VIROLOGY 4 - THE INFECTIOUS CYCLE AND BASIC NOTIONS OF INFECTION Flashcards
what are the steps of the basic mechanism of viral multiplication?
similar for all viruses;
1. attachment/adsorption of virion to receptor site on host cell
2. entry/penetration of virion or its genome into host cell
3. uncoating (for animal viruses) capsid separates and releases viral genome
4. biosynthesis of new nucleic acids and viral proteins by the host cell’s enzymes and ribosomes
5. assembly of new viral particles within host cell
6. release of newly formed virions into host cell
what are the two broad phases of the viral infectious cycle?
early phase:
attachment to delivery of the genome
“early proteins” synthesized soon after infection are usually needed to take control of the host cell and for viral genome replication (for example RdRp)
late phase:
“late proteins” are the virus structural proteins such as capsomeres
final steps of the infectious life cycle are assembly and release
what are the two different cycles for bacteriophages?
lytic cycle: ends with the lysis and death of the host cell
lysogenic cycle: host cell remains alive, the phage DNA is incorporated into host DNA
how does attachment/absorption happen for bacteriophages?
viruses cannot diffuse through membranes
must attach to receptors on host cell
bacteriophages have a chemical attraction between the viral protein and cell proteins on the cell wall
some use fibers at the end of their tails to attach
how does attachment/absorption happen for animal viruses?
have glycoproteins spikes (enveloped virus) or other attachment molecules on the capsid (naked virus) that attach to plasma membrane
viruses bind to specific proteins on host cells
host/range specificity depends on viral and host surface molecular interactions (covid binds to ACE2 receptors)
understanding this can lead to therapies
how does entry/penetration happen for bacteriophages?
bacteriophages release a protein enzyme, lysozyme that weakens bacterial cell wall
phage’s tail pierces through wall
nucleic acid from capsid passes through tail and enters
only the nucleic acid enters
how does entry/penetration happen for animal viruses?
not as well understood but there are at least 3 different mechanisms
direct penetration: naked viruses, capsid attaches and sinks into membrane, makes a pore (poliovirus)
membrane fusion: viral envelope and host membrane fuse, release capsid into cytoplasm (measles)
endocytosis: enveloped and naked viruses, after attachment, cell engulfs the virus (adenovirus, hepatitis C)
how does uncoating happen?
only for animal viruses
the capsid must be removed to release the genome before biosynthesis
varies with the type of virus but in general:
action of lysosomal enzymes of the host cell
acidification of the endosome which disassembles the capsid
for some viruses uncoating happens in cytoplasm while for some its in the nucleus
how does biosynthesis happen for bacteriophages?
many copies of phage DNA using host cell nucleotides and several enzymes
viral proteins are made using host cell ribosomes and amino acids
how does assembly happen for bacteriophages?
how does assembly happen for animal viruses?
how does release happen for bacteriophages?
lysis: host cell opens to release the new phages and the cell dies (lytic cycle)
how does release happen for animal viruses?
lytic vs lysogenic life cycle
examples of lysogeny in animal viruses?
what are patterns of viral infection
what are the three types of viral tropisms?
what are direct methods of transmission from person to person?
what are the 4 types of transmission
what are factors that contribute to viral disease emergence?
