Viruses Part I Flashcards
Virus vs. virion
Viruses are obligate intracellular parasites which can only replicate inside host cell
- nucleic acid (RNA or DNA) enveloped inside a protein capsid coat
- host cells supply building blocks, machinery and energy to replicate virus
Virons are inert infectious particles
Define:
Capsid
Capsomeres
Nucleocapsid
Envelope
Capsid = protein coat around nucleic acids
Capsomeres = proteins which make up capsid
Nucleocapsid = simple virion
Envelope = lipid layer embedded with virus-specific proteins
- in animal viruses the envelope is derived from cytoplasmic membrane of host
Virus shapes and examples
Helical (typical of plants) - viral RNA in a helix with identical capsomeres backbone
Ex. tobacco mosaic virus
Polyhedral (typical of animal viruses) - symmetrical faces, 20-sided icosahedron is most common
Ex. human papilloma virus
Complex (typical of bacteriophages) - head, tail, tail fibers for injecting genome into bacteria
Note: shape depends on capsomeres
Prions
Misfolded protein (PrP^Sc) which causes the misfolding of other proteins (PrP^C) - does not actually replicate
Accumulation of PrP^Sc kills cells
Primarily found in neurons
Ex. Mad cow disease
Viroids
Viroids (only in plants) - closed circle of single stranded RNA 240-380 nucleotides long
- disease state caused by viroid genome taking over host genome machinery
Ex. Cadang cadang coconut disease and potato spindle tuber
Virus taxonomoy naming scheme
Family: -viridae
Genus: -virus
Species: species name in English
Classification based on: nature of host, type of disease caused, life cycle, naked or enveloped, nucleic acid type and strandedness
Baltimore Classification Scheme
Plus vs minus config
Classification based on type of genome which dictates replication mechanism
Plus configuration: same strand as mRNA –> can be directly translated
Minus configuration: complementary mRNA strand which requires transcription before translation
Life cycle stages of viruses
1) Adsorption: virus attachment to specific receptors on host surface
- in plants often done damage or insect vectors
2) Penetration: virus entry into the host cell
- envelope –> left at surface and nucleocapsid enters
- naked –> capsid left at surface
3) Uncoating: Removal of envelope and/or capsid by host enzymes (ex. lysosomes in animals)
4) Replication of nucleic acids - transcription and protein synthesis
- host cannot help but do this
5) Maturation - assembly of components to form new virions, often spontaneous
6) Release - virions exit by budding or lysis of cell
- or by vectors in plant viruses
Viropexis
Endocytosis of enveloped virus in eukaryotes
- double envelope fuses with lysosomes for degradation and then uncoating of capsid for genome release into cytoplasm
Naked viruses can also be taken up by endocytosis
Latent period and rise period of viral replication
Latent period = eclipse + maturation
Eclipse = time for host cells to replicate viral genome and synthesize viral components
Maturation = time needed for assembly
Rise = virions detected outside the cell once release has begun
- Burst size = number of virions released
Mechanism of budding release
Budding is how viruses get their envelope
Viral proteins are recognized and viral capsid fuses on cytoplasmic side of membrane to bud and be released
- Relatively slow process
Does not cause the cell to lyse right away
Ex. Herpes stays in you forever and occasionally release new virus like this
Bacteriophages
Virulent vs temperate
Genome mostly dsDNA, structure mostly naked
Virulent: infection leads to lytic pathway
Temperate: can do both lytic and lysogenic pathways and genome is incorporated into host cell’s genes
- can lie dormant and then return to lytic pathway
Bacteriophage T4 replication
Virulent bacteriophage
1) Adsorption via T4 tail fibers attaching to core region of LPS
2) Tail sheath contracts forcing central core of phage through outer membrane
3) Tail lysozymes digest peptidoglycan to form pore (gram-neg)
4) Phage DNA injected through pore into host cell
Bacteriophage Lambda replication phases and mechanisms
Involves prolonged latent stage or lysogeny
1) Infection of viral genome into host cell
2) Lysogenic pathway: viral DNA is integrated into host cell forms –> Lysogen (contains Prophage)
- Homologous region in host genome to the viral dsDNA with cohesive ends that fit together
- viral DNA integrated at specific site using lambda enzyme integrase (encoded by phage genome)
3) Lysogens undergo cell division and replication of prophage
4) Induction: excision of viral genome out of host DNA to enter lytic pathway
5) Production of new viral particles and host cell lysis
