Viruses Flashcards
Size compared to bacteria, fungi & parasites?
Smaller! Unable to be seen without electron microscope
Obligate Intracellular Parasite (definition)
Viruses are completely dependent upon a host cell for replication; replication occurs inside the cells of other organisms
Not technically “alive”
Acellular!!!
Viruses must carry or encode necessary enzymes for replication or use what is inside the cells they infect
Virus Morphology – 4 basic virus shapes
- Spherical
- Polyhedral
- Helical
- Complex (bacteriophage)
What are viral particles also known as?
Virions
Genetic Material of Virus
- Name
- RNA or DNA
- Envelope?
- Naked?
Genetic material of a virus is called its “genome”
Can be made of RNA or DNA
Some viruses have an envelope of phospholipids, derived from previously infected host cells
Non-enveloped viruses are called “naked viruses”
Glycoprotein Spikes
Enveloped viruses typically have glycoproteins that protrude from the envelope
-Important for attachment & entry into host cells
Matrix Proteins
Some virions contain structural proteins called matrix proteins, provide organization and structural integrity to the virion
-Supports the viral envelope
Capsid
- What is it?
- Made of?
- Different shapes?
Capsid = protein coat that surrounds and protects the genome
-Made of subunit proteins called capsomeres
Shapes:
1. Helical
2. Polyhedra
3. Bullet
4. Spherical
Viral Genome Properties: (they are complex!) Nucleic Acids Shape of Genome Number of Strands Sense
Nucleic Acids - DNA or RNA
Shape of Genome - linear, circular, segmented
Number of Strands - ss or ds
Sense - Positive Sense (+) or Negative Sense (-)
What does “Sense” mean? (+ or -)
Sense pertains primarily to RNA viruses, which includes the MAJORITY of viruses on Earth
Recall: Translation = mRNA –> proteins
(+) sense vs (-) sense
Positive Sense = mRNA
-Can use their genomes to make proteins as soon as they are in the host cytosol!
Negative Sense = RNA that is complimentary to mRNA
-RNA must first be converted into + sense RNA by an enzyme called RNA-dependent RNA-polymerase (RdRP) before being translated to a protein
Blatimore Classification System – Class VII
DNA virus with a reverse transcription (RT) step!
Example = Hepatitis B
- Integrate into human genome
- Transcription of viral genome results in production of RTase
- Virally encoded RTase converts RNA back to DNA
Life Cycle of a Virus: 7 Steps
- Attachment
- Entry (penetration)
- Uncoating
- Replication of the Viral Genome
- Protein Translation
- Virus Assembly
- Egress
Step 1: Attachment/Adsorption
Viral ligands (typically surface glycoprotein) binds to attachment receptor(s) on the target cell
Step 2: Entry/Penetration
Viral structures, notably envelope glycoproteins, may also be needed to bind to additional cell-surface entry receptors
- Some viruses use the same receptors for attachment & entry
- Entry may also depend on accessory proteins
Examples of Entry Accessory Proteins
Attachment & entry of the COVID-19 virus is facilitated by the entry receptor angiotensin converting enzyme 2 (ACE 2) To enter respiratory epithelium, it must first be primed by transmembrane protease, serine 2 (TMPRSS2) Primes spike (S) protein to bind ACE 2
Mechanisms of Entry
- Fusion
- Receptor-Mediated Endocytosis
- Fusion = enveloped viruses can directly fuse w/ the host cell plasma membrane if stabilized and brought close enough together. DIRECT entry into cytosol
- Receptor-Mediated Endocytosis = viral proteins bind entry receptor, resulting in internalization of viral particle into an endosome. Viral envelope may then fuse with endosome membrane, or viral proteins may facilitate migration into the cytosol
Step 3: Uncoating
Process by which the viral capsid proteins deteriorate/disassemble to that the viral genome can be release
HOW? Low pH (endosomes), proteins at the nuclear pore, viral proteases carried in virion
Nucleic acids can then be transported to replication sites
Step 4: Replication of the Viral Genome
Goal = multiply the viral genome
DNA viruses typically replicate in the cell’s nucleus
Viruses that replicate in the nuclei use the host’s nuclear import machinery to shuttle their genomes into the nucleus
Replication is an enzymatically regulated process, takes place of the course of hours –> days, depending on the virus
Replication may require host enzymes (DNA viruses), or those carry and/or encoded by viruses
Step 5: Protein Translation
Some viruses make few proteins, others make hundreds
Viral mRNA needs to be translated to support replication or to assemble new virions
Translation of viral proteins occrus at host cell ribosomes
Polycistronic mRNA (definition)
Transcripts that code for MORE than one protein
Spliced into different sized transcripts for translaiton
Polyprotein is produced that must then be cleaved into mature proteins (post-translation)
Step 6: Virus Assembly
Involves interactions b/w viral structural and non-structural proteins and the coordinated action of host factors
Step 7: Egress
- Budding
- Cell Lysis
- Exocytosis
Budding: through the cell envelope, using the cell’s membrane for the virus itself, is most effective for viruses that need an envelope… will slowly use up the membrane & eventually lead to cell death
Lysis: release of virus from the infected cell by bursting its membrane & kills cell
Exocytosis: accumulate the virus particles in vesicles & releasing them (energy-dependent)
Cyopathic Effects = CPE = damage incurred by infection
- Holes in monolayer of cells (plaques)
- Rounding of infected cell
- Fusion with adjacent cells to form syncytia
- Nuclear or cytoplasmic inclusion bodies
- Cell clumping
- Infection may also change normal cell physiology, and biosynthetic pathways
Lytic Vs Lysogenic Cycles in Phages
Lytic
- Phage infects cell
- Phage DNA circularizes, remaining separate from the host DNA
- Phage DNA replicates and phage proteins are made. New phage particles are assembled
- THe cell lyses, releasing the phage
Lysogenic
- The phage infects a cell
- The phage DNA becomes incorporated into the host genome
- The cell divides, and the prophage DNA is passed onto daughter cells
- Under stressful conditions, the phage DNA is excised from the bacterial chromosome and enters the lytic cycle
Latency (definition)
Viral latency is the ability of a virus to remain dormant within the infected host cell, sometimes establishing lifelong infection
During latency, most viral genes are NOT active, except for latency associated transcripts (LATs), which facilitate the maintenance of latency and allow infected cell to avoid detection by immune system
-Reactivation may occur periodically
Ex - Herpesviruses
Viral Tropism: 2 Features
- Susceptibility
- Permissivity (permissiveness)
Viral Tropism = range of cels or organisms that are susceptible to infection with a microorganism
Susceptibility - whether a virus can bind to a cellular receptor and ENTER (infect) that cell
Permissivitiy - refers to the ability of a virus to REPLICATE in a cell
Zoonotic Infection
An animal infection that can be transmitted to humans
