Viruses: Nature and Properties
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Questions and Answers

What is the role of matrix proteins in the viral lifecycle?

  • They facilitate viral entry into the host cell.
  • They encode the viral genetic material.
  • They protect the viral capsid from degradation.
  • They form links between the nucleocapsids and the envelope. (correct)

    • Which statement accurately describes complex viruses?

  • They have uniform capsomeres and a simple structure.
  • They only consist of a nucleocapsid and nucleic acids.
  • They contain various capsomeres and a membrane derived from the host. (correct)
  • They lack an envelope and rely solely on the capsid.

    • What distinguishes naked viruses from other types of viruses?

  • They solely consist of a nucleocapsid. (correct)
  • They have no capsid surrounding the nucleic acid.
  • They contain an envelope made of lipids.
  • They require host cell machinery for replication.

    • What is a characteristic feature of viral capsids?

    <p>They provide the shape and structure to the viral particle.</p> Signup and view all the answers

    What is one feature of the viral envelope?

    <p>It is derived from the host cell membrane.</p> Signup and view all the answers

    What is the main function of spike proteins on viruses?

    <p>They determine how the virus interacts with host cell receptors.</p> Signup and view all the answers

    Which type of viral capsid is characterized by a three-dimensional, 20-sided structure?

    <p>Icosahedral capsid</p> Signup and view all the answers

    How do viruses typically reproduce?

    <p>By hijacking the reproductive machinery of living host cells.</p> Signup and view all the answers

    What is the significance of capsomeres in viral structure?

    <p>They are the building blocks that form the viral capsid.</p> Signup and view all the answers

    What is the average number of viruses produced by a single infected cell?

    <p>50 million</p> Signup and view all the answers

    What is the size range of viruses?

    <p>15nm - 800nm</p> Signup and view all the answers

    What distinguishes a virion from a virulent infectious agent?

    <p>Virions consist of a capsid and can establish infection.</p> Signup and view all the answers

    What is an important characteristic of viruses related to their replication?

    <p>Viruses require a specific host cell for multiplication.</p> Signup and view all the answers

    How do viruses adhere to their host cells?

    <p>Via a lock-and-key recognition mechanism on their surface.</p> Signup and view all the answers

    Which of the following is NOT a known property of viruses?

    <p>Viruses can reproduce independently of a host.</p> Signup and view all the answers

    Which statement best describes the capability of viruses regarding metabolism?

    <p>Viruses completely lack enzymes for most metabolic processes.</p> Signup and view all the answers

    What role do vaccines play concerning viruses?

    <p>Vaccines can protect against viral infections and related cancers.</p> Signup and view all the answers

    Which organisms can be infected by viruses?

    <p>All types of cells, including bacteria, algae, fungi, protozoa, plants, and animals.</p> Signup and view all the answers

    What is the primary role of spikes in viral envelopes?

    <p>To facilitate attachment to the host cell</p> Signup and view all the answers

    Which characteristic distinguishes complex capsids found in bacteriophages from simpler viral capsids?

    <p>They possess multiple protein types and irregular shapes</p> Signup and view all the answers

    What are viral capsids primarily composed of?

    <p>Proteins</p> Signup and view all the answers

    How do viral envelopes acquire their membrane composition?

    <p>They are derived from the host cell's membrane system</p> Signup and view all the answers

    What feature of glycoproteins is specifically noted in the description of viral spikes?

    <p>They have a glucosamine coating</p> Signup and view all the answers

    What is the composition of a viral genome?

    <p>Either DNA or RNA but not both</p> Signup and view all the answers

    Which of the following enzymes is responsible for synthesizing DNA from RNA?

    <p>Reverse transcriptase</p> Signup and view all the answers

    What distinguishes positive-sense RNA from negative-sense RNA?

    <p>Positive-sense RNA can be directly translated into proteins.</p> Signup and view all the answers

    How many genes are typically present in a viral genome compared to a human cell?

    <p>Fewer than human cells</p> Signup and view all the answers

    In which cellular location can X DNA viruses be replicated?

    <p>In the nucleus</p> Signup and view all the answers

    Which phase of the viral life cycle is characterized by the virus attaching to the cell membrane?

    <p>Adsorption</p> Signup and view all the answers

    What best describes the term 'host range' in relation to viruses?

    <p>The spectrum of host species a virus can infect</p> Signup and view all the answers

    Which virus is an example of a broad host range virus?

    <p>Rabies virus</p> Signup and view all the answers

    In which phase does a virus replicate its genetic material and proteins within the host cell?

    <p>Synthesis</p> Signup and view all the answers

    Which of the following accurately describes a 'restricted host range'?

    <p>A virus that infects only specific cell types within one species</p> Signup and view all the answers

    What process allows the entire virus to be engulfed by the host cell?

    <p>Endocytosis</p> Signup and view all the answers

    Which statement correctly describes the role of reverse transcriptase?

    <p>It allows RNA to be made into DNA.</p> Signup and view all the answers

    Where do RNA viruses primarily replicate within the host cell?

    <p>Cytoplasm</p> Signup and view all the answers

    What is the mechanism through which mature viruses are released from host cells?

    <p>Viral budding or exocytosis</p> Signup and view all the answers

    In which scenarios can viruses remain dormant in a host cell?

    <p>Due to nutrition, carcinogens, or stress</p> Signup and view all the answers

    What is the significance of inclusion bodies in the context of cytopathic effects?

    <p>They consist of concentrated viruses or organelles and reveal viral infection.</p> Signup and view all the answers

    How do oncogenic viruses contribute to cancer development?

    <p>They carry genes or proteins that disrupt normal cell growth regulation.</p> Signup and view all the answers

    What distinguishes a provirus from active viral infections?

    <p>Proviruses remain dormant within the host DNA without causing acute infection.</p> Signup and view all the answers

    What phenomenon describes the fusion of multiple host cells into a single large cell?

    <p>Syncytia formation.</p> Signup and view all the answers

    What role do oncogenes play in cell division?

    <p>They can accelerate the rate of cell division by increasing protein quantity.</p> Signup and view all the answers

    What distinguishes the lytic phase of the bacteriophage life cycle from the lysogenic cycle?

    <p>The lytic phase results in the death of the host cell.</p> Signup and view all the answers

    Which T-even bacteriophage is primarily associated with E. coli infection?

    <p>T-2</p> Signup and view all the answers

    Which statement is true regarding the impact of bacteriophages on bacteria they infect?

    <p>They can make bacteria more pathogenic to humans.</p> Signup and view all the answers

    What was a significant milestone in the discovery of bacteriophages?

    <p>The identification of the first bacteriophage in 1915.</p> Signup and view all the answers

    What characteristic is common to both the lytic and lysogenic cycles of bacteriophages?

    <p>Both cycles can impact the pathogenicity of the bacteria.</p> Signup and view all the answers

    What outcome is commonly observed as a result of viral growth in cell culture?

    <p>Degeneration and lysis of infected cells</p> Signup and view all the answers

    What role does lysogenic conversion play in human diseases?

    <p>It facilitates the gain of new pathogenic traits from temperate phages.</p> Signup and view all the answers

    Which statement best describes the nature of satellite viruses like adeno-associated virus (AAV)?

    <p>They can infect cells that have disrupted DNA, not solely dependent on adenovirus.</p> Signup and view all the answers

    What is a defining characteristic of prion infections?

    <p>They consist solely of protein and replicate without nucleic acids.</p> Signup and view all the answers

    What do missing proteins from infectious agents indicate about their dependence on host cells?

    <p>They must rely on already infected cells for replication.</p> Signup and view all the answers

    Study Notes

    Filterable Virus

    • Virus smaller than bacteria
    • Found by passing infectious fluids through filters designed to trap bacteria
    • Filtered fluid remained infectious

    Size of Virus

    • 15nm - 800nm

    Viruses on the Biological Spectrum

    • Infect every type of cell including bacteria, algae, fungi, protozoa, plants and animals

    Early Discoveries

    • Louis Pasteur developed a rabies vaccine
    • Pasteur coined the term "virus" (Latin for poison)

    Virion vs. Viroid

    • Virion: A fully formed virus able to establish infection in a host. Consists of an infection agent and all plant pathogens
    • Viroid: lacks a capsid and consists of a close circular RNA molecule

    Properties of Viruses

    • Infectious particles
    • Active or inactive
    • Obligate intracellular parasites: Cannot multiply unless they invade a specific host cell; must instruct the host cell's genetics and metabolic machinery to make and release new viruses

    Viruses and Recognition

    • Viruses follow a lock-and-key mechanism for entry in host cells.
    • Some viruses have broader recognition abilities, infecting more than one type of cell.

    Unique Properties of Viruses

    • Are not cells. They are obligate intracellular parasites.
    • Do not independently fulfill the characteristics of life.
    • They are inactive macro-molecules outside their host.
    • Cannot replicate biological function independently of a host organism.
    • Can have either DNA or RNA, ranging from 20 to 450 nm
    • May have a double-stranded DNA, a single-stranded DNA, a single-stranded RNA or a double-stranded RNA.
    • Contain molecules on the surface that determine specificity for a host.
    • Carry machinery to make essential proteins.
    • Lack enzymes for most metabolic processes.

    Viral Components

    • Viruses are not cells
    • Lack protein-synthesizing machinery
    • Contain the parts needed to invade and control a host cell
      • External coating
      • Core containing nucleic acids

    Capsid

    • Shell that surrounds the nucleic acid (protein)
    • Nucleocapsid: capsid and nucleic acid together

    Envelope

    • Usually a modified piece of the host cell membrane
    • Matrix proteins: proteins that form a layer on the inside of the viral envelope. They play important roles in virus assembly, forming links or bridges between nucleocapsids/cores and the envelope

    Complex Viruses

    • Has different capsomeres
    • The envelope will have phospholipids like all membranes

    The Capsid

    • Made of protein
    • Naked viruses will only have a nucleocapsid
    • Complex viruses will have a nucleocapsid covered by a membrane

    Naked Viruses

    • Consists only of a nucleocapsid
    • Made of proteins with similar capsomeres

    Spikes

    • Found on both naked and enveloped viruses
    • Project from either the nucleocapsid or envelope
    • Allow viruses to dock with their host cells

    Spike proteins

    • Give the viruses its identity
    • 20 spike proteins react to different cell receptors
    • Tell the host cell which spike proteins to produce

    The Viral Capsid

    • The protective outer shell. It is made up of capsomeres, identical protein subunits, that spontaneously self-assemble to form the capsid
    • Helical capsid: Rod-shaped capsomeres that form a continuous helix around the nucleic acid
    • Icosahedral capsid: Three-dimensional, 20-sided figure with 12 evenly spaced corners

    Viruses and Reproduction

    • Viruses reproduce through an alive host cell
    • The host cell provides the virus with ATP, chemicals, and allows the virus to produce its own protein (RNA or DNA)

    Viral Production in a Cell

    • The average cell produces ~50 million viruses
    • Viral infections are typically short-term and can resolve in, if the body can handle it, 3-4 days

    The Viral Capsid

    • The viral capsid is a protein shell that encloses the viral genetic material
    • Complex Capsids: Found in bacteriophages (viruses that infect bacteria). They have multiple protein types and take shapes that are not symmetrical.

    The Viral Envelope

    • Composed of the membrane system of the host cell (cell membrane or nuclear membrane)
    • Regular membrane proteins are replaced with viral proteins
    • Spikes: Protruding glycoproteins essential for attachment to the host cell
    • Glycoproteins: Proteins with a glucosamine coating

    Nucleic Acids

    • Genome: the full complement of DNA and RNA carried by a cell
    • Viruses contain either DNA or RNA, but not both
    • The number of viral genes is small compared to that of a cell
    • They possess only the genes necessary to invade host cells and redirect their activity. 31 viral genome: 4 or 5 genes; humans: 27,000; prokaryotic cells: 4,000-5,000
    • The viral genome tells the host cell what it needs to produce.

    Nucleic Acids - Positive-sense RNA

    • Single-stranded RNA genomes ready for immediate translation into proteins.

    Nucleic Acids - Negative-sense RNA

    • RNA genomes that need to be converted into the proper form to be made into proteins.

    Eukaryotic cells

    • Do not have replicative RNA, so the virus would need to bring the enzyme itself.

    Other Substances in the Virus Particle

    • Enzymes for specific operations within the host cell:
      • Polymerases: synthesize DNA and RNA
      • Replicases: copy RNA
      • Reverse transcriptase: synthesizes DNA from RNA

    X DNA virus replication

    • Can only be replicated in the nucleus.
    • The only time DNA is in the cytoplasm is during the cell cycle

    Multiplication Cycles in Animal Viruses

    • General phases in the life cycle of animal viruses:
      • Adsorption
      • Penetration
      • Uncoating
      • Synthesis
      • Assembly
      • Release from the host cell

    Adsorption and Host Range

    • Invasion begins when the virus encounters a susceptible host and adsorbs specifically to receptor sites on the cell membrane.
    • Adsorb: to attach (similar to a virus)
    • Absorb: to soak in (similar to a paper towel)

    Host Range

    • A virus can invade its host cell only through making an exact fit with a specific host molecule.
    • Restricted host range: Hepatitis B only infects liver cells of humans.
    • Intermediate host range: Poliovirus infects intestinal and nerve cells of primates.
    • Broad host range: Rabies virus infects various cells of all mammals.

    Virus Entry and Replication

    • Penetration and Uncoating of Animal Viruses:
      • Endocytosis: The entire virus is engulfed by the cell and enclosed in a vacuole or vesicle.
      • Uncoating: Enzymes in the vacuole dissolve the envelope and capsid, releasing the virus into the cytoplasm.
      • Reverse transcriptase allows RNA to be made into DNA.

    Entry of a Virus into Host Cell

    • Viruses damage the host cell by taking parts of its membrane.

    Replication and Protein Production

    • Viral nucleic acid takes control over the host's synthetic and metabolic machinery
    • Mechanism depends on whether the virus is DNA or RNA
    • RNA viruses replicate in the cytoplasm
    • DNA viruses replicate in the nucleus.

    Synthesis of DNA Viruses

    • The virus must be converted to DNA to replicate
    • Proteins are formed by the cytoplasm and rRNA is made in the nucleus
    • Viruses can be dormant in the host cell's cytoplasm, only in the DNA, where they insert their genome into the host cell's DNA. This can be due to nutrition, carcinogens, or stress.

    Release of Mature Viruses

    • Viral budding or exocytosis: Nucleocapsid binds to the membrane and viruses are shed gradually without destruction of the cell.

    Damage to the Host Cell and Persistent Infections

    Cytopathic Effects (CPEs)

    • Virus-induced damage to the cell that alters its microscopic appearance
    • Cells can become disoriented, undergo major changes in shape or size, or develop intracellular damage

    Damage to the Host Cell (cont'd)

    • Cytopathic effects (cont'd):
      • Inclusion bodies: Compacted masses of viruses or damaged cell organelles in the nucleus or cytoplasm
      • Syncytia (singular, syncytium): Fusion of multiple host cells into single large cells containing multiple nuclei

    Persistent Infections

    • A carrier relationship that develops in some cells
    • The cell harbors the virus and is not immediately lysed

    Provirus

    • Viral DNA incorporated into the DNA of the host.
    • Chronic latent state: periodic activation after a period of viral inactivity

    Viruses and Cancer

    • Oncogenic viruses: Experts estimate that up to 20% of human cancers are caused by viruses.
    • Transformation:
      • Virus carries genes that directly cause cancer
      • Virus produces proteins that induce a loss of growth regulation in the cell
      • Will change identity in the cell

    Viral Movement Between Cells

    • In eukaryotic cells, a virus can move from one cell to another and move DNA between them, called transformation. This is called transduction in prokaryotic cells.

    Oncogenes

    • Produce the same protein, but increase the number causing the cell to divide rapidly.

    Viruses that Infect Bacteria

    • Bacteriophage (bacteria eater):
      • Discovered in 1915
      • Parasitize every known bacterial species
      • Often make the bacteria they infect more pathogenic for humans

    Bacteriophage Life Cycle

    • Bacteriophage (T-even):
      • "T-even" bacteriophage infect E.coli
      • T-2 and T-4
      • Most widely studied bacteriophage
      • Go through similar stages as animal viruses

    Bacteriophage Life Cycle (Lytic Phase)

    • Lytic phase or lytic cycle: life cycle of bacteriophage that ends in destruction of the bacterial cell.

    Bacteriophage Life Cycle (Lysogenic Cycle)

    • Lysogenic cycle: bacteriophage becomes incorporated into the host cell DNA.

    The Danger of Lysogeny in Human Diseases

    • Lysogenic conversion:
      • The acquisition of a new trait from a temperate phage
      • Responsible for the diphtheria toxin, cholera toxin, and botulism toxin

    Using Cell Culture Techniques

    • Detecting viral growth in culture:
      • Degeneration and lysis of infected cells.
      • Plaques:
      • Clear, well-defined patches in the cell sheet.
      • Develops when viruses released from an infected cell radiate out to surrounding cells.
      • Infection spreads gradually and symmetrically from the original point of infection

    Satellite viruses

    • Dependent on other viruses for replication
      • Adeno-associated virus (AAV)
        • Originally thought that it could only infect cells infected with the adenovirus.
        • Now found to infect cells infected with other viruses or had their DNA disrupted through other means.

    Other Noncellular Infectious Agents

    • Prion infection:
      • Exact mode of infection is unknown.
      • Protein composition.
      • Questions about how prions replicate given that they have no nucleic acid.

    Other

    • Missing proteins: Essentially are missing their proteins; depend upon cells that have already been infected.

    Additional Information

    • Do not have DNA or RNA, but are still able to replicate.
    • All affect brain tissue.

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    Description

    Explore the fascinating world of viruses, from their size and structure to their role as obligate intracellular parasites. This quiz covers early discoveries in virology, including key terminology and the differences between virions and viroids. Test your knowledge on how viruses interact with various types of cells.

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