40 Questions
What is the main difference between DNA and RNA viruses?
DNA viruses have a molecule of DNA, while RNA viruses have a molecule of RNA
What is the function of the protein coat in a virus?
To protect the genome and enable binding to host cells
What is the term for a complete virus particle?
Virion
What determines the pathogenicity of a virus?
A variety of structural and functional characteristics
What is the role of the host cell in viral replication?
To provide the virus with enzymatic machinery
What is the outcome of a viral infection for the host cell?
The host cell undergoes rapid lysis or gradual release of viral particles
What is a key characteristic of viruses?
They are obligate intracellular parasites
What is the term for the outer layer of a virus that is derived from the host cell's plasma membrane?
Envelope
What is the first step in the virus replication cycle?
Adsorption
What is the function of the receptor molecules on the host cell membrane?
To serve as receptors for a particular virus
What determines the tissue specificity within a susceptible host species?
The presence or absence of host cell receptors
What is the result of antibody binding to the viral structures required for adsorption?
Neutralization of viral infectivity
What is the primary difference between helical and icosahedral symmetry in viral capsids?
The number of polypeptide species
What type of viruses have specialized attachment structures in their envelopes?
Rhabdoviruses and coronaviruses
What is the function of the protein shell in a virus?
To protect the genome from environmental damage
What is the fate of the viral genome after penetration?
It is released from the capsid
What is the term for the complex of protomers and nucleic acid in helical viruses?
Nucleocapsid
Which type of viruses always contain RNA and are enveloped?
Helical viruses
What is the role of the unique folding of capsid proteins in some viruses?
To form attachment sites
What is the outcome of the absence of host cell receptors for a particular virus?
The virus cannot infect the host cell
What is the origin of the lipid-containing membrane surrounding the nucleocapsid?
Host cell membranes
What is the characteristic shape of enveloped viruses?
Flexible and coiled
What is the primary function of the envelope in enveloped viruses?
To protect the nucleocapsid
What type of symmetry do capsids with repeated units of a single polypeptide species exhibit?
Helical
What is the primary reason for dividing viruses into different species?
Differences in the host range and serologic reactions
What term is used to describe viruses of the same species isolated from different geographic locations that differ in nucleotide sequence?
Strains
What is the most common form of viral genome found in nature?
ss RNA
What is the characteristic of a positive-strand RNA virus?
The RNA genome is of messenger RNA sense
Which of the following is a characteristic used to define virus families?
Type of nucleic acid
What is the main difference between a positive-strand and a negative-strand RNA virus?
The polarity of the RNA genome
Which of the following is NOT a characteristic used to define virus genera?
Type of symmetry of the virus capsid
What is the term used to describe a group of related viruses?
Family
Where does the assembly of nucleocapsids generally take place?
In the cytoplasm for most RNA viruses and in the nucleus for most DNA viruses
How do progeny viruses get released from the cell in naked viruses?
Passively, as a result of the disintegration of the dying cell
What happens to the host cell glycoproteins when virus-specific glycoproteins are inserted into the host cell membrane?
They are displaced by viral glycoproteins
What is the result of the envelopment of the nucleocapsid by the process of 'budding'?
The continuous release of progeny viruses
Where is the dsDNA integrated into the cell genome in retroviruses?
In the nucleus
What is the role of the matrix proteins in enveloped viruses?
To associate with the nucleocapsid and bind to viral glycoproteins
When do progeny viruses get released from the cell in enveloped viruses?
When the cell loses its ability to maintain the integrity of the plasma membrane
What is a consequence of the mechanism of viral replication in enveloped viruses?
All infectious progeny are extracellular
Study Notes
Characteristics of Viruses
- Viruses are obligate intracellular parasites, meaning they must infect a host cell to replicate.
- A virus consists of a molecule of DNA or RNA, surrounded by a protein coat called a capsid.
- Some viruses also have an envelope, which is derived from the host cell membrane.
- Viruses contain the genetic information necessary for their own replication, but require the host's cellular structures and enzymatic machinery to complete the process.
Structure of a Virus
- A complete virus particle is called a virion.
- A virion consists of a genome (either RNA or DNA) surrounded by a protein-containing structure called a capsid.
- The capsid can be either helical or icosahedral in shape.
- Some viruses also have an envelope, which is a lipid bilayer derived from the host cell membrane.
Characteristics Used to Define Virus Families, Genera, and Species
- Viruses are divided into families, genera, and species based on characteristics such as:
- Type and structure of the viral nucleic acid
- Replication strategy
- Type of symmetry of the virus capsid
- Presence or absence of a lipid envelope
- Host range
- Serologic reactions
- Amino acid sequences of viral proteins
- Degree of nucleic acid homology
Genome
- The type of nucleic acid found in a virus particle is perhaps the most fundamental and straightforward of viral properties.
- Viral genomes can be either RNA or DNA, and can be single-stranded or double-stranded.
- Single-stranded viral RNA genomes can be further divided into those of "positive polarity" (messenger RNA sense) and those of "negative polarity" (antisense).
Capsid Symmetry
- The protein shell enclosing the genome can have either helical or icosahedral symmetry.
- Helical symmetry is found in viruses such as paramyxoviridae, and consists of repeated units of a single polypeptide species that self-assemble into a helical cylinder.
- Icosahedral symmetry is found in viruses such as picornaviruses, and consists of several different polypeptides grouped into structural subassemblies called capsomers.
Envelope
- An envelope is a lipid-containing membrane that surrounds the nucleocapsid in some viruses.
- The envelope is derived from the host cell membrane.
- Enveloped viruses can be either icosahedral or helical in shape.
Steps in the Replication Cycles of Viruses
- The individual steps in the virus replication cycle are:
- Adsorption: attachment of the virus particle to the host cell
- Penetration: entry of the viral genome into the host cell
- Uncoating: release of the viral genome from the capsid
- Gene expression and replication
- Assembly and release of viral progeny
Adsorption
- The initial attachment of a virus particle to a host cell involves an interaction between specific molecular structures on the virion surface and receptor molecules in the host cell membrane.
- Attachment sites on the viral surface can include specialized structures such as glycoprotein spikes, or unique folding of the capsid proteins.
- Host cell receptor molecules are specific for each virus family and can be molecules that usually carry out normal cell functions.
Learn about the basics of viruses, including their structure, characteristics, and types. This lecture is based on Lippincott's Illustrated Reviews Microbiology.
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