Module 3: Nonliving pathogens

virus

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composition/activity

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what is it?

acellular infectious agent that has one or more pieces of nucleic acid (viral genome)

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lack cytoplasmic membrane, no cytosolic organelles (not living)

no metabolic activity

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a pathogenic nucleic acid

virion

virus outside of a cell

virus types

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most common distinction of viruses

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replication events

viruses can only infect specific hosts

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between viruses that infect bacteria (bacteriophages) and those that infect animals (animal viruses)

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similar for both types of viruses, however each type has differences in structure that allow them to infect their specific host

viral structures

within a cell, a virus is just a piece of foreign nucleic acid

outside of the cell, the nucleic acid is fragile, viruses have structures to protect core and facilitate entry into the host cell

external structures of virion

protein or lipid

protect viral genome

allow for transmission + infection

Capsid

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composition

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types of virion shapes

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naked virions

external structure that protects the viral nucleic acid

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made up of individual protein units (capsomeres)

gives virus its general shape

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helical and polyhedral

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when virions consist only of a capsid and a genome

the envelope

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what do viral envelopes contain?

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what can they have mult of?

viruses can envelope themselves in remnants of the host’s cytoplasmic membrane (enveloped virions)

almost exclusively viruses that infect eukaryotes

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proteins (spikes) that allow virus to recognize the host cell

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spikes, which can change due to mutation of viral genome

viral genome

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interesting thing about ssDNA and dsRNA

viruses have genomes made up of nucleic acid

small than hosts

can have ssDNA, dsDNA, ssRNA, dsRNA

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almost nonexistent in cells

genomes of viruses can take variety of shapes

Viral transmission

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what do common viruses cause?

viruses are no alive, however they can be transmitted like any other pathogen

contact transmission usual route

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respiratory infections, so droplets are inhaled or hands are contaminated through fomites, touch eyes and face

viral transmission process steps

attachment (virion attachments to host cell membrane)

entry (viral genome goes into cell)

replication (new genome, new external)

assembly release (reform back to virion)

release (damage, cell dies, causes s/sx)

attachment

viruses are not alive, can only come into contact with host cells by random collision (random chance)

animal viruses attach to a host cell by binding receptors on the cell surface

receptors can bind the capsomere proteins of a naked virion

binding of a virus to a receptor provides tissue specificity

entry for naked virions

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entry for enveloped virions

inject their genome into the cell through the plasma membrane or be taken by endocytosis

capsid out of cell injects genome, now inside cell and can replicate

endocytosis creates virion within cell

but not genome so no replication

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endocytosis and membrane fusion

only enveloped virions can enter host cell by membrane fusion

uncoating

any virus that enters a cell through fusion or endocytosis must be uncoated before replication

capside is removed and viral genome is released into cytoplasms

replication

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how does a piece of nucleic acid replicate itself?

living organism has ability to make energy, grow, divide

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hijacking the normal growth process of the host cell

allows virus to accomplish 2 things

replicate viral geome

replicate other structures of the virus (proteins)

DNA replication

DNA → DNA polymerase → DNA

viruses take enzyme from us

how do our cells make proteins

translation

RNA → ribosome → proteins

making RNA

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cells

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viruses

transcription

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DNA → RNA polymerase → RNA

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RNA → RNA polymerase → RNA

DNA viral replication genome

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other structures

DNA moves into nucleus

genomes of DNA viruses often enter nucleus of cell

protects viral genome

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DNA genome → transcription → mRNA → translation → replicated proteins

RNA viral replication

RNA viruses have genomes that stay in the cytoplasm