Accelular Flashcards
+ssRNA
+ssRNA (Positive-sense single-stranded RNA)
- Definition: Viral RNA that functions directly as mRNA
- Function: Can be immediately translated by host ribosomes
- Examples: Poliovirus, SARS-CoV-2
-ssRNA
-ssRNA (Negative-sense single-stranded RNA)
- Definition: Viral RNA that is complementary to mRNA
- Function: Must be transcribed into +ssRNA by a viral RNA-dependent RNA polymerase before translation
- Examples: Influenza virus, rabies virus
bacteriophage
Bacteriophage
- Definition: A virus that infects and replicates within bacteria
- Life Cycles: Lytic (kills host) or lysogenic (integrates into host genome)
- Example: T4 phage
biosynthesis
Biosynthesis (in virology)
- Definition: The stage in the viral replication cycle where viral nucleic acids and proteins are synthesized using the host cell’s machinery
- Includes: Genome replication and viral protein production
- Depends on: Type of viral genome (DNA or RNA, + or -)
BSE
BSE (Bovine Spongiform Encephalopathy)
- Definition: A prion disease in cattle, also known as “mad cow disease”
- Cause: Misfolded prion proteins (PrP^Sc)
- Transmission: Through consumption of infected tissue
- Human Concern: Can be transmitted to humans as variant CJD
CJD
CJD (Creutzfeldt-Jakob Disease)
- Definition: A rare, fatal neurodegenerative disorder caused by prions
- Forms: Sporadic, inherited, or acquired (e.g., variant CJD from BSE)
- Symptoms: Rapid cognitive decline, motor dysfunction, death within months
DNA virus
DNA Virus
- Definition: A virus with a DNA genome (either ssDNA or dsDNA)
- Replication Site: Usually in the nucleus of the host cell
- Examples: Herpesviruses (dsDNA), Parvoviruses (ssDNA)
dsDNA
dsDNA (Double-stranded DNA)
- Definition: Two complementary DNA strands forming the viral genome
- Function: Can be transcribed into mRNA by host enzymes
- Examples: Herpes simplex virus, adenovirus
dsRNA
dsRNA (Double-stranded RNA)
- Definition: Viral genome composed of two complementary RNA strands
- Replication: Requires viral RNA-dependent RNA polymerase to transcribe mRNA from the dsRNA
- Example: Rotavirus
hemagglutinin (HA)
Hemagglutinin (HA)
- Definition: A surface glycoprotein found on influenza viruses
- Function: Binds to sialic acid on host cells to mediate viral entry
- Importance: Target of immune response and vaccines; subject to antigenic drift
host specificity
Host Specificity
- Definition: The range of host species or cell types a virus can infect
- Determined by: Viral surface proteins and host cell receptors
- Example: HIV infects human CD4+ T cells due to gp120-CD4 interaction
RNA virus
RNA Virus
- Definition: A virus with an RNA genome (can be +ssRNA, -ssRNA, or dsRNA)
- Replication: Often occurs in the cytoplasm; requires viral RNA polymerase
- Examples: SARS-CoV-2 (+ssRNA), influenza (-ssRNA)
virus
Virus
- Definition: A non-cellular infectious agent composed of genetic material (DNA or RNA) enclosed in a protein coat (capsid), sometimes with an envelope
- Replication: Only inside a host cell (obligate intracellular parasite)
- Components: Genome, capsid, +/- envelope and enzymes
Name and describe the general characteristics of all viruses.
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Acellular:
- Viruses are not cells (not alive); exist as virions outside a host
- Virions are inert with no metabolism or active enzymes outside the host
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No Metabolism:
- Cannot generate ATP or carry out metabolic functions
- Enzymes (if present) are inactive outside host cells
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Obligate Intracellular Parasites:
- Require a host cell to replicate
- Use host structures (ribosomes, enzymes, ATP, phospholipids) to build new viruses
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High Host Specificity:
- Most infect a single species and even a specific cell type
- Example: HPV targets epithelial cells; Hepatitis A targets liver cells
- Bacteriophages can be strain-specific
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Structure:
- May be enveloped or non-enveloped
- May contain glycoproteins - all enveloped viruses contain
- Contain either DNA or RNA (never both) as their nucleic acid genome
Describe structural components of viruses
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Capsid: - IN ALL VIRUSES
- Definition: Protein shell that surrounds and protects the viral genome
- Made of: Subunits called capsomeres
- Function: Protects nucleic acid and aids in host cell attachment for non-enveloped viruses
- Shapes: Icosahedral, helical, or complex
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Nucleic Acid (Genome): - IN ALL VIRUSES
- Type: Either DNA or RNA, single- or double-stranded, linear or circular
- Function: Carries genetic instructions for viral replication and protein synthesis
- Note: A virus never contains both DNA and RNA
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Nucleocapsid: - IN ALL VIRUSES
- Definition: The combined structure of nucleic acid + capsid
- Function: Core infectious unit; may be the entire virus in non-enveloped types
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Envelope:
- Definition: A lipid bilayer derived from the host cell membrane that surrounds the nucleocapsid (only in enveloped viruses). NOT A CELL MEMBRANE
- Function: Helps virus enter host cells via membrane fusion or endocytosis
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Glycoproteins (Spikes):
- Embedded in: The viral envelope
- Function: Mediate host cell recognition and entry by binding to specific receptors
- Examples: Hemagglutinin (HA) in influenza, gp120 in HIV
- Clinical Relevance: Major targets for host immune response and vaccines
Enveloped vs non-enveloped viruses
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Capsid:
- Both: Contain a protein capsid surrounding the nucleic acid
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Nucleic Acid:
- Both: Contain DNA or RNA, but never both
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Envelope:
- Enveloped Viruses: Have a lipid envelope derived from the host cell membrane
- Non-Enveloped Viruses: Lack an envelope; outermost layer is the capsid
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Glycoproteins:
- Enveloped Viruses: Always present; embedded in the envelope and form spikes used for host cell attachment
- Non-Enveloped Viruses: May have capsid-associated proteins or receptor-binding domains, but true glycoprotein spikes are uncommon
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Stability:
- Enveloped Viruses: Less stable in the environment; sensitive to drying, detergents, and heat
- Non-Enveloped Viruses: More stable; resist harsh conditions and common disinfectants
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Entry into Host Cell:
- Enveloped Viruses: Enter via membrane fusion or endocytosis
- Non-Enveloped Viruses: Enter via endocytosis or direct penetration
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Examples:
- Enveloped: Influenza virus, HIV, herpesviruses
- Non-Enveloped: Norovirus, poliovirus, adenovirus
Host Speceficity
- Viruses infect only specific species or cell types based on the interaction between viral surface proteins and host cell receptors
- This interaction determines which cells a virus can enter and replicate in
- Specificity can be narrow (e.g., HIV infects only human CD4+ T cells) or broad (e.g., some influenza strains infect birds, pigs, and humans)
Examples of Virus-Host Recognition:
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HIV (Human Immunodeficiency Virus):
- Viral glycoprotein: gp120
- Host receptor: CD4 (on helper T cells)
- Co-receptors: CCR5 or CXCR4
- Host specificity: Infects only human CD4+ T cells and some macrophages, causing immune system failure
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Influenza Virus:
- Viral glycoprotein: Hemagglutinin (HA)
- Host receptor: Sialic acid on respiratory epithelial cells
- Host specificity: Depends on the type of sialic acid linkage (α-2,6 in humans, α-2,3 in birds), influencing which species or tissues are infected
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SARS-CoV-2 (COVID-19):
- Viral glycoprotein: Spike (S) protein
- Host receptor: ACE2 (angiotensin-converting enzyme 2), present on lung, gut, and heart cells
- Host specificity: Infects humans (and some animals); receptor distribution influences symptoms and tissue tropism
Lytic Replication - bacteriophage
- Definition: A replication process where a virus quickly hijacks a host cell, produces new virions, and destroys the host cell upon release
1. Attachment
- Virus binds to specific receptors on the surface of the host cell
- Determines host specificity
2. Entry
- Viral genome enters the host cell
- The phage injects its DNA into the bacterial cytoplasm - a protein tube penetrates through the cell wall injecting DNA
3. Synthesis (Biosynthesis)
- Viral genes are expressed using host machinery
- Genome replication and viral protein synthesis occur
- Depends on genome type (+ssRNA, -ssRNA, DNA, etc.)
4. Assembly
- Newly synthesized viral genomes and proteins are assembled into new virions
- Takes place in the cytoplasm or nucleus (depending on the virus)
5. Release
- Virally-encoded lysozymes break down peptidoglycan of host cell. Host undergoes lysis due to osmotic pressure against wakened cell wall. New bacteriophages released upon lysis
Lysogenic Replication - bacteriophage
- Definition: A bacteriophage replication pathway where the phage genome integrates into the host chromosome and remains dormant as a prophage, without destroying the host cell immediately
1. Attachment
- The phage attaches to specific receptors on the surface of a bacterial cell
2. Entry
- The phage injects its DNA into the bacterial cytoplasm - a protein tube penetrates through the cell wall injecting DNA
- The capsid remains outside the cell
3. Integration and Lysogeny
- Instead of replicating immediately, the viral DNA integrates into the bacterial genome, forming a prophage
- The cell enters lysogeny, a dormant state where the prophage is replicated along with the bacterial chromosome during normal cell division
- During lysogeny, no synthesis, assembly, or release occurs
4. Induction
- In response to environmental stress (e.g., UV light, DNA damage), the prophage is excised from the bacterial genome
- This process is called induction and marks the transition from lysogeny to the lytic cycle
5. Synthesis (Biosynthesis)
- After induction, the phage begins to replicate its DNA and produce capsid proteins and enzymes using host cell machinery
6. Assembly
- New viral components are assembled into complete phage particles
7. Release
- The host cell is lysed using phage-encoded lysozyme, which breaks down the bacterial peptidoglycan wall, releasing new phages
Describe Bacteriophage
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Definition: Viruses that infect bacteria
- Highly species- and strain-specific
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Structure:
- Made of a nucleocapsid: protein capsid + nucleic acid genome
- Most lack an envelope
- Some (e.g., T4 phage) have a complex structure: capsid head, sheath, tail fibers
- Just protein and nucleic acid
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Infection Evidence:
- Plaques on agar plates represent zones where bacteria were lysed by phage infection
Types of Bacteriophage (based on replication strategy):
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Lytic Phages:
- Use only the lytic replication cycle
- Always cause lysis (destruction) of the host cell
- Replication steps: attachment, entry, biosynthesis, assembly, release
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Lysogenic Phages:
- Can use lysogenic or lytic replication
- In lysogeny, phage DNA integrates into the host chromosome as a prophage
- Host survives and divides normally with prophage DNA
- Can later be induced to enter the lytic cycle
Key Notes:
- Bacteriophage therapy is limited by viral specificity
- Not all bacteriophages have complex tails—structure may vary
Describe Lysogenic Conversion, list examples
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Lysogenic Conversion:
- Occurs when a lysogenic phage integrates into a bacterial genome and alters the host cell’s genotype and phenotype
- The prophage DNA carries genes that can encode new traits, such as toxins
- These changes can turn non-pathogenic bacteria into dangerous pathogens
- Only occurs with lysogenic phages (phages that enter the lysogenic cycle and form a prophage)
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Mechanism:
- The phage genome becomes part of the bacterial chromosome
- Prophage genes are expressed, giving the bacterium new capabilities
- These changes are heritable as long as the prophage remains in the genome
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Examples of Prophage-Encoded Toxins:
- Clostridium botulinum → botulinum toxin
- Corynebacterium diphtheriae → diphtheria toxin
- Escherichia coli O157:H7 → Shiga-like toxin
- Staphylococcus aureus → exfoliatin
- Streptococcus pyogenes → pyrogenic toxin
- Vibrio cholerae → cholera toxin
Describe Animal Virus Attachement
Non-enveloped virus
- Capsid protein chemistry compatible with host OR
- Glycoprotein spike chemistry compatible with host
Enveloped virus
- glycoprotein spike chemistry compatible with host (all enveloped viruses have a glycoprotein)
1. Influenza: Hemagglutinin targets sialic acid
2. HIV: GP12 targets CD4/CCR5 or CD4/CXCR4
3. COVID: S ‘spike’ protein targets ACE2
Describe Animal Virus Entry
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Non-Enveloped Viruses:
- Endocytosis: Virus is engulfed into an endosome; capsid then escapes or disassembles to release the genome
- Direct Injection: Some non-enveloped viruses inject their genome directly into the cytoplasm through the host membrane (similar to bacteriophage mechanism)
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Enveloped Viruses:
- Endocytosis: Virus is taken into an endosome; acidification triggers fusion of viral envelope with endosomal membrane, releasing the nucleocapsid
- Membrane Fusion: Viral envelope fuses directly with the plasma membrane, releasing the nucleocapsid into the cytoplasm (e.g., HIV)
Describe viral replication locations
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DNA Viruses:
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Location:
- Viral DNA is synthesized in the nucleus using host DNA-dependent DNA polymerases
- Viral proteins are synthesized in the cytoplasm using host ribosomes (either free-floating or attached to RER)
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Key Enzymes:
- Uses host enzymes for genome replication and transcription
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Location:
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RNA Viruses:
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Location:
- Viral RNA is typically synthesized in the cytoplasm, though some replicate in the nucleus
- Proteins are always synthesized in the cytoplasm using host ribosomes
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Key Enzymes:
- Require RNA-dependent RNA polymerase (either carried in or encoded by the virus)
- Eukaryotic host cells do not have enzymes that can make RNA from RNA, so this enzyme is essential for RNA viruses
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Location:
