Lecture 6 Virus Flashcards
Key concepts
➢ A virus consists of a nucleic acid
surrounded by a protein coat
➢ Viruses replicate only in host cells
➢ Viruses and prions are formidable
pathogens in animals and plants
A Glimpse of History in tobacco
▪ Tobacco mosaic disease (1890s)
* D. M. Iwanowsky, Martinus Beijerinck determined caused
by “filterable virus” too small to be seen with light
microscope, passed through filters for bacteria
* F. W. Twort and F. d’Herelle discovered “filterable virus”
that destroyed bacteria
* Virus means “poison”
Viruses: Obligate Intracellular Parasites
▪ Genetic information: DNA or RNA contained within
protein protective coat
* Inert particles: no metabolism, replication, motility
* Genome hijacks host cell’s replication machinery
* Inert outside cells; inside, direct activities of cell
* Infectious agents, not organisms
* Can classify generally based on type of cell they infect:
eukaryotic or prokaryotic
- General Characteristics of Viruses
Virion (viral particle) is nucleic acid, protein coat
* Protein coat is capsid: protects nucleic acids
○ Carries required enzymes
○ Composed of identical subunits called capsomeres
* Nucleocapsid = NA + capsid
* Matrix protein between nucleocapsid and envelope
* Enveloped viruses have lipid bilayer envelope
* Non-enveloped (naked) viruses lack envelope; more resistant to disinfectants
virus size/ shape
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Viral Structure
▪ Viral genome is either DNA or RNA, never both
* Useful for classification (that is, DNA or RNA viruses)
* Genome linear or circular
* Double- or single-stranded
▪ Viruses have protein components for attachment
* Phages have tail fibers
* Many animal viruses have spikes
* Allow virion to attach to specific receptor sites
Viral Shape
▪ Generally four different shapes
* Icosahedral (Adenovirus)
* Helical or rod (TMV)
* Complex (bacteriophages like T2)
* Spherical (influenza, coronavirus)
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- Replicative cycle
Generally four steps
* Attachment and Entry
* Synthesis
* Assembly of particles
* Exit of the cell
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Replication of Bacteriophages and the types of relationships
▪ Viruses that infect bacteria
▪ 3 general types of bacteriophages based on relationship
with host
* Lytic phages
* Temperate phages
* Filamentous phages
Lytic Phage Infections
● Lytic or virulent phages exit host
● Cell is lysed
○ Productive infection: new particles formed
● T4 phage (dsDNA) as model; entire process ~30 min
● Five step process
○ Attachment
○ Genome entry
○ Synthesis
○ Assembly
○ Release
Lytic Cycle
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Lytic Cycle (time lapse)
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Lysogenic Cycle
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Bacterial Defenses Against Phages
- Preventing Phage Attachment (NO RECEPTORS)
- Restriction enzymes (restrict a phage’s ability to replicate
within the bacterium - CRISPR-Cas system
- Preventing Phage Attachment
Alter or cover specific receptors on cell surface
* May have other benefits to bacteria
* S. aureus produces protein A, which masks phage receptors;
also protects against certain human host defenses
* Capsules, slime layers (biofilms) also mask receptor
- Restriction-Modification Systems: two enzymes
Restriction enzymes (RE)
● Restrict a phage’s ability to replicate within the bacterium
● Recognize, cut short nucleotide sequences (phage DNA)
● Bacteria have different versions; hundreds of varieties
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- Restriction-Modification Systems: two enzymes
Methylase
Methylate host sequences normally recognized by RE
* Restriction enzymes do not recognize bacterial DNA
* Enzymes methylate (CH3) bacterial DNA
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3.CRISPR-Cas system
- Bacterial immune system
- CRISPR - clustered regularly interspaced short palindromic repeats
- Cas (CRISPR-associated protein) degrades RNA
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Replication of Animal viruses
Key Characteristics of animal viruses
● Genome structure
- Nucleic acid (RNA or DNA)
- Strandedness: single-strand(ss) or double-strand (ds)
● Enveloped vs non enveloped
● Hosts infected
● Other characteristics
- viral shape
- disease symptoms
Central dogma of Biology
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think of what should the host provide and what should the virus provide
Baltimore classification System of Viruses
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+ strand used as a template
Synthesis of viral genetic material
● Expression of viral genes to produce viral structural and
catalytic genes (for example, capsid proteins, enzymes
required for replication)
● Synthesis of multiple copies of genome
● Most DNA viruses multiply in nucleus
● Enter through nuclear pores following penetration
● Three general replication strategies depending on type of
genome of virus
○ DNA viruses ( Herpes, Pox, HPV)
○ RNA viruses (Flu, Covid)
○ Reverse transcribing viruses (Retroviruses)-HIV
Sense vs nonsense review
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What is Antisense RNA
Antisense RNA is a single-stranded RNA that is complementary to the messenger RNA (mRNA) strand transcribed within a cell
they are introduced in a cell to inhibit the translation machinery by base pairing with the sense RNA and activating the RNase H, to develop a particular novel transgenic
mRNA = Sense
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