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Questions and Answers
What is a characteristic of mature virion release in non-enveloped viruses?
What is a characteristic of mature virion release in non-enveloped viruses?
Which statement accurately describes chronic infections?
Which statement accurately describes chronic infections?
How do enveloped viruses differ in their release mechanism compared to non-enveloped viruses?
How do enveloped viruses differ in their release mechanism compared to non-enveloped viruses?
What defines a persistent viral infection?
What defines a persistent viral infection?
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Which of the following viral infection types can reactivate after a latent period?
Which of the following viral infection types can reactivate after a latent period?
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What is a defining characteristic of cellular life as described?
What is a defining characteristic of cellular life as described?
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How do viruses differ in their origin compared to cellular life?
How do viruses differ in their origin compared to cellular life?
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Which of the following states is true of the virion stage of a virus?
Which of the following states is true of the virion stage of a virus?
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What is a common method of viral transmission represented in the informal taxonomy?
What is a common method of viral transmission represented in the informal taxonomy?
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What limits the amount of information in a viral genome?
What limits the amount of information in a viral genome?
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In the infected cell stage, what typically happens to a virus?
In the infected cell stage, what typically happens to a virus?
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Which of the following is NOT true of viruses according to their characteristics?
Which of the following is NOT true of viruses according to their characteristics?
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What is a NOT a characteristic of the proteins used by viruses?
What is a NOT a characteristic of the proteins used by viruses?
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What is the primary role of the protein capsid in viruses?
What is the primary role of the protein capsid in viruses?
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Which of the following shapes is NOT one of the three basic shapes of viruses?
Which of the following shapes is NOT one of the three basic shapes of viruses?
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What is a virion composed of?
What is a virion composed of?
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Enveloped viruses acquire their lipid bilayer from which source?
Enveloped viruses acquire their lipid bilayer from which source?
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Which type of bacteriophage relationship involves always rupturing the host cell?
Which type of bacteriophage relationship involves always rupturing the host cell?
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What makes enveloped viruses more susceptible to disinfectants like alcohol?
What makes enveloped viruses more susceptible to disinfectants like alcohol?
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What are capsomeres?
What are capsomeres?
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Which of the following best describes temperate phages?
Which of the following best describes temperate phages?
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What is the primary characteristic of a lytic phage infection?
What is the primary characteristic of a lytic phage infection?
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What role does phage lysozyme play in the lytic infection process?
What role does phage lysozyme play in the lytic infection process?
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During which stage of a lytic phage infection are new viral components assembled?
During which stage of a lytic phage infection are new viral components assembled?
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What happens to the host cell at the release stage of the lytic infection?
What happens to the host cell at the release stage of the lytic infection?
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What type of DNA does the T4 phage have?
What type of DNA does the T4 phage have?
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What happens to the host cell's genome during the synthesis stage of lytic infection?
What happens to the host cell's genome during the synthesis stage of lytic infection?
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What distinguishes a temperate phage from a lytic phage?
What distinguishes a temperate phage from a lytic phage?
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Which condition does NOT facilitate the infection process in lytic phage such as T4?
Which condition does NOT facilitate the infection process in lytic phage such as T4?
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What is a significant consequence of the lack of proofreading during RNA virus replication?
What is a significant consequence of the lack of proofreading during RNA virus replication?
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What does antigenic drift in viral replication refer to?
What does antigenic drift in viral replication refer to?
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How can a single cell infected by multiple influenza viruses affect the resulting virus particles?
How can a single cell infected by multiple influenza viruses affect the resulting virus particles?
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What unique enzyme does the retrovirus utilize to convert its RNA genome into DNA?
What unique enzyme does the retrovirus utilize to convert its RNA genome into DNA?
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What happens to the DNA produced by reverse transcriptase in retroviruses?
What happens to the DNA produced by reverse transcriptase in retroviruses?
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How do capsomeres contribute to the assembly of viruses?
How do capsomeres contribute to the assembly of viruses?
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What is the primary consequence of antigenic shifts in influenza viruses?
What is the primary consequence of antigenic shifts in influenza viruses?
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What characteristic of the viral genome allows some viruses to use it as a scaffold during assembly?
What characteristic of the viral genome allows some viruses to use it as a scaffold during assembly?
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What is the result of the host polymerases completing the second strand of DNA?
What is the result of the host polymerases completing the second strand of DNA?
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What role does the replicative form (RF) of DNA play in phage replication?
What role does the replicative form (RF) of DNA play in phage replication?
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How are capsomeres involved in the assembly of M13 phage?
How are capsomeres involved in the assembly of M13 phage?
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What is one application of the SEA-PHAGES program described in the content?
What is one application of the SEA-PHAGES program described in the content?
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What do virus attachment proteins bind to on the host cell surface?
What do virus attachment proteins bind to on the host cell surface?
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Why do particular viruses only attach to specific receptors?
Why do particular viruses only attach to specific receptors?
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What common feature do many animal viruses share concerning their infection process?
What common feature do many animal viruses share concerning their infection process?
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How does the structure of animal cells complicate viral infection compared to bacterial cells?
How does the structure of animal cells complicate viral infection compared to bacterial cells?
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Study Notes
Origin Story - Cells
- Cellular life arose once.
- Evolution was linear, with branches.
- Cellular life forms share a common heritage, allowing for a timeline to be mapped.
Origin Story - Viruses
- Viruses arose multiple times, from diverse cell types.
- They use various strategies and components.
- Viruses are frequently restricted to a limited range of host cells.
- Viruses use nucleic acids (RNA or DNA) to store information.
- Viruses use the same genetic codes as living cells and evolve through genetic mutations.
- The size of their genomes influences the amount of information they can store.
- All viruses use proteins but the specific proteins and how they are used vary widely.
- Virus classification has a naming convention, but it is less useful compared to the system for cellular life.
Informal Taxonomy
- Groups of unrelated viruses share similar routes of infection.
- Enteric viruses use the oral-fecal route.
- Respiratory viruses spread via inhalation.
- Arboviruses are spread by vectors.
- Sexually transmitted infections (STI/STD)
- Zoonotic viruses spread from animals.
Two General Stages of Viruses
- Virion: Consists of protein and nucleic acid.
- It is the infective stage and primarily protein based.
- Some virions have a membrane envelope.
- Viruses do not have metabolic functions and can persist for many years.
- Infected cell: Viruses primarily hijack cell systems and their nucleic acids enter the cell. Replications require a host cell.
Structure - Protein Capsid
- Viruses have a limited number of proteins for their small size.
- Capsid structures are constructed using "lego-like" proteins.
Three Basic Shapes of Viruses
- Icosahedral (20 flat triangles)
- Helical
- Complex (bacteriophages, often ancillary proteins)
Virion Structure Example 1
- Virion is a viral particle, a nucleic acid surrounded by a protein coat (capsid).
- The capsid is made of identical subunits called capsomeres.
- The capsid and nucleic acids together make a nucleocapsid.
- Some viruses are surrounded by a lipid bilayer (envelope) from the host cell.
First Focus: Bacteriophages
- Lytic and Temperate phages, which interact with organisms in different ways.
- Lytic phages always result in cell destruction.
- Temperate phages can transition between lytic and lysogenic states.
- Filamentous phages produce virus continuously without killing the cell.
Diagram of Three Types of Bacteriophage Infection
- Lytic infections eventually kill host cell.
- Lysogenic infections insert genetic material into the host genome.
- Productive Infections have continuous production of virus, without destroying the cell.
Lytic Phage Infection (Example T4)
- T4 is a complex phage with double-stranded DNA.
- Attachment is by tail fibers.
- Genome entry involves a small hole created in the cell wall by a phage lysozyme.
- Only the DNA travels into the cell.
Lytic Phage Infection (cont.)
- Synthesis stage: Early genes are transcribed and translated following entry of the genome.
- Host cell resources are used to create new viral components.
- The host DNA is destroyed by phage enzymes.
- New DNA polymerases are created for viral replication.
Lytic Phage Infection (cont.)
- Assembly stage: Viral components self-assemble into new phages.
- Hundreds of infectious virions are assembled inside the host cell.
Lytic Phage Infection (cont.)
- Release stage: Lysis (destruction) of the host cell releases viral particles.
- A phage enzyme degrades the cell wall.
Temperate Phage (Example Lambda Phage)
- Double-stranded DNA that infects E. coli
- Attachment and genome entry are similar to T4.
- After entry, the linear chromosome circularizes.
- The pathway can transition to lytic infections.
Lysogenic Infection
- The phage genome integrates into the host chromosome.
- The phage DNA becomes a prophage.
- Lysogen maintains the prophage, and multiplies with the host cell.
- Certain stressors cause viruses to enter a lytic infection cycle.
Lysogenic Infection (cont.)
- Prophage can remain latent for multiple host cell division cycles.
- Phage can be induced to exit the lysogenic cycle under stressful conditions, such as exposure to UV light.
- Lysogeny provides immunity to superinfection, which is a crucial feature for virus persistence.
Filamentous (Helical) Phage - Example M13.
- Single-stranded (+) DNA genome.
- Filamentous phages infect the F-pilus.
- DNA is enclosed within an icosahedral head.
Assembly - M13
- Capsomeres insert into cytoplasmic membrane.
- Phage proteins span the cell wall and the outer membrane.
- DNA is continually released while the cell stays alive.
SEA-PHAGES
- Freshman students' research isolating and sequencing bacteriophages.
- Phage research is growing as a model for phage therapy.
Animal Viruses
- Viruses that infect animals, including humans.
- Infection and enzymes are a critical detail for understanding transmission and developing antiviral agents.
- Cellular organization is complex in animals compared to bacteria.
Animal Viruses - Attachment (Adsorption)
- Viruses attach to specific receptors on the host cell surface, often glycoproteins on the cytoplasmic membrane.
- More than one receptor is typically needed for successful infection.
- Viral attachment is critical. The receptor specificity limits the range of cells that can be infected.
Enveloped Viruses (Option 1 - Membrane Fusion)
- Viral infection begins with the virus envelope fusing with the host cell membrane.
- The nucleocapsid is released inside.
- Viruses then use host cell machinery to replicate.
Option 2 - Enveloped or Non-enveloped.
- Enveloped viruses fuse with the host cell membrane.
- Non-enveloped viruses are engulfed by the host cell or enter using other means.
Next Steps
- Viruses replicate their genomes and produce viral proteins based on the type of nucleic acid present.
A Moment of Explanation on (+) and (-)
- Single-stranded RNAs have either (+) or (-) designation.
- (+) ssRNA is read directly by ribosomes to make protein.
- (-) ssRNA requires synthesis into (+) form.
Animal Virus Synthesis
- Expression of viral genes in order to make structural and catalytic viral genes.
dsDNA
- dsDNA is replicated to form viral genome and transcribed to produce mRNA in order to allow translation to form viral proteins.
- Complementary strand is transcribed.
ssDNA, (+) and (-)
- Replication process for ssDNA viruses, which includes the production of complementary strands.
Replication of RNA Viruses
- The replication of RNA viruses occurs in the cell cytoplasm rather than nucleus.
- RNA viruses encode replication enzymes.
- ssRNA(+) serve as mRNA, unlike ssRNA(-) which require transcription to form mRNA for translation processes.
Replication of ssRNA(-) Viruses
- Replication enzyme is needed for RNA genome replication.
- (+) strand is synthesised and used to make viral mRNA.
Replication of dsRNA Viruses.
- Replication enzyme is required to facilitate dsRNA replication.
- (+) strand copies are produced for viral mRNA synthesis.
RNA Viruses Encourage Mutations
- Replicases lack proofreading, which leads to more mutations for RNA viruses.
- Frequent mutations make previous immunity less effective.
Retroviruses... Special ssRNA(+) Viruses
- Contain RNA-dependent DNA polymerases (reverse transcriptase).
- RNA is transcribed to DNA and integrated into the host's DNA.
Assembly of Virons
- Viruses assemble their components into new virions.
- Some use their genome as a scaffold for assembly
- Others assemble a procapsid which encapsulates their components.
Release of Virions – Non-enveloped Viruses
- Mature virions assemble inside the host cell.
- Cell lysis releases virions.
Release of Virions – Enveloped Viruses
- Viral proteins penetrate the cytoplasmic membrane.
- Virus capsids become surrounded by host cellular membranes.
- New enveloped viruses extrude through the membrane.
Kinds of Viral Infections in Humans
- Acute infections: Rapid onset, short duration.
- Persistent infections: Last for a long time.
Persistent Infections
- Chronic infections: continuous low-level production of viruses.
- Latent infections: viral genomes persist in a latent state.
Latent Infections
- Virus integrates into host chromosomes or replicates separately, often in a non-productive way.
- Infections can reactivate and cause new rounds of productive infection.
- Examples are HSV and VZV
Viroids
- Small circular RNA molecules that cause disease in plants.
- Replicate by interacting with host cell mechanisms.
Prions
- Infectious proteins that cause disease in humans and animals.
- Linked to slow, fatal diseases.
- Composed entirely of proteins, without nucleic acids.
In Disease, Prion Proteins Accumulate in Neural Tissue
- Neurons die due to prion accumulation.
- Tissues develop holes due to prion aggregation.
- Brain function deteriorates..
- Characteristic appearance in diseased brains.
Mode of Action - Folding Rearrangement
- Normal cellular proteins (PrP^c) can be mutated to misfolded versions leading to prion aggregation.
- Infectious prion proteins (PrP^Sc) cause misfolding of normal proteins.
- Misfolded proteins aggregate, damaging tissues.
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Description
Explore the fascinating stories behind the origins of cellular life and viruses. This quiz delves into evolutionary paths, the common heritage of cellular organisms, and the diverse strategies employed by viruses. Understand how viruses categorize themselves and the implications of their genetic makeup.
