Exam 1: Intro to Virology Flashcards
Viruses
Overview
- Obligate intracellular existence
- Dependent on host cells for energy, metabolism, protein synthesis, and replication
- Not visible by LM ⇒ 20-300 nm diameter
- Ultimate parasites
A single virus is called a…
virion
Nucleic Acid
Structure
- Innermost virion component
- DNA or RNA ⇒ not both
- Linear or circular
- Single or double stranded
- Segmented or non-segmented
- More segmented = more complex lifecycle
Capsid
Outermost covering of viruses
protomers ⇒ capsomers ⇒ capsid
Functions:
-
Protection
- Against DNase and RNase
-
Attachment
- For non-enveloped viruses
- Antigenicity
Protomers
Structural unit of the capsid.
Viral proteins.
Capsomers
Morphological unit of the capsid.
Formed from one or more types of protomers.
Capsid Symmetry
Determines by the organization of capsomers.
Used for classification.
- Helical
-
Icosahedral
- Pentons ⇒ always surrounded by 5 other capsomers
- Hexons ⇒ always surrounded by 6 other capsomers
-
Complex
- Seen in POX viruses
- Rare
Nucleocapsid
Nucleic acid + capsid
Naked viruses more stable to ___ than enveloped viruses.
environmental stress
Envelope
- Derived from host lipid bilayer
- Protects nucleocapsid
- Contains peplomers (“spikes, glycoprotein spikes”)
- Virus-encoded
- Functions
- Attachment
- Entry
- Antigenicity
- Enzymatic activity
Viral Budding
How a virus become enveloped:
- Virus encoded proteins become associated with host plasma membrane
- Can get membrane from organelles also
- Exit through exocytosis
- Glycoprotein spikes incorporated into membrane
- Viral nucleocapsid assembled near the membrane
- Budding begins
- Viral spikes further inserted into membrane
- Buding completed releasing a free virion
Virus
Classification
Only 6 ways to classify:
-
Nucleic acid type
- DNA or RNA
-
Nucleic acid structure
- SS vs DS
- Circular vs linear
- Segmented or non-segmented
-
RNA polarity ⇒ for RNA viruses only
- Positive sense vs negative sense
- Enveloped or Naked
-
Symmetry
- Helical, icosahedral, or complex
- Strategy for genome replication
Size & Morphology
- Differ widely in shape and size
- Picornavirus ⇒ 28 nm, size of a ribosome
- Poxvirus ⇒ 250x300 nm, ⅓ size of mitochondrion
RNA Virus
Classification
RNA Viruses
Families
DNA Virus
Classification
DNA Viruses
Families
Viral Replication
Steps
- Recognition of target cell
- Attachment
- Penetration
- Uncoating
- Macromolecular synthesis
- Assembly of virus
- Budding of enveloped virus
- Release of virus
Recognition and Attachment
“Adsorption”
-
Recognition
- Enveloped viruses ⇒ use peplomers aka viral glycoprotein or spikes
- Nake viruses ⇒ use capsid proteins
- Attach to cell surface via receptors or co-receptors
Tropism
Use of different receptors or co-receptors to attach and enter cells.
Ex. HIV is T-cell tropic
Penetration
Translocation of entire viron or portion of viron.
-
Enveloped viruses
- Receptor mediated endocytosis
- Fusion
-
Naked viruses
- Receptor mediated endocytosis
- Directly cross plasma membrane
Uncoating
Release of viral nucleic acid from capsid.
Makes genome accessible to cellular machinery.
- Can occur simultaneously with penetration or with receptor induced conformational changes
- Some naked viruses need host enzymes to remove capsid
-
Three strategies:
- At plasma membrane
- Within endosomes
- At nuclear membrane
Macromolecular Synthesis
Varies depending on type of viral genome.
Tightly regulated through expression of early and late gene products to regulate host cell cycle.
Generally:
- Early mRNA and non-structural protein synthesis
- Replication of viral genome
- Late mRNA and structural protein synthesis
- Post-translational modification of protein
Viruses can use their molecular machinery to help with ___ but cannot perform ___.
DNA/RNA replication
protein synthesis
Most RNA viruses replicate in the ___ except for ___ and ____.
cytoplasm
Exceptions:
- Influenza ⇒ replicates in the nucleus
- Retroviruses ⇒ integrates DNA copy into host chromosome
RNA-dependent RNA-Polymerases
(RdRp)
- Not present in eukaryotic cells
- Viral encoded
- Can be produced within host cells for SS ⊕-sense RNA viruses
- Must be brought along within capsid for SS ⊖-sense RNA viruses
- Other viral proteins may be required as accessory factors for RNA synthesis
RNA Processing
Addition of 5’ cap and 3’ poly-A tail required.
Neccessary for recognition by host translation machinery.
Examples of SS ⊕-sense RNA viruses are…
Poliovirus & West Nile Virus
SS ⊕-sense RNA viruses
Protein Synthesis
- Functions directly as mRNA
- Translated by cellular ribosomes into viral proteins
- Can be translated as one large polyprotein
- Cleaved by viral proteases to make individual proteins
SS ⊕-sense RNA viruses
RNA Production
- Parental ⊕ ssRNA replicated by viral RNA-dependent RNA-polymerase (RdRp)
- Makes ⊖ ssRNA
- Used by RdRp to make more ⊕ ssRNA
- Serves as additional mRNA for protein synthesis by cellular ribosomes
- Packaged into new virions
- Used by RdRp to make more ⊕ ssRNA