Chapter 13: Viruses, Viroids, and Prions

What are viruses?

• genetic material (DNA or RNA) contained within a protein coat (capsid)

• have no metabolism but can reproduce after infecting another cell

What is a bacteriophage?

A virus that infects a bacteria

What are enveloped and naked viruses?

• enveloped- virus surrounds itself in a lipid bilayer taken from the host cell, easier to kill with disinfectants

• naked- no envelope, way more resistant to disinfectants

What are the three shapes of a virus?

• icosahedral (20 flat triangles)

• helical (looks like a rod)

• icosahedral nucleocapsid head and helical protein tail (shape of a bacteriophage)

What are the two types of phage replication?

• productive infection- new viral particles are produced

• latent phase- when the viral genome remains silent in a host cell until it emerges one day and enters the productive infection phase

What are the 5 steps of infection?

Attachment, genome entry, synthesis of phage proteins and genome, assembly (maturation), and release through host cell lysis

What is a lytic phage infection?

• lytic phage exit the host by lysing and killing the host cell

• can release approximately 250 phages

What is a temperate (lysogenic) phage infection?

• phage uses integrase to insert phage DNA into a specific site on the host chromosome and makes a prophage

• lysogen then replicates the prophage and passes it on to progeny through vertical gene transfer

• prophage genes are silenced by a repressor protein until the lysogen enters the lytic stage

What is a filamentous phage infection?

• phage lands on the F pilus and injects its genetic information

• the phage replicates in the bacteria and then exits through the F plus, and it gains a capsid as it exits

• the host bacteria does not lyse

What are different methods to study phages?

• plaque assays- a bacterial lawn forms and phage culture forms clearings (plaques) in the bacteria; each plaque represents one phage

• tissue culture using tumor cells

• hemagglutination- mix viruses with red blood cells, the highest dilution with maximum agglutination is the titer of the virus

What is a viroid?

• RNA molecules without a capsid

• only infect plants

• scientists do not understand how they reproduce 

What is a prion?

• a misfolded protein that makes the proteins around it misfold

• very resistant to heat and chemicals

• Mad Cow Disease and Creutzfeldt-Jakob disease