Lecture 6, viruses and vaccine Flashcards
General properties of viruses
- Virion -> Complete virus particle
– Consists of at least 1 molecule of DNA or RNA (not both) enclosed in coat of protein
– May have additional layers
– Cannot reproduce independent of living cells nor carry out cell division
– Cannot produce the energy
– (much) Smaller than cells
– But can exist extracellularly!
Acellular Agents
- Viruses: protein and nucleic acid
- Viroids: only RNA
- Satellites: only nucleic acids
- Prions: proteins only
Types of viruses - Single stranded DNA (ssDNA):
Parvoviruses
Types of viruses - double stranded DNA (dsDNA):
Herpesviruses
Adenoviruses
Poxviruses
Hepadnaviruses* (Partially double stranded)
Virion
an entire virus particle,
consisting of an outer protein shell called a capsid and an nucleocapsid which is composed of nucleic acid (either RNA or DNA).
Some viruses consist only of a nucleocapsid, others have additional components.
Types of viruses - Single stranded RNA (ssRNA)
– Picornaviruses (+)
– Retroviruses (+)
– Rhabdoviruses (-)
Types of viruses - double stranded RNA (dsRNA)
Reoviruses
Capsids
- Large macromolecular structures which serve as protein coat of virus
- Protect viral genetic material and aids in its transfer between host cells
- Made of protein subunits called protomers
Capsids are helical, icosahedral, or complex
Helical capsids
Shaped like hollow tubes with protein walls
Protomers self assemble
Size of capsid is a function of nucleic acid
Examples: Tobacco Mosaic Virus, Ebola virus
Icosahedral Capsids
- An icosahedron is a regular polyhedron with 20 equilateral faces and 12 vertices
- Build out of capsomers
- ring or knob-shaped units made of 5 or 6 protomers – pentamers (pentons) – 5 subunit capsomers
- hexamers (hexons) – 6 subunit capsomers
Examples: Polioviruses, herpesvirus
Viral Envelope
- Many viruses are bound by an outer, felxible, membranous layer called the envelope.
- Animal viral envelopes usually arise from the host cell plasma or nuclear membranes.
- Naked viruses lack envelopes.
Viral envelope proteins
- Envelope proteins, which are viral encoded, may project from the envelope surface as spikes or peplomers
- Involved in viral attachment to host cell
- used for identification of virus
- may have enzymatic or other activity,
- may play a role in nucleic acid replication
Virion Enzymes
It was first erroneously thought that all virions lacked enzymes. Now accepted that a variety of virions have enzymes. Some are associated with the envelope or capsid but most are within the capsid
Viral Multiplication
Mechanism used depends on viral structure and genome
However steps are similar:
- attachment to host cell -> entry
- uncoating of genome -> synthesis
- assembly
- release
Attachment step of viral multiplication
Specific receptor attachment
Receptor determines host preference
- There may be more than one host
- May be more than one receptor
- May be in lipid rafts providing entry of virus
Viral Entry and Uncoating
- Entire genome or nucleocapsid
- Varies between naked or enveloped virus
Three methods used
1) fusion of the viral envelope with host membrane; nucleocapsid enters
2) endocytosis in vesicle; endosome aids in viral uncoating
3) injection of nucleic acid
Synthesis
- Genome dictates the events
- ds DNA
- RNA viruses:virus must carry in or synthesize the proteins necessary to complete synthesis (reverse transcriptase)
- Early period: Synthesis of proteins required for replication of viral genetic material.
- Late period: Nucleic acid replication and synthesis of capsid and envelope proteins
Assembly
Late proteins are important in assembly
Assembly is complicated but varies:
- bacteriophages – stages
- some are assembled in nucleus
- some are assembled in cytoplasm
- may be seen as paracrystalline structures in cell
Virion Release
Nonenveloped viruses (Naked viruses and pox viruses) lyse the host cell
Enveloped viruses use budding:
- viral proteins are placed into host membrane
- nucleocapsid may bind to viral proteins
- envelope derived from host cell membrane, but may be Golgi, ER, or other
- virus may use host actin tails to propel through host membrane
Bacterial and Archaeal Viral Infections
Virulent phage –> one reproductive choice
- multiplies immediately upon entry and lyses bacterial host cell
Temperate phages have two reproductive options
- reproduce lytically as virulent phages do or
- remain within host cell without destroying it
many temperate phages integrate their genome into host genome (becoming a ‘prophage’ in a ‘lysogenic bacterium’) in a relationship called lysogeny
Lytic and lysogenic cycle
Lytic cycle: replicates, synthesizes virus particels and lysis the cell
Lysogenic cycle: stays latent as a prophage and replicates with the devision of the cell.
Infection in Eukaryotic Cells
- Cytocidal infection results in cell death through lysis
- Persistent infections may last years
- Cytopathic effects(CPEs) –> degenerative changes (swelling or shrinkage, rounding, lysis, plaques, clumping, syncytia, and inclusions)
- Transformation to malignant cell
Possible Mechanisms by Which Viruses Cause Cancer
- Viral proteins bind host cell tumor suppressor proteins
- Carry oncogene into cell and insert it into host genome
- Altered cell regulation
- Insertion of promoter or enhancer next to cellular oncogene
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