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Flashcards in Introduction to Virology Deck (37):

Description of viruses? (4)

1. small

-filterable- small enough to pass through filter that bacteria cannot

-not visible by light microscopy

2. obligate intracellular pathogens

-replicate only in living cells

-cannot be cultured like bacteria

-no sub cellular organelles

3. progeny visions are assembled, the virus does not divide


How do viruses infect cells?

1. attachment

-can't infect if it can't bind cell

2. entry/coating

-genome needs to be released into host cell

3. macromolecular synthesis

-genome gets copies

-viral protein gets transcribed/translated

4. assembly and release

-new viral particles self assemble

-released from host cell and spread to infect new cells


What are viruses composed of? (7)

1. nucleic acid- genome

-DNA or RNA, but not both

-genome and associated enzymes make up viral core

2. protein shell called capsid

-helical or icosahedral (crystal-20 sided)

-protects genome

-core + capsid = nucleocapsid

3. envelope

-capsid surrounded by lipid bilayer


Viral envelope?

1. host derived lipid membrane

2. viral encoded proteins

-matrix proteins (assembly, acts as bridge)

-surface glycoproteins (viral attachment-VAPs, viral cell fusion-fusion between envelope and lipid membrane of cell, allows capsid to get released )


How to classify viruses?

1. Type of genome


2. presence or absence of a viral envelope

-naked or envelope

3. type of capsid

-helical or icosahedral


DNA vs RNA virus?

1. DNA:

-genome can persist over time, not readily degraded

-genome resides in nucleus, may integrate

-can use host polymerase, some encode their own (DNA dependent DNA pol made by host)

-interacts with host transcription factors

2. RNA

-genome is labile, degraded sooner

-genome remains in cytoplasm

-has to encode their own polymerase (RNA dependent RNA pol)

-more prone to mutations


Clinical consequences of type of genome?

1. DNA and retroviruses

-transformation- trigger cancer

-latent infections- remains dormant

2. RNA viruses

-variability- quasi species (variance of initial infection strain), can escape immune response, antigenic variation

-more labile


Shapes of capsid?

1. helical

2. icosahedron

3. icosadeltahedron- herpes


Functions of viral capsids?

1. protects genome

2. involved in attachment- naked viruses

-viral attachment protein (VAP)

3. involved in entry/uncoating

4. involved in assembly

-packaged viral enzymes if necessary


Properties of naked icosahedral viruses?

1. can dry out and retain infectivity

2. can survive acidic conditions of GI tract

3. resistant to temp extremes, detergents, poor sewage treatment

4. released by cell lysis- allows it to spread


Clinical properties of naked icosahedral viruses?

1. survive in the GI tract enables transmission via fecal oral route

-shed in stool

-present in sewage contaminated water

2. survival in environment enables transmission by fomites (infecting object then someone touches object)

3. responsible for most cases of viral gastroenteritis


Properties of enveloped viruses?

1. must stay wet to retain infectivity

2. cannot survive in GI tract (acid labile)

3. infectivity is destroyed by organic solvents

4. need not kill infected cell to spread- can bud out

5. some induce cell to cell fusion


Clinical properties of enveloped viruses?

1. transmitted through droplets/secretions

-respiratory route, blood, organ transplants

2. cannot survive in GI tract

3. need not kill infected cells to spread

-virus can be shed over time


Attachment of virus to cell? (19)

-mediated by surface glycoproteins of enveloped viruses

-mediated by capsid proteins of naked viruses

-important determinant viral tropism- ability of virus to infect a certain type of cell


Viral receptors?

-proteins or carbohydrates on glycoproteins or glycolipids

-physiological role in host cell, some other role, virus takes advantage of pre existing cell receptors

-types of receptors:

-molecules involved in cell to cell interactions

-hormone, cytokine, complement receptors



Examples of viral receptors? (22)

see table

-cell receptor determines what cells can be infected


Entry of virus to cell? (19)

1. through plasma membrane

-viral cell fusion (enveloped viruses)

-hydrophobic interactions create a channel through the membrane (naked virus)

2. receptor mediated endocytosis

-most common route of entry

-used by both enveloped and naked viruses


Viral cell fusion at plasma membrane? (24)

-viral attachment protein binds to host cell receptor

-lipid bilayers mix, makes pore

-capsid gets released into cytoplasm


Receptor mediated endocytosis? pH dependent entry? pH independent?

-viral attachment protein binds to host cell receptor


-pH dependent entry:

-virus is internalized into endosome

-endosome becomes acidic and fuses with lysosome, dumps its enzymes in to activate fusion proteins

-acidic environment activates fusion activity of viral fusion protein

-fusion occurs between viral envelope and endosomal membrane

-more common


-pH independent:

-virus in internalized into vesicle that does not fuse with lysosomes and there is no acidification of vesicle

-the viral fusion protein is active at a neutral pH and can mediate fusion without exposure to an acidic environment


Entry of naked icosahedral viruses?

-plasma membrane or receptor mediated endocytosis

-virus forms a pore through the membrane

-virus lyses the membrane (endosome)

-conformational change in capsid proteins:

-expose hydrophobic region (make a pore)

-dissociate to release genome (uncoating)


What is uncoating? (27)

-release of genome into cell


-get rid of capsid

-get RNA into cytoplasm- uncoating coincides with entry, happen at same time

-get DNA into nucleus (two separate events, entry then uncoating at nucleus)

-triggers for uncoating include:

-binding to receptor

-change in pH

-proteolytic degradation


Taking over the host cell?

-transcription of viral mRNA (viruses doesn't have ribosomes)

-replicate genome

-controls protein synthesis:

-make viral proteins

-shut down synthesis of host proteins


Transcription of viruses general properties?

1. early transcripts

-encode regulatory enzymes and proteins:

-control transcription of viral mRNA

-initiate replication of viral genome

-shut down host protein synthesis

2. late transcripts

-encode viral structural proteins


DNA viruses?

-transcription and replication occurs in nucleus (exception= pox viruses)

-genome = template for viral mRNA

-genome is infectious

-virus uses host cell DNA dependent RNA polymerase- more complex DNA viruses will encode their own polymerase


replication of DNA viruses? (31)

see pic


+ strand RNA virus? (32)

+ strand has same sequence as mRNA

1. genome can function as mRNA

-translated as polyprotein, then cleaved into viral proteins

2. virus encodes an RNA dependent RNA polymerase

3. transcription produces (-) strand RNA

-template for mRNA

-template for genomic RNA

4. genome is infectious


(-) strand RNA virus? (33)

1. virus encodes an RNA dependent RNA polymerase

-core protein

2. genome is template for mRNA

3. + strand copy of genome is template for genomic RNA


Retroviruses? (35)

+ strand RNA is transcribed into DNA

-reverse transcriptase (RNA dependent DNA polymerase) is core enzyme- makes DNA from RNA

-DNA travels to nucleus where it integrates into host chromosome

-transcription of DNA produces mRNA and genomic RNA


Assembly and release? (37)

1. capsids self assemble

-package genome and core enzymes

2. naked virions are released upon cell lysis

-DNA virus pass through nuclear pore and is released upon cell lysis

3. enveloped viruses bud

-matrix proteins determine site of budding

-plasma membrane, ER, Golgi, nuclear membranes, released via exocytosis

-act as a bridge between nucleocapsid and surface glycoproteins


Types of viral infections at cell level?

1. productive infection

-progeny virus is produced

-infected cells are permissive for that virus

2. non productive infection

-no progeny are produced

-cells are non permissive


Types of productive infection?

1. lytic infection

-destruction or lysis of host cell

-naked virus produces lytic infections

2. persistent (chronic) infection

-continual shedding of virus

-no destruction of host cell (budding)

-envelope virus


Types of nonproductive infection?

1. abortive

-host cell cannot support viral replication

-viral genome is lost

2. latent infection

-virus is dormant in host cell

-viral genome is maintained

-viral transcripts may be detected, few or no viral proteins are expressed

-occurs in DNA or retroviruses

-reactivation of a lytic infection is possible

3. immortalizing and transforming infections

-one or few viral genes are expressed

-immortalized cells continually go through cell cycle- may or may not be transformed

-transformed cells are phenotypically altered

-cells are considered semi permissive for virus


Viruses that can cause latent infections? (41)

see chart


Possible consequences of transformation?

1. uncontrolled cell growth

2. alteration of morphology and metabolism

-decreased requirement for serum growth factors

-alterations in cell surface components

-increased metabolic rate

3. loss of contact inhibition of growth


Examples of transforming viruses? (44)

see chart


Summary of viral infections at cell level? (45)

see chart

-latent is a way to escape host immune response


Consequences of viral replication? (46)

see chart