Lecture 1 - Virology: Introduction, Structure and Taxonomy Flashcards
(9 cards)
What are the main characteristics of viruses?
Viruses are the smallest infectious agents (20–300 nm), smaller than bacteria and filterable.
They contain only one type of nucleic acid (DNA or RNA) inside a protein shell (capsid), which protects the genome from enzymes like nucleases.
Some viruses have an outer envelope made of lipid and protein.
Free viral nucleic acids are unstable without the capsid.
Viruses are obligate intracellular parasites—they cannot reproduce on their own.
They use the host cell’s energy and ribosomes to make viral macromolecules.
Viruses do not carry genes for energy production, ribosomal proteins, mRNA, or tRNA.
They have a unique replication cycle, entirely dependent on the host cell.
How is viral morphology studied and what is icosahedral symmetry?
Viral Morphology refers to the shape and structure of viruses, often visualized using electron microscopy.
Icosahedral/Cubical Symmetry: A common viral shape with 20 triangular faces; provides stability and efficient packaging of genetic material.
Negative Staining: A technique where the background is stained (not the virus), allowing clear visualization of surface structures.
Positive Staining: Stains the virus particles directly to show internal details, often used to examine internal components.
What are the main structural components of a virus?
Nucleic Acid – the genetic material (DNA or RNA).
Capsomeres – protein subunits that join to form the capsid.
Capsid – protein shell that protects the nucleic acid.
Nucleocapsid – the capsid + nucleic acid together.
Envelope – a lipid layer surrounding some viruses, derived from host cell membranes.
Envelope Glycoproteins (Peplomeres) – spike-like structures on the envelope, important for host cell attachment.
Virion – a complete infectious virus particle made up of the nucleic acid, capsid (or nucleocapsid), and envelope (if present).
What are the key features of viral nucleic acids?
Viral genomes can be DNA or RNA, single- or double-stranded, but never both in one virus.
RNA genomes can be:
Positive-sense (+): Same direction as mRNA (can be directly translated).
Negative-sense (–): Must be converted to mRNA first.
Some RNA viruses have segmented genomes (each segment usually codes for one protein).
Viral nucleic acid determines how the virus replicates and produces proteins.
Most viruses are haploid (1 copy of each gene), except retroviruses, which are diploid.
Genome size:
DNA viruses: 1.7 kb – 200+ kb
RNA viruses: 1.7 kb – 33 kb
What are the main types of viral morphology?
Helical Symmetry: The nucleic acid is surrounded by a spiral-shaped capsid (e.g. tobacco mosaic virus).
Icosahedral Symmetry: Virus has a 20-faced structure, common in many animal viruses.
Binal Symmetry: Found in bacteriophages, combining an icosahedral head (DNA storage) with a helical tail (used for injection into host).
Complex Symmetry:
Seen in large viruses like poxviruses.
Orthopoxviruses (e.g. cowpox): Brick-shaped, covered with tubules.
Parapoxviruses (e.g. orf virus): Oval-shaped, with a basketweave surface pattern.
These viruses have large genomes coding for ~100 proteins.
What are viral envelopes and what is their significance?
Some viruses mature by budding through a host membrane and acquire a lipid envelope of host origin.
Viral glycoproteins are embedded in the envelope and play a key role in virus-host interactions.
Large envelope glycoproteins are called spikes or peplomers (e.g. in Coronaviridae).
The envelope is usually essential for viral infectivity.
What are the different hosts that viruses infect?
Animals, including protozoa
Plants
Bacteria (called bacteriophages or phages)
Fungi
What are arboviruses?
Arbovirus = Arthropod-borne virus
Not a taxonomic term, but a general term for viruses transmitted by arthropods.
Replicate in both animal hosts and arthropod vectors (e.g. mosquitoes, ticks).
To be classified as an arbovirus, the virus must be biologically transmitted, not just mechanically.
Mechanical vectors: Some arthropods only carry viruses without them replicating in the vector.
Arboviruses can belong to several different viral families.
What are prions and what diseases do they cause?
Prions: Infectious agents causing subacute spongiform encephalopathies (brain diseases).
Diseases caused by prions:
Scrapie in sheep
Creutzfeldt-Jakob disease in humans (associated with cannibalism)
Bovine spongiform encephalopathy (BSE) – mad cow disease
Prions do not contain nucleic acid (no DNA or RNA).
The abnormal prion protein, PrPsc, accumulates in the brain, replacing the normal prion protein, PrPc.
