Lecture 2: Viral Definition Origins and Classification Flashcards
How was a virus first defined?
It was discovered in 1884-92 by Ivanovski and thought of as a filterable agent (not so much nowadays) and named Virus for “poison” in Latin.
As of now, 5,000 or so distinct viruses are known (many others exist) but these are detectable by their nucleic acid genomes.
What is the physical definition of a virus?
Set of nucleic acid (RNA or DNA) genes packaged within a protein coat (mostly smaller, and fewer genes than, the simplest cellular organism).
Thus, viruses are the smallest and simplest of all biological entities.
What is the functional definition of a virus?
A molecular genetic parasite dependent on its host for replication, transmission, and/ or maintenance (S. Luria 1952).
What is likely the best functional definition?
An obligate intracellular parasite.
Since viruses can only replicate inside living cells and the host cell machinery is required for virus reproduction (mRNA translation and energy metabolism).
Is a virus an living organism based on the 7 characteristics of living things theory?
In terms of the 7 characteristics of living things, NO!
Grow- No
Reproduce (undergo replication)- Yes
Responsiveness/”irritability”: No
Organized into cells: No
Evolve: Yes
Respiration: No, excretion, movement, metabolism.
Free living thing: No
What is the living systems theory?
Living systems can be defined as those systems that are able to process and be maintained by flows of:
Matter: (atoms, molecular “metabolism”).
Energy (respiration)
Information (genetics)
What is Prof Gershon’s definition?
Require water (as the universal solvent).
Are based on carbon-containing molecules
Exhibit a continuum of complexity (from simple viruses to humans).
Is a virus a living organism in terms of the living system theory?
Flow of matter- Yes (encode biosynthetic enzymes).
Flow of energy: No (don’t encode enzymes of energy metabolism)
Flow of information: Yes (pass on their genes)
Is a virus a living organism in terms of Gershon criteria?
Require water: Yes
Based on carbon: Yes
Viruses are declared to be not alive by who and when?
In 2000, the International Committee on Taxonomy of Viruses officially declared that viruses are NOT alive.
Because: we can now synthesize a virus in a test tube and count the atoms).
Viruses are declared to be not alive by who and when?
In 2000, the International Committee on Taxonomy of Viruses officially declared that viruses are NOT alive.
Because: we can now synthesize a virus in a test tube and count the atoms).
Perhaps inside the cell = Alive
Outside the cell = Non-living
What are giant viruses?
Discovered in 2004, giant viruses are non-filterable (Mimivirus, Megavirus). Have genomes larger/more genes than genomes of many bacteria.
Therefore: “Alive”? Well size does not alter any of the other criteria.
What are viruses in their simplest form?
Simple nucleic acid genome in a protein coat.
What are viroids?
a small piece of nucleic acid (no protein encoded).
Viroids are the smallest known infectious pathogens.
Ex. Cause transmissible disease in plants! (transmitted via leaf-leaf contact, aphids, plasmodesmata.
A viroid may hitch a ride with a virus.
In animals (humans) hepatitis D is a viroid.
Hepatitis D: Packaged in the protein coat of Hepatitis B virus: Hep D is a parasite of a virus.
Where is the boundary between life and non-life since the simplest transmissible disease causing entity is a single molecule? Is it at the level of?
Organism (clearly living)
Single Cell? (clearly living: bacteria/ our cells can happily divide indefinitely in a Petri dish).
Virus: not free living perhaps the boundary is here
Single molecule viroid whose only life like property is to reproduce.
Maybe life is a progressively emerging property therefore no hard boundary exists.
What is the RNA world argument?
RNA world is an appealing hypothesis in which life originated as a self-replicating molecule that carried only information required for its own replication. Note: Planet = 4.54 billion years old.
Viroids and Viruses Origin
Perhaps they started as very crude RNA-based self-replicators. Later after the advent of cellular life/proteins/central dogma could only compete with cells by parasitizing them (and hijacking/hacking) the cellular machinery to make virus proteins?
What is the phylogenetic argument?
How organisms change over time/order of events. The simplest order of descent to account for three closely related genomic sequences found today *Blue mutation first and then red added later). (refer to slide in power point).
When we do phylogeny for all known DNA polymerase genes, which both cells and many DNA based viruses have in common ___ virus families appear to have very ___ and __ origins
different, different, ancient
What is the branch for DNA pols?
There is no single branch for DNA pols of all DNA viruses. Viruses don’t have a common origin unless you extrapolate right back to the very root of the tree where everything first diverged.
DNA pol genes orientate at the very deepest roots of the tree of life.
Viruses may be as ancient as life itself.
Since viruses don’t all fit nicely in one branch on the universal tree of life, are viruses a 4th kingdom of life?
Giant DNA viruses might be regarded as the 4th domain of life. In general though all organisms have either ribosomes (eukaryotes, eubacteria, archaea) or a capsid (viruses).
