptionbinding of virus to specific

molecule on host cell

2. penetrationgenome enters host cell3. replicationviral components produced

4. assembly - viral components assembled

5. maturationcompletion of viral formation

6. releaseviruses leave cell to infect other cells


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Fig 6.11


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Penetration


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Bacteriophage assembly line


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Not all bacteriophages lyse cells

Temperate phages insert their viral DNA into thehost chromosome & viral replication stops at there

until some later time.

Lysogeny- bacterial chromosome carries phageDNA

Impact of Bacteriophages


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Animal Virus Replication


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Stages of Animal virus replication

1. adsorption

2. penetration/

uncoating of genome

3. duplication/synthesis

4. assembly

5. release


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Virus Adsorption


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Virus Adsorption

Once in the host, the virus must encounter

( recognize) a susceptible cell for the infection

cycle to proceed

Susceptible host cells must :

Contain receptors for the virus

Be permissive or capable of

supporting viral replication


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Virus Adsorption..

Host cell receptors must match the virusrecognition sites for the virus to subsequently

adsorb into the host cell

Most virus receptors are surface glycoproteins

Host receptors

Virus receptors( glycoproteins)


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Adsorption


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Virus Penetration


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Virus penetration

Viral entry into host cells occurs throughone of the following methods:

Endocytosis

Direct fusion

Nucleic acid translocation


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Endocytosis

All of the virus is engulfedand enclosed in a vacuole


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Direct Fusion

Host cell membrane fuses with the virus

N l i id t l ti


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Non enveloped virus injecting itsnucleic acid to the host cell


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Viral Uncoating


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Viral Uncoating


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Summary of Penetration/uncoating


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Duplication and Synthesis


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Duplication and Synthesis

DNA and RNA viruses replicate indifferent manner.


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The Most Common DNA Viruses

Adenoviruses Cytomegalovirus

Epstein-Barr virus

Hepatitis B virus

Herpes simplex Types 1 &2

Papovavirus

Varicella-Zoster virus

DNA Vi R li ti 1


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DNA Virus Replication-1

DNA dependent RNA polymerase is needed for the

Transcription of mRNA from the DNA strand

DNA Vi R li ti 2


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DNA Virus Replication.2


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DNA Virus Replication3

The virus needs to code for proteinsand enzymes that it can use in theSubsequent stages of its replication

And assembly ( e.g. DNA polymerase,Capsid proteins.

DNA Virus Replication 4


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DNA dependent DNA polymerase isProduce so that the DNA of the virusCan be replicated.

DNA Virus Replication 5


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RNA Virus Replication


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RNA Virus Replication

RNA viruses enter the host cell already in an RNA

form and the virus cycle occurs entirely in the

cytoplasm.

RNA viruses bring with them one of the following

genetic messages:

a positive sense genome ( +RNA)

a negative sense genome ( -RNA)

a ds RNA ss RNA which is converted to DNA

( latent retroviruses)

RNA Virus Replication.1


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RNA Virus Replication.1

RNA Vi R li ti 2


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RNA Virus Replication2

They have their own RNA dependentRNA Polymerase which it uses to transcribe

from The

RNA the + RNA strandwhich is use In the making the protein

RNA Vi R li ti 3


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The + RNA strand is transcribe, theComplimentary copy is now a negative strand.

RNA Virus Replication 4


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It cant

It cant


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RNA Virus Replication (Retrovirus)

RNA Vi R li ti (R t i )


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RNA Virus Replication (Retrovirus)..

RNA Virus Replication (Retrovirus)


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RNA Virus Replication (Retrovirus)..


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Viral assembly


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Viral Release

Vi l R l


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Viral Release

Naked-nonenveloped andComplex viruses are releasedby cell lysis.

Vi l R l


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Viral Release.

Viral Release.


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Release by budding

Viral Release


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Viral Release.

ReceptorsHost cytoplasms


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RNA

spikes

SUMMARY OF VIRAL REPLICATION


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Host range

Spectrum of cells a virus can infect

cell has to have a specific structure (receptor) on itssurface for viral attachment

cell has to contain all of the enzymes and materials

needed to produce new virions May be one species or many

HIV (only humans) vs rabies (many animals)

May be one tissue or many within a host

Hepatitis (liver) vs polio (intestinal & nerve cells)

Diff b t h d i l


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Differences between phage and animal

virus replication

1. Animal virus replication is more complex

than phage replication because host cellsare more complex.

2. Animal viruses cannot inject their DNA.

3. Lysogeny for phage, latency for animalviruses


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VIRAL CPE


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Cytopathic effects- virus-induced

damage to cells1. changes in size & shape

2. cytoplasmic inclusion bodies

3. nuclear inclusion bodies

4. cells fuse to form multinucleated cells

5. cell lysis

6. alter DNA

7. transform cells into cancerous cells


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Cytopathic changes in cells


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Inclusion body

Alteration of host membrane by insertion of


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viral proteins that trigger CMIR

responses against the host cell

Cytotoxic T cell will kill virus infected cells

Transformation or conversion of cell division to cells


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Transformation or conversion of cell division to cells

that divide indefinitely ( tumors ,warts)

Induces hyperplasia of the host cells


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Host Response to Viral Infections


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Host Cell response includes :

Antibody response

Cellular Immune response

Interferon production

A ib d R


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Antibody Response



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Interferons


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Interferons


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Diagnostic Virology


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How do we grow viruses?

Obligate intracellular parasites

require appropriate cells to replicate.


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Growing animal viruses

1. live animals

2. bird embryoschicken, duck; intact, self-

supporting unit, sterile, self-nourished3. cell culture- cell lines

Virus laboratory


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Established human cell lines


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One of the earliest human cell lines,

descended from Henrietta Lacks, who died

of cervical cancer .

The cultured HeLacells shown below have

been stained with Hoechst turning their

nuclei blue.

Tissue Culture
http://en.wikipedia.org/wiki/Image:HeLa_Hoechst_33258.jpg


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Tissue Culture

Tissue Culture


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Tissue Culture


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No virus virus

E I l ti


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Egg Inoculation

E i l i


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Egg inoculation


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Diagnosis of viral diseases

More difficult than other agents

Consider overall clinical picture

Take appropriate sampleInfect cell culture- look for characteristic

cytopathic effects

Screen for parts of the virus

Screen for immune response to virus(antibodies)


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diagnosis


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Thank you end of lecture !

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