Origin of Viruses
Three theories have been put forward to explain the origin of viruses. These theories are highly speculative and are as follows :
Survivors of Pre-Cellular First Living Inhabitants of the Earth
This theory intimately rests on the theory of origin of life on Earth. Life, according to this theory, originated from simple inorganic compounds by a slow biochemical evolution of “ordinary” chemical reactions spread over millions and millions of years.
It is speculated that during the course of origin of life on Earth somewhere at the stage when complex chemical molecules united to form still more complex molecules which could mate with still other metastable molecules till a relatively large molecule (like nucleoprotein) capable of growth and division, a simple virus or a protovirus may have originated (Haldane, 1954; Fraser, 1967). This theory, however, enjoys some insurmountable objections.
Three theories have been put forward to explain the origin of viruses. These theories are highly speculative and are as follows :
Survivors of Pre-Cellular First Living Inhabitants of the Earth
This theory intimately rests on the theory of origin of life on Earth. Life, according to this theory, originated from simple inorganic compounds by a slow biochemical evolution of “ordinary” chemical reactions spread over millions and millions of years.
It is speculated that during the course of origin of life on Earth somewhere at the stage when complex chemical molecules united to form still more complex molecules which could mate with still other metastable molecules till a relatively large molecule (like nucleoprotein) capable of growth and division, a simple virus or a protovirus may have originated (Haldane, 1954; Fraser, 1967). This theory, however, enjoys some insurmountable objections.
Present day viruses are all obligate parasites and it is difficult to conceive of their origin before the origin of their hosts (cells) which are at a far higher scale of evolution. Viruses use the same genetic code as cellular organisms and depend solely and entirely on ribosomes, transfer RNAs and enzymes of the host cell for protein biosynthesis. Moreover, viral nucleic acid has the same properties and the same mode of replication as the nucleic acid of cellular organisms.
Regression from More Highly Evolved Free-Living Microorganisms/Cells
Viruses are considered to have originated by retrogressive evolution from free-living cells, according to this theory. A parasite evolves retrogressively as it takes the ready made metabolites from its host instead of synthesizing them himself.
Regression from More Highly Evolved Free-Living Microorganisms/Cells
Viruses are considered to have originated by retrogressive evolution from free-living cells, according to this theory. A parasite evolves retrogressively as it takes the ready made metabolites from its host instead of synthesizing them himself.
If the parasite continues to evolve retrogressively then, to save labour and energy, it would slowly loss some of its physiological, morphological and even genetical functions that become super-numerary in its new ecological niche and new mode of biological existence. A parasite would, therefore, get regressed to a much simpler organism. Obligate intracellular parasitism is the most specialized type of parasitism and such a parasite would ultimately possess only the bare minimum and this minimum is the possession of a nuclei acid (to ensure genetic continuity) enclosed within a protein shell (to ensure safety of the nucleic acid).
Shedding of all unnecessary morphological, physiological and genetical materials would necessarily reduce the size. A formerly free-living organism would thus be transformed into a virus. Green (1935), Laidlow (1938) and Burnet (1945) support the theory of retrogressive origin of viruses but the same is opposed by Luria and Darnell (1967) and Fenner (1968).
Shedding of all unnecessary morphological, physiological and genetical materials would necessarily reduce the size. A formerly free-living organism would thus be transformed into a virus. Green (1935), Laidlow (1938) and Burnet (1945) support the theory of retrogressive origin of viruses but the same is opposed by Luria and Darnell (1967) and Fenner (1968).
Derived from Normal Constituents of the Cell
An eukaryotic cell possesses organelles like chloroplasts and mitochondria which self-replicating semi-autonomous structures and reproduce their like. Chloroplasts and mitochondria increase in size and then divide while kinetosome is synthesized near a pre-existing one by assembly from the tubular materials. Besides they possess DNA which is functional and possesses its own mutational history, codes for the synthesis of mRNA and also presumably for protein.
Mitochondria mutate to nonfunctional forms in Neurospora and yeasts, while chloroplasts mutate to undeveloped, colourless proplastids in algae and higher plants. These mutations are based on genetic changes. Cells also contain certain organelles that exceptionally undergo autonomous unrestricted replications; examples are the centrioles in Marsilea and sperms and nuclear genes in the amphibian oocytes.
An eukaryotic cell possesses organelles like chloroplasts and mitochondria which self-replicating semi-autonomous structures and reproduce their like. Chloroplasts and mitochondria increase in size and then divide while kinetosome is synthesized near a pre-existing one by assembly from the tubular materials. Besides they possess DNA which is functional and possesses its own mutational history, codes for the synthesis of mRNA and also presumably for protein.
Mitochondria mutate to nonfunctional forms in Neurospora and yeasts, while chloroplasts mutate to undeveloped, colourless proplastids in algae and higher plants. These mutations are based on genetic changes. Cells also contain certain organelles that exceptionally undergo autonomous unrestricted replications; examples are the centrioles in Marsilea and sperms and nuclear genes in the amphibian oocytes.
Viruses resemble above mentioned cellular organelles in chemical composition, possession of nucleic acid, genetic continuity of genome, independent mutational history, capacity to replicate independently under their own genetic control and also under the overall regulatory control of and within the premises of the host cell.
Many of the cellular organelles or factors possess some of the distinctive characters of viruses, or more specifically, of viral genetic determinant (reproductive independence, evolutionary independence, independent cell to cell transfer and infectivity and pathogenicity) while others could be conferred on them by a specific arrangement of nucleotides. Viruses could, therefore, be derived from any or several of these cellular components and it is possible that different viruses have originated differently. Some of the possibilities are given below:
Many of the cellular organelles or factors possess some of the distinctive characters of viruses, or more specifically, of viral genetic determinant (reproductive independence, evolutionary independence, independent cell to cell transfer and infectivity and pathogenicity) while others could be conferred on them by a specific arrangement of nucleotides. Viruses could, therefore, be derived from any or several of these cellular components and it is possible that different viruses have originated differently. Some of the possibilities are given below:
1. Primordial self-replicating molecules may have mutated to regain the capacity to enter ('infect') the cell and then integrate with it. Such a molecule would be a 'virus'.
2. Some plasmids or even chromosomal segments may have evolved by merging of some primitive self-replicating molecules with the cellular genome. If this s true then it conceivable that some of the genes or groups of genes may revert to their ancestral habit and may have regained/evolved the genetic independence and independence and independent transfer of this genetic material. This would result in the ‘virus’.
3. Some genes of the cell could have escaped of the control mechanisms of the cell and may have acquired the capacity of autonomous replication independent of the division of the cell and capacity of independent transfer. Integration of these genes with the host genome would give us a prophage. Origin of bacteriophage from such prophage DNA has been outlined by Lindegren (1962). Luria and Darnell (1967) also suggest that bacteriophages containing DNA may have evolved from a number of genetic transfer elements (like F factor, bacteriocinogenic factor, etc.) occurring in the prokaryotic cells.
4. DNA viruses of eukaryotic cells may have originated from the functional DNA of cellular organelles (e.g., mitochondria and chloroplast) rather than for nuclear DNA (Matthews, 1970).
5. Origin of DNA plant viruses is somewhat more difficult to understand since there is no definite information with respect to the integration of plant viral nucleic acid into the genome of the plant cell. There is, however, some suggestive evidence in this connection. Some experimental evidence suggests that cut surfaces of barley seeds take up the DNA of the bacterium Micrococcus lysodeikticus and that it integrates into nuclear DNA of barley and replicates (Ledous and Huort, 1968).
In short, therefore, viruses may have originated from cell constituents which escape the control mechanisms of the cell, regained/developed the capacity of autonomous self-replication and ability to mediate their own independent cell to cell transfer and could enter or infect cells to which they did not belong.
2. Some plasmids or even chromosomal segments may have evolved by merging of some primitive self-replicating molecules with the cellular genome. If this s true then it conceivable that some of the genes or groups of genes may revert to their ancestral habit and may have regained/evolved the genetic independence and independence and independent transfer of this genetic material. This would result in the ‘virus’.
3. Some genes of the cell could have escaped of the control mechanisms of the cell and may have acquired the capacity of autonomous replication independent of the division of the cell and capacity of independent transfer. Integration of these genes with the host genome would give us a prophage. Origin of bacteriophage from such prophage DNA has been outlined by Lindegren (1962). Luria and Darnell (1967) also suggest that bacteriophages containing DNA may have evolved from a number of genetic transfer elements (like F factor, bacteriocinogenic factor, etc.) occurring in the prokaryotic cells.
4. DNA viruses of eukaryotic cells may have originated from the functional DNA of cellular organelles (e.g., mitochondria and chloroplast) rather than for nuclear DNA (Matthews, 1970).
5. Origin of DNA plant viruses is somewhat more difficult to understand since there is no definite information with respect to the integration of plant viral nucleic acid into the genome of the plant cell. There is, however, some suggestive evidence in this connection. Some experimental evidence suggests that cut surfaces of barley seeds take up the DNA of the bacterium Micrococcus lysodeikticus and that it integrates into nuclear DNA of barley and replicates (Ledous and Huort, 1968).
In short, therefore, viruses may have originated from cell constituents which escape the control mechanisms of the cell, regained/developed the capacity of autonomous self-replication and ability to mediate their own independent cell to cell transfer and could enter or infect cells to which they did not belong.
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