Viruses: Structure and Properties

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Questions and Answers

What type of viral genome is directly ready for translation into proteins?

Double-stranded RNA

Negative-sense RNA

Single-stranded DNA

Positive-sense RNA (correct)

Which characteristic distinguishes the genomes of viruses from those of prokaryotic cells?

Prokaryotic cells exclusively have RNA genomes

Viruses have a larger number of genes

Viruses can contain either DNA or RNA, not both (correct)

Prokaryotic cells lack a genome

What enzyme is responsible for synthesizing DNA from an RNA template?

Helicase

Polymerase

Reverse transcriptase (correct)

Replicase

Where do DNA viruses replicate within a host cell?

Cell nucleus Signup and view all the answers

What size range do viruses typically fall within?

15nm - 800nm Signup and view all the answers

What is the primary function of a viral genome in a host cell?

To instruct the host on what to produce Signup and view all the answers

Which of the following is a characteristic of viruses?

They are obligate intracellular parasites. Signup and view all the answers

How do viruses typically infect host cells?

By using a mechanism similar to lock-and-key recognition. Signup and view all the answers

What is the first step a virus takes to enter a host cell?

Endocytosis Signup and view all the answers

Where do RNA viruses primarily replicate within the host cell?

Cytoplasm Signup and view all the answers

What is meant by the term 'cell-free filtered fluid' in relation to viral research?

Fluid that contains infectious agents smaller than bacteria. Signup and view all the answers

What distinguishes viruses from living organisms?

Viruses do not independently fulfill the characteristics of life. Signup and view all the answers

What process allows mature viruses to be released from a host cell without causing its destruction?

Viral budding or exocytosis Signup and view all the answers

In which cellular structure is rRNA produced for DNA viruses?

Nucleus Signup and view all the answers

What must happen to RNA viruses before they can replicate within a host cell?

They must convert their RNA into DNA Signup and view all the answers

What best describes naked viruses?

They consist only of a nucleocapsid. Signup and view all the answers

What is the main function of spike proteins in viruses?

To allow viruses to dock with their host cells. Signup and view all the answers

Which type of capsid is characterized as a three-dimensional, 20-sided figure?

Icosahedral capsid Signup and view all the answers

How do viruses reproduce?

By using an alive host cell. Signup and view all the answers

What is a typical outcome of a viral infection regarding duration?

Most infections resolve in 3-4 days. Signup and view all the answers

What primary function does the capsid serve in a virus?

Protecting and encasing the nucleic acids Signup and view all the answers

Which statement accurately describes the structure of naked viruses?

They consist only of a nucleocapsid. Signup and view all the answers

What is the role of matrix proteins in viral structure?

They help in viral assembly by linking nucleocapsids to the envelope. Signup and view all the answers

Which of the following statements is true about viral envelopes?

The envelope typically contains phospholipids similar to host cell membranes. Signup and view all the answers

What characterizes complex viruses?

They possess both nucleocapsid and an envelope. Signup and view all the answers

Which phase comes immediately after adsorption in the viral life cycle?

Penetration Signup and view all the answers

What characterizes a virus with a restricted host range?

It specifically infects cells in one particular organ or tissue. Signup and view all the answers

Which statement about the viral multiplication cycle is true?

Assembly involves the formation of new virions. Signup and view all the answers

In which phase does the virus release new virions from the host cell?

Release Signup and view all the answers

What does the term 'adsorb' refer to in the context of viral infection?

The attachment of the virus to specific receptors on the cell membrane. Signup and view all the answers

What is the most important aspect of constructing a meaningful stem in a multiple choice question?

It must present a definite problem that is meaningful by itself. Signup and view all the answers

When creating options for a multiple choice question, what is a key principle to follow?

Distractors should represent common misconceptions to challenge the student. Signup and view all the answers

What should be avoided when writing the options for a multiple choice question?

Including distractors that could also be seen as correct. Signup and view all the answers

What is the purpose of providing additional context in the 'info' section of a multiple choice question?

To clarify why the chosen answers are correct. Signup and view all the answers

Which of the following practices is essential for ensuring the clarity of options in multiple choice questions?

Ensuring options do not give away the correct answer through hints. Signup and view all the answers

What is lysogenic conversion responsible for producing?

Diphtheria toxin Signup and view all the answers

What do plaques in cell culture indicate?

Viral lysis of infected cells Signup and view all the answers

Which of the following describes satellite viruses?

Dependent on other viruses for replication Signup and view all the answers

What is a unique characteristic of prion infections?

The exact mode of infection is unknown Signup and view all the answers

What is true about the cells infected by satellite viruses?

They are missing proteins and depend on infected cells Signup and view all the answers

What are prions primarily known for causing?

Spongiform encephalopathies Signup and view all the answers

What is a notable characteristic of the brain tissue affected by prion diseases?

It appears spongy and devoid of structure Signup and view all the answers

What is a key aspect of how prions replicate?

They convert normal proteins into prion forms Signup and view all the answers

Which disease is not associated with prions in animals?

Avian flu Signup and view all the answers

What challenge does prion contamination present in clinical environments?

Prions can remain infectious despite being exposed to high heat Signup and view all the answers

What occurs during the lytic phase of a bacteriophage's life cycle?

The bacterial cell is destroyed to release new phages. Signup and view all the answers

Which of the following best describes the lysogenic cycle of a bacteriophage?

The phage DNA remains dormant within the bacterial genome. Signup and view all the answers

What is a notable characteristic of T-even bacteriophages?

They are widely studied and infect E. coli. Signup and view all the answers

What effect do bacteriophages generally have on the bacteria they infect?

They can make certain bacteria more pathogenic to humans. Signup and view all the answers

Which structural components are associated with bacteriophages?

Capsid and tail fibers Signup and view all the answers

Study Notes

Filterable Virus

Viruses are smaller than bacteria, as demonstrated by the fact that infectious fluids passed through filters designed to trap bacteria remained infectious.
Viruses range in size from 15nm to 800nm.
Viruses can cause cancer, but vaccines can immunize individuals against certain cancers.

Viruses on the Biological Spectrum

Viruses can infect every type of cell, including bacteria, algae, fungi, protozoa, plants, and animals.

Properties of Viruses

Viruses are infectious particles, not living organisms.
They can be active or inactive, but not alive or dead.
Viruses are obligate intracellular parasites, meaning they cannot multiply unless they invade a specific host cell.
Viruses adhere to a lock-and-key recognition model with their host cells, although some viruses can infect more than one type of cell.

Unique Properties of Viruses

Viruses are not cells.
They are obligate intracellular parasites.
They do not independently fulfill the characteristics of life.
They have unique structures and functions within a host cell.

Viral Components

Capsids: The external coating of a virus that surrounds the nucleic acids.
Nucleocapsid: Composed of the capsid and the nucleic acid.

Viral Size Range

Viruses come in a variety of sizes.

Viral Components

Envelope: A modified piece of the host cell membrane that can surround the nucleocapsid.
Matrix proteins: Found on the interior of the viral envelope, playing a crucial role in assembly.

The Capsid

The capsid is made of protein.
Naked viruses only have a nucleocapsid.
Complex viruses have a nucleocapsid covered by a membrane.

Envelope

Complex viruses contain various envelope components and capsomeres.
The envelope is composed of phospholipids, similar to other cell membranes.

Viral Components

Naked Viruses: Consist only of a nucleocapsid, composed of proteins with similar capsomeres.
Spikes: Project from the nucleocapsid or envelope of both naked and enveloped viruses, allowing them to dock with host cells. They are responsible for the virus's identity and determine which cell receptors the virus can interact with.

The Viral Capsid: The Protective Outer Shell

Capsomeres: Identical protein subunits that spontaneously self-assemble forming the capsid.
Helical Capsid: Rod-shaped capsomeres forming a continuous helix around the nucleic acid.
Icosahedral Capsid: Three-dimensional, 20-sided figure with 12 evenly spaced corners.

Virus Reproduction

Viruses reproduce only through a living host cell.
The host cell provides the virus with ATP, chemicals, and the means to produce its own protein, RNA, or DNA.

Viral Production

An average cell can produce 50 million viruses.
Viral infections can resolve in 3-4 days if the body is able to handle them.

Nucleic Acids: At the Core of a Virus

Genome: The complete set of DNA or RNA within a cell.
Viruses: Contain either DNA or RNA, but not both.
Viral Genes: Fewer than cellular genes, with only the genes necessary for invading and controlling the host cell.

Nucleic Acids

Positive-sense RNA: Single-stranded RNA genomes that are ready for direct translation into proteins.
Negative-sense RNA: RNA genomes that require conversion into a usable form for protein production.

Viral Genome

Human: Carries 4-5 genes.
Prokaryotic Cells: Contain 4,000-5,000 genes.
Viral Genome Function: The viral genome provides instructions for the host cell to produce viral components.

Synthesis of + Strand RNA Viruses

- Strand RNA viruses synthesize a complementary strand for protein synthesis.

Assembly of Animal Viruses

Assembly of animal viruses occurs within the host cell.

Eukaryotic Cells and RNA Viruses

Eukaryotic cells lack the enzymes necessary to replicate viral RNA, meaning the viruses must bring their own replication machinery into the cell.

Other Substances in the Virus Particle

Enzymes: Essential for specific viral processes within the host cell.
Polymerases: Synthesize DNA and RNA molecules.
Replicases: Copy RNA.
Reverse Transcriptase: Synthesizes DNA from RNA.

DNA Viruses and Cellular Replication

Replication Location: DNA viruses replicate only in the cell nucleus.
Cytoplasmic DNA: DNA is only present in the cytoplasm during certain cellular phases, such as the cell cycle.

Multiplication Cycles in Animal Viruses

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

Viral Multiplication

Viral entry into a cell involves attachment, penetration, viral replication, and the assembly of new virions.

Adsorption and Host Range

Invasion: Starts when a virus encounters a susceptible host cell and adsorbs to receptor sites on the cell membrane.
Adsorb: To attach (like a virus).
Absorb: To soak in (like a paper towel).

Host Range

Host range: A virus can invade a host cell only by making a perfect fit with a specific host molecule.
Restricted Host range: For example, hepatitis B infects only liver cells in humans.
Intermediate Host range: Poliovirus infects intestinal and nerve cells of primates.
Broad Host range: For example, rabies virus infects various cells of all mammals.

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.

Entry of a Virus into Host Cell

Viral entry into a host cell involves various stages.
Viruses can damage the host cell by taking parts of its membrane.

Replication and Protein Production

Viral nucleic acid takes control of the host's synthetic and metabolic machinery.
The mechanism varies depending on whether the virus is a DNA or RNA virus:
- RNA viruses: Replicate in the cytoplasm.
- DNA viruses: Replicate in the nucleus.

Synthesis of DNA Viruses

RNA viruses must be converted to DNA to replicate.
Proteins are formed in the cytoplasm, and rRNA is made in the nucleus.

Viruses can be Dormant

Viruses can remain dormant in the host cell's cytoplasm or DNA.
They can be reactivated by factors such as nutrition, carcinogens, or stress.

Release of Mature Viruses

Viral budding or exocytosis: The nucleocapsid binds to the membrane, and viruses are shed gradually without destroying the cell.

The Danger of Lysogeny in Human Diseases

Lysogenic Conversion: The acquisition of a new trait from a temperate phage, responsible for 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 that develop when viruses released from an infected cell infect surrounding cells.

Satellite Viruses

Dependent on other viruses for replication.
For example, adeno-associated virus (AAV) was initially thought to only infect cells infected with the adenovirus. It has since been found to infect cells infected with other viruses or have their DNA disrupted by other means.

Other Noncellular Infectious Agents

Prion infection: The precise mode of infection is unknown, although the agent is known to be composed of protein. There are questions regarding how prions replicate given that they have no nucleic acid.

Other Noncellular Infectious Agents: Spongiform Encephalopathies

Implicated in chronic, persistent diseases in humans and animals.
Brain tissue removed from affected animals resembles a sponge.

Prions

A common feature of spongiform encephalopathies.
Distinct protein fibrils deposited in brain tissue of affected animals.

Other Noncellular Infectious Agents: Creutzfeldt-Jakob Disease (CJD)

Affects the central nervous system of humans, causing gradual degeneration and death.
Transmissible by an unknown mechanism.
Several animals are victims of similar diseases:
- Scrapie: Sheep, mink, elk.
- Bovine Spongiform Encephalopathy: Cows.

Viruses and Human Health: Interferon (IFN)

A naturally occurring human cell product used with some success in preventing and treating viral infections.

Bacteriophage: Viruses That Infect Bacteria

Bacteriophage: Discovered in 1915, parasitic to every known bacterial species, and often make the bacteria they infect more pathogenic for humans.
"T-even" bacteriophage infect E.coli, such as T-2 and T-4, the most widely studied bacteriophage, undergoing similar stages as animal viruses.

Bacteriophage Life Cycle

Lytic phase or lytic cycle: Bacteriophage life cycle that ends in the destruction of the bacterial cell.
Lysogenic cycle: The bacteriophage becomes incorporated into the host cell's DNA.

Bacteriophage Life Cycle Diagrams

Images illustrate a bacteriophage's life cycle, showing its entry into the bacterial cell, replication of phage parts, and the eventual lysis of the bacterial cell to release new phages.

Other Noncellular Infectious Agents: Spongiform Encephalopathies

Implicated in chronic, persistent disease in humans and animals.
Brain tissue removed from affected animals resembles a sponge.

Prions

A common feature of spongiform encephalopathies.
Distinct protein fibrils deposited in brain tissue of affected animals.

Creutzfeldt-Jakob Disease (CJD)

Affects the central nervous system of humans, causing gradual degeneration and death.
Transmissible by an unknown mechanism.
Several animals are victims of similar diseases:
- Scrapie: Sheep, mink, elk.
- Bovine Spongiform Encephalopathy: Cows.

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Description

This quiz explores the unique characteristics of viruses, including their size, structure, and the way they interact with host cells. Understand the implications of viruses on living organisms, from bacteria to humans. Test your knowledge on the biological properties that make viruses distinct entities.