GENETICS OF VIRUSES
Definition: Obligate intracellular parasite
● Viruses lack enzymes for most metabolic processes and machinery for protein synthesis, thus
are dependent on their living host cells for building-blocks (e.g. amino acids, nucleotides),
protein-synthesising machinery (e.g. ribosomes) and energy (ATP) -> viruses can only
reproduce within a living host cell
● Virion: extracellular state of virus; metabolically inert
Arguments for viruses being living organisms Arguments for viruses being non-living organisms
(1) Viruses can reproduce (in the intracellular state) (1) Viruses are not cells
● However, a new virion is never formed directly ● Do not have protoplasm/organelles
from a pre-existing virion (replication involves
synthesis of viral components followed by
assembly into virions)
(2) Viruses are able to direct metabolic processes (in the(2) Viruses lack some of the characteristics of living
intracellular state) organisms (in their extracellular state)
● Unable to carry out metabolic processes
(3) Viral genomes can evolve
(e.g. nucleic acid/protein synthesis)
● Different types of viruses vary greatly in their
● Do not require nutrition (e.g. amino
structural and genetic complexity and no single
acids/nucleotides)
gene is shared by all viruses/viral lineages (do
● Unable to synthesise their own ATP as
not share a common ancestor)
energy source
● Viruses evolve with their host and acquire their
● Unable to respond to stimuli
metabolic and translational genes from host cells
● Neither grow nor excrete
● Genetic recombination can change viral
genomes
How viruses challenge the cell theory:
1) Cells are the smallest unit of life
● Viruses lack the necessary molecular machinery to conduct many biochemical
reactions; yet like cells, they contain the genetic material necessary to form the next
generation and are able to evolve in response to environment
2) All cells come from pre-existing cells
● Viruses are able to replicate, but rely on host cells to provide energy and materials
needed to replicate genomes and synthesise proteins -> cannot replicate unless they
have entered a suitable host cell
3) All living organisms are composed of cells
● Viruses are acellular and do not have protoplasm/organelles; virions are
metabolically inert and do not carry out respiration/biosynthesis
Conclusion: viruses are infectious particles which are active as intracellular virus state, or inactive as
extracellular virion state
Copyright © 2019 tonyndr
Definition: Obligate intracellular parasite
● Viruses lack enzymes for most metabolic processes and machinery for protein synthesis, thus
are dependent on their living host cells for building-blocks (e.g. amino acids, nucleotides),
protein-synthesising machinery (e.g. ribosomes) and energy (ATP) -> viruses can only
reproduce within a living host cell
● Virion: extracellular state of virus; metabolically inert
Arguments for viruses being living organisms Arguments for viruses being non-living organisms
(1) Viruses can reproduce (in the intracellular state) (1) Viruses are not cells
● However, a new virion is never formed directly ● Do not have protoplasm/organelles
from a pre-existing virion (replication involves
synthesis of viral components followed by
assembly into virions)
(2) Viruses are able to direct metabolic processes (in the(2) Viruses lack some of the characteristics of living
intracellular state) organisms (in their extracellular state)
● Unable to carry out metabolic processes
(3) Viral genomes can evolve
(e.g. nucleic acid/protein synthesis)
● Different types of viruses vary greatly in their
● Do not require nutrition (e.g. amino
structural and genetic complexity and no single
acids/nucleotides)
gene is shared by all viruses/viral lineages (do
● Unable to synthesise their own ATP as
not share a common ancestor)
energy source
● Viruses evolve with their host and acquire their
● Unable to respond to stimuli
metabolic and translational genes from host cells
● Neither grow nor excrete
● Genetic recombination can change viral
genomes
How viruses challenge the cell theory:
1) Cells are the smallest unit of life
● Viruses lack the necessary molecular machinery to conduct many biochemical
reactions; yet like cells, they contain the genetic material necessary to form the next
generation and are able to evolve in response to environment
2) All cells come from pre-existing cells
● Viruses are able to replicate, but rely on host cells to provide energy and materials
needed to replicate genomes and synthesise proteins -> cannot replicate unless they
have entered a suitable host cell
3) All living organisms are composed of cells
● Viruses are acellular and do not have protoplasm/organelles; virions are
metabolically inert and do not carry out respiration/biosynthesis
Conclusion: viruses are infectious particles which are active as intracellular virus state, or inactive as
extracellular virion state
Copyright © 2019 tonyndr
