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Questions and Answers
What is the primary function of the capsid?
What is the primary function of the capsid?
What material composes the viral envelope?
What material composes the viral envelope?
Which type of symmetry is exhibited by the influenza virus?
Which type of symmetry is exhibited by the influenza virus?
What is the correct term for the protein coat that encases the nucleic acid of a virus?
What is the correct term for the protein coat that encases the nucleic acid of a virus?
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Which one of these viruses is non-enveloped?
Which one of these viruses is non-enveloped?
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Which feature distinguishes most RNA animal viruses from DNA animal viruses?
Which feature distinguishes most RNA animal viruses from DNA animal viruses?
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Which of the following is NOT a property of viruses?
Which of the following is NOT a property of viruses?
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Study Notes
Definition and Properties of Viruses
- Viruses are the smallest infectious agents, containing one kind of nucleic acid (RNA or DNA) in their genome, usually as a single molecule.
- Viruses are obligatory intracellular parasites.
Properties of Viruses
- Very small in size, requiring an electron microscope to be seen (except for poxvirus).
- Can pass through bacterial filters.
- Obligate intracellular parasites, replicating only inside host cells due to lack of machinery for replication.
- Cannot be grown on artificial culture media, only on living cells (tissue culture, embryonated eggs, or living animals).
- Genome is either DNA or RNA, never both.
Structure of Viruses
- The virus particle (virion) consists of a nucleic acid core called the "virus genome" surrounded by a protein coat called the capsid.
- Viral nucleic acid:
- Contains a single kind of nucleic acid (RNA or DNA) that encodes genetic information for replication.
- May be single-stranded or double-stranded.
- Most RNA animal viruses have single-stranded RNA genomes, while most DNA animal viruses have double-stranded DNA genomes.
- Functions of viral nucleic acid:
- Essential infectious component of the virion.
- Encodes genetic information for replication.
- Carries heritable characteristics of the virus.
Capsid
- Protein coat surrounding the nucleic acid, made of tiny protein subunits called "capsomeres".
- Nucleic acid surrounded by its protein coat is called the nucleocapsid.
- Functions:
- Protects nucleic acid from damage.
- Facilitates attachment to host cell during infection.
- Source of viral antigens.
Symmetry of Virus Capsids
- Arrangement of capsomers determines virus symmetry, which may be:
- Icosahedral or cubical symmetry (e.g., herpesviruses and adenoviruses).
- Helical symmetry (e.g., influenza virus).
- Complex symmetry (e.g., poxvirus and bacteriophages).
Envelope
- Lipoprotein membrane composed of lipid from the host cell membrane and virus-specific protein.
- Frequently contains glycoproteins in the form of spike-like projections that attach to host cell receptors.
- Matrix protein mediates interaction between capsid proteins and the envelope.
Morphology
- Most animal viruses are spherical.
- Poxviruses are brick-shaped.
- Influenza virus is spherical or filamentous.
- Bacteriophages are spermatozoa-like.
Metabolism
- Viruses are obligatory intracellular parasites.
- Contain no enzymes associated with energy metabolism, relying on host cells for replication.
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Description
Learn about the fundamental characteristics of viruses, including their structure, size, and properties as infectious agents.
