Virus Life Cycle Flashcards
How do viruses grow?
- inverted microscopes are used to observe virus and cell cultures
- viruses are NOT visible by light microscopy
- virus infection may cause cytopathic effects that can be used to study virus replication and infectivity (Not all viruses cause CPE)
Key concepts about viruses
- obligate intracellular parasites
- no genes for biosynthetic machinery
- assemble from host cell molecules
- evolve rapidly compared to host
What does a virus need?
- the right host (tropism)
- cells with the right receptors (susceptible)
- appropriate intracellular environment (permissive)
- biosynthesis machinery
- abundant building blocks- nucleotides (RNA, DNA), amino acids, ATP, lipids, sugars, etc.
- time to finish replication
Steps in Virus Replication
1) Recognition of the target cell
2) Attachment
3) Entry: Penetration or Fusion
4) Uncoating
5) Transcription of mRNA
6) Protein synthesis
7) Replication of the genome
8) Assembly of virions
9) Egress: lysis, budding, exocytosis
Steps 1 and 2: Recognition and Attachment
- recogntion: interactions between virions and tissues
- attachment: binding of a virion surface molecule to its specific cellular receptor
- Entry: Internalization of the virion into the cell. Herpes recognizes the extracellular matrix, then attaches to specific protein receptors
Host Range
-the preferred species
Tissue tropism
-the preferred cell type
Susceptible
-cells that a virus can enter
Permissive
-cells that support virus replication and virion synthesis
Abortive infection
-replication is incomplete
Step 3: Entry
- virions may use multiple routes to enter cells
- varies by cell type
- may have consequences for disease outcome
- penetration: engulfment of entire virion into cell
- receptor-mediated endocytosis
- pinocytosis
- phagocytosis
-fusion: virion envelop fuses with plasma membrane, leaving parts of the virion behind
Step 4: Uncoating
- release of the genome into the cell
- for infection to begin, capsids must open to release the genome into the cytoplasm or nucleus
- uncoating marks the beginning of the “eclipse phase”
Mechanisms of entry and uncoating:
- at the plasma membrane
- within endosomes
- at the nuclear pore
Step 5: Transcription of mRNA
- all viruses must make mRNA
- viral genome is the template for transcription
- viral and host transcription factors regulate mRNA synthesis- RNA-dependent RNA polymerase (RDRP)
- mRNA is made by viral or host polymerases
Step 6: Protein Synthesis
- viral mRNAs are translated into protein by the host machinery- ribosomes, tRNAs, amino acids
- viral proteins are sorted to site of virion assembly
- capsid proteins interact with the newly made genomes
- membrane proteins traffic through the secretory pathway
- cytosolic proteins accumulate next to the membrane
Step 7: Replication of the Genome
- viral genomes come in many types
- RNA double or single stranded, + or - sense
- DNA double or single stranded, + or - sense
- Linear, circular, segmented, sealed ends, etc
- polymerases make new genomes using host cell nucleotides
- viral RNA dependent RNA polymerase (RDRP)
- viral DNA polymerase
- host cell DNA polymerase
- host cell RNA Pol II (RNAP II)
