Microbiology Chapter 13: Viruses

Definition of virus

virus means "poison". viruses are NOT alive; they are defined as either active or inactive.

What does it mean to be an obligate intracellular parasite?

cannot multiply unless they invade a host cell. they require being inside cells (intracellular) and must commandeer the host cell's organelles.

What kind of nucleic acid core do viruses possess?

contain a single type of nucleic acid: DNA or RNA (never both). this can be single-stranded (ss) or double-stranded (ds).

What do viruses possess for protection?

capsid

What is a capsid?

a protein coat that protects the nucleic acid, composed of protein subunits called capsomeres

What is the function of the capsomeres?

the arrangement of capsomeres determines the shape

Do viruses possess enzymes or organelles?

have few or no metabolic enzymes of their own. they lack mitochondria, ribosomes, and other cellular components.

Effects of antibiotics on viruses

antibiotics do not work against them because the virus has no target components (like ribosomes).

How is the host range of the virus determined?

determined by the virus's specific attachment to cell surface receptors. viruses infect specific types of cells (e.g., bacteriophage infects bacterial cells).

Definition of virion

the complete, fully developed infectious particle

How are virions classified?

by the shape of the protein coat

What is the viral envelope?

consists of embedded lipids, protein, and carbohydrates that may be covered with spikes

What are the viral spikes composed of?

carbohydrate protein complexes

What is the function of the viral spikes?

attachment and identification

What is a requirement for growing viruses in the lab?

a host cell

What does a bacteriophage infection appear as on an agar plate?

on an agar plate that has a lawn of host cells, the infection is visible as clear plaques.

What do the plaques on the plate represent?

millions of bacteria that have died

What are the main components of a bacteriophage?

DNA, capsid, and sheath

What are the most common T-even bacteriophages?

T2, T4, T6

What is the end goal of the Lytic Cycle?

to lyse the cell

1st Step of Lytic Cycle

virus attaches to a complementary receptor site on the bacterial cell wall (attachment/adsorption)

2nd Step of Lytic Cycle

virus injects nucleic acid into the host cell (penetration)

3rd Step of Lytic Cycle

virus induces degradation of host DNA and host cell becomes a factory for viral particles by synthesizing viral nucleic acids and proteins (biosynthesis)

4th Step of Lytic Cycle

viral DNA and capsids are assembled into complete virions (maturation)

5th Step of Lytic Cycle

cell lyses and dies, releasing new bacteriophages to infect new cells

How long does it take for the lytic cycle to complete?

less than an hour -- thousands of bacteriophages can be produced in hours

What is the end goal of the lysogenic cycle?

the cell stays alive.

Definition of prophage

viral DNA that has integrated into the host DNA. it replicates with every cell division of the host

What differentiates the lysogenic cycle from the lytic cycle?

the phage remains latent (inactive/not being expressed), and continues to reproduce normally

When the prophage excides from the bacterial chromosome, what initiates as a result?

the virus is induced to switch to the lytic cycle. this switch is irreversible because the end result is cell death

Where are the attachment sites for animal viruses found?

attachment sites are the spikes on the envelope surface

How do animal viruses enter host cells?

by receptor-mediated endocytosis or by fusion

Which animal virus entry is the more common one?

receptor-mediated endocytosis

How do animal viruses utilize fusion to enter the host cell?

the viral envelope fuses with the plasma membrane and releases the capsid into the cytoplasm

After the attachment and entry of a virus into a host cell, what is the next step in the multiplication of animal viruses?

the separation of viral DNA/RNA from the capsid (uncoating). the capsid is degraded by host enzymes

After uncoating, the viral DNA needs to be...

copied, then expressed

How does the virus prioritize its gene expression?

divides genes into "early" and "late" genes

What do the "early" genes typically code for?

enzymes required for viral multiplication

What do the "late" genes typically code for?

the virus's capsid and other structural components

What does it mean for a viral infection to be latent?

the virus remains in the host in an inactive state and can be reactivated

What are 2 examples of latent viral infections?

Herpes, chickenpox/shingles

What does it mean for a viral infection to be persistent?

disease occurs gradually over a long period; infections are detectable, and the virus gradually builds up

What are 2 examples of persistent viral infections?

HIV, HPV

Definition of a prion

a proteinaceous infectious particle.

How are prions characterized?

characterized by an abnormal (misshaped) protein (PrPsc).

How do prions differ from viruses?

prions have no nucleic acids (no genetic component).

How are prions caused?

caused by transmissible spongiform encephalopathies

What do transmissible spongiform encephalopathies lead to?

causes plaques to develop in the brain, leading to loss of nerve cells and motor function.

What is an example of a prion disease in sheep?

scrapie

What is an example of a prion disease in cattle?

bovine spongiform encephalopathy (BSE), or "mad cow disease"

What are two examples of prion diseases in humans?

Creutzfeldt-Jakob disease (CJD), and kuru

Prognosis of prion diseases

prion diseases are always fatal -- brain progressively deteriorates, causing a loss of motor function and dementia