Ch 12 - Viruses, Viroids, & Prions

What are viruses?

An overarching term, can infect basically anything, not technically alive, cannot replicate on their own or produce their own ATP

What are virions?

A complete virus particle

What is a bacteriophage?

A virus that can infect bacteria

What does it mean if something is an obligate intracellular parasite?

It needs a host for energy and substrates, will hijack a host cell’s replication machinery and direct it to express viral genes and proteins

What is a host range?

The range of hose species a virus can infect

What is tissue trophism?

The range of tissue types a virus can infect

What are the main components of a virus (~3)?

Nucleic acid (viral genome), protein capsid (shell protecting genome), and envelope (lipid bilayer, not all have)

What does the nucleocapsid of a virus contain?

Nucleic acid and the capsid

What is nucleic acid?

Viral genome, can be DNA or RNA, can be linear, circular, or segmented, can be double or single-stranded

What determines the shape of a virus?

Its capsid

What is a capsomere?

Repeating or identical protein subunits

What is the purpose of a capsid?

To protect the genome

What is a naked virus?

A virus without an envelope, only consist of a nucleocapsid, more resistant to disinfectants

What are enveloped viruses?

Viruses whose nucleocapsid in enclosed within a lipid bilayer, tegument (contents like proteins) may exist between the envelope and capsid

How do viruses attach to host cells?

Via spike proteins (on enveloope), tail fibers, or fiber proteins (on capsid)

What determines a viruses host range and tissue trophism?

Its attachment proteins on the surface of the virus, can also be used for identification, help the virus recognize and attach to host cells

What part of the virus does the immune system normally recognize?

The attachment proteins on the surface of the cell, can also be used for viral identification

What is an icosahedral?

A viral shape with 20 faces, exhibits radial symmetry

Whgat is a helical/filamentous virus?

A viral shape with a helical capsid tube surrounding the genome, generates a flexible filament, envelope masks the shape

What is a complex/amorphous virus?

A viral shape that has no symmetrical form, the core contains the genetic material, it has a flexible “core wall”

What are the 3 parts of a bacteriophage?

The head, tail, and tail fibers

What is the head of a bacteriophage?

Its an icosahedral that contains the genetic material

What is the purpose of the tail on a bacteriophage?

It is used to inject the bacteriophages genome into the host virus cell

What is the tail fiber on bacteriophages used for?

Attachment

What part of the virus do we use to classify viruses?

The genome

What are the 5 steps of viral replication?

1. Host recognition and attachment

2. Genome entry

3. Biosynthesis

4. Assembly/maturation

5. Release and transmission

What happens during the host recognition and attachment (1st) step of viral replication?

The viral surface protein binds to the host receptor

What happens during the genome entry (2nd) step of viral replication?

Viral nucelid acid needs to enter the host for replication, the whole virus particle may enter the host cell or just the viral genome

What happens during the biosynthesis (3rd) step of viral replication?

The viral genome is replicated first followed by viral proteins (mechanism of replication depends on the viral genome)

What happens during the assembly/maturation (4th) step of viral replication?

New virions are built (typically occurs before they are released from the host cell but can occur after they are released)

What happens during the release and transmission (5th) step of viral replication?

The virions leave the host and find a new host (replication process repeats), leaving may or may not kill the host

What is a productive/lytic bacteriophage?

A bacteriophage that enters and hijacks the replication machinery to make more of the bacteriophage, destroys/lyses host cell to release new virions (phages)

What types of phage are productive (2)?

Virulent (lyse host) and filamentous (do not lyse host) phages

What is a latent/lysogenic phage?

A phage whose viral genome integrates into the host genome, no virions (phages) get produced, remains dormant in host cell w/o immediately destroying it, can switch to lytic cycle

What type of phage is latent/lysogenic?

Temperate phages

What is burst size?

Number of virions (phages) released after lysing (~100s)

What is a plaque?

A clearing on a lawn of bacteria, 1 plaque = 1 active virion

What is a filamentous phage?

A phage that often uses host pili to enter and secretion systems to exit, DOES NOT kill the host cell

What is a prophage?

A phage genome that has been successfully integrated into the host genome

What is induction?

When a phage switches to the lytic cycle and the prophage is cut out, mistakes may occur (specialized transduction)

What can temperate phages do that others can’t?

Temperate phages can switch between a lysogenic and lytic life cycle

What are some bacterial defenses against phages (3)?

Restriction enzymes (endonucleases), methylation of bacterial genome to mark self, and CRISPR

How do restriction enzymes help protect bacteria against phages?

Endonucleases can recognize short DNA sequences and cut out foreign DNA

How does CRISPR help protect bacteria against phages?

Bacteria can store short phage sequences in their own genome after surviving infection, crRNAs transcribe from these sequnces, CAS proteins (endonucleases) cleave DNA when bound to a crRNA (can specifically target phage DNA), phage protection can get passed on to daughter cells, utilized by scientists to target and cut specific DNA sequnces

How can viruses enter host cells (2)?

Endocytosis or fusion

How do viruses use endocytosis to enter host cells?

Virus-host attachment triggers endosome formation

How do naked viruses enter host cells?

Via endocytosis ONLY

How do viruses use fusion to enter host cells?

The bacterial envelope fuses with the host plasma membrane allowing the nucleocapsid to enter the cytoplasm

How do enveloped viruses enter host cells?

They can enter via endocytosis OR fusion

What does uncoating mean?

Once inside a host, a viruses capsid breaks down to release the viral genome within

What are the 2 main tasks of animal viral biosynthesis?

To express genes (produce viral proteins) and replicate the viral genome (reproduction)

How is DNA polymerase used in biosynthesis of animal viruses?

DNA dependent DNA synthesis (make DNA from DNA)

How is RNA polymerase used in biosynthesis of animal viruses?

DNA dependent RNA synthesis (make RNA from DNA)

How are ribosomes used in biosynthesis of animal viruses?

Make proteins from + sense RNA

How is RNA-dependent RNA polymerase (RdRp) used in biosynthesis of animal viruses?

Uses replicase to duplicate RNA from an RNA template (mRNA)

What is replicase?

A viral enzyme, more error-prone (antigenic drift)

How is reverse transcriptase used in biosynthesis of animal viruses?

Reads RNA, makes DNA, peels away RNA, then makes dsDNA, has a higher error rate, used by retroviruses, gets replicated as DNA within the host cell using DNA polymerase

What is a provirus?

A viral genome that has permanently integrated itself into host DNA that was originally single-stranded RNA but got replicated into dsDNA

How are enveloped viruses released from the host cell?

They can use exocytosis (secretory pathway) or budding (get envelope from host membrane)

How are naked viruses released from the host cell?

Typically kill the host cell via apoptosis

What are viroids?

Infectious agents in which an RNA is the entire infectious particle, no protective capsid, no ORFs or protein coding genes, mainly infect plants

What are prions?

Infectious agent made of ONLY protein, no nucleic acid, abnormal folding of a protein, VERY stable, lead to aggregates in the brain, can be inherited or transmitted