Three different theories have been proposed to explain the origin of virues.The first , the regressive theory of virus origins, proposes that viruses arise from free-living organisms like bacteria that have progressively lost genetic information ----- to the point where they become intracellular paprasties dependent upon their hosts to supply the functions thay have lost.
The second theory proposes that viruses arises from the host cell DNA or RNA molecules , which gain a self-replicative but parasitic existence . One or few genes ----- or the mRNA from one or few genes ----- acquires the ability to replicate and evolve ( change its neuclotide sequences or orginization ) independantly of its host cell.
The third theory proposes that viruses originated and evolved along with the most primitive molecules that first contained self-replicating abilities . While some of the molecules were eventually collected into units of organization and duplication termed cells , other molecules were packaged onto virus particles that coevolved with cells and parasitized them.
Possibility 1 : Do virus originate by Regression from Free-living organisms?
Several examples of present-day intracellular parasites are clearly derived from free-living bacteria. While a good case can be made that the cell organelles of higher organisms may have originated from bacteria by gegeneration , it is less clear that viruses arose in this way. Viruses never encode ribosomal RNAs. Further, we know of no living intermediates between chlamydiae and viruses. The eukaryotic cell organisma in common with the bacteria , have only DNA genome so it remains unclear from this theory hoe RNA viruses might arise. Finally , the regressive theory may explain how a DNA genome and an intercellular parasitic life could begin but it does little to suggest how the next step -----the gormation of a coat or a shell about this DNA genome ----- might be acconplished. The genetic information in a cell might be employed as a starting material to package the viral DNA but exactly what might drive this process and what its selective advantages could be remain less clear. The degenerative theory of virus origin to leave number of questions unanswered.
Possibility 2 : Do Viruses Originate from componenets of Cellullar DNA or RNA ?
This theory postulates that a portion of the genetic information found in the host cell or an mRNA copy of this DNA can acquire the ability of relicate itself and then evolve independently of the original host-cell genome. This constitutes an origin event. The question then becomes : What is needed to create a self-replicative molecule that store their information ( DNA or RNA ). They do so by an ordered process that starts at s unique site on the polymers and ends when all thw neuclotides are duplicated ijn the correct sequences. The start site position has been termed an origin of replication, and every self- replicating molecule must have one. The neuclotide structure at the origin is recgonized by one proteins whose functions is to initiate polymerization od the neuclotides into a complementry molecule. By obtaining a new neuclotide sequence recgonzed as an origin of relication, an RNA or DNA molecule synthesized by a host cell may be acquired the ability to duplicate itself independently.
If we look at the molecules wth these properties we can find them easily in bacterial cells ; they are called episomes. Episomes are circular DNA molecules that can exist in an autonomous state ----- replicating freely in the in the cytoplasm, where they are trmed plasmids ----- or in an intergrated state contiguous with the bacterial chromosome. In this sense they are similiar to the lambda bacteriophage which exist in both states ; its DNA is also termed an episome . Whether the episome is free or integrated into the DNA of its host , its neuclotide sequence duplicates once per host-cell generation and is segregrated into the two daughter cells.
Possibility 3 :Do Viruses Originate from Self-Replicating Molecules ?
As we have seen, it appears that the earliest self replicating information systems were composed of RNA polymers are promoted by the RNA molecules themselves., the rate at which this happens is slow ----- only about two monomeric nucleotides are added to a growing chain per minute at 25 degrees celcius. This contrasts with the rate of nucleotide addition into RNA copied from a DNA template by the enzyme RNA polymerase ----- fifty neuclotides per second at the same temperature. The protein is more efficient catalyst: 1500 times faster.
Proteins were also among the chemical components of the early wound when life began and evolved. This theory proposes that because proteins are more efficient in promoting important chemical reactions useful to life forms, the initial RNA information-duplication systems evolved into RNA-protein information-replication processes. Probably for the sake of better stability and productibility, DNA came to replace RNA as the information storagemolecule and RNA assumed the role of intermediate or messenger while proteins carried out the functions. Once the sequence of neuclotides in DNA determined the sequence of amino acids in proteins, a genetic code common to all life forms on Earth was fixed , becoming the language used by this two-part system of life.
As the series of living molecules became more complex, it was advantageous to package the genetic information and its readout systems. Lipida, insoluble in the aqueous cell's enviroment, surrounded these molecules, seprating them from the outside: in this way, some life forms became cells. Other life forms may well have been simple neucleic acids surrounded by protein coats. To duplicate themselves, these molecules entered a cell. In this way, host cells and viruses might both have arisen in the same primordial 'soup.' It is reasonable, according to this view , to suppose that such self-replicating molecules ---- the virues ----- and their host cells coevolved. While new origin events may still occur to generate viruses, spontaneous origin origin events that result in more complex organisms like bacteria no longer occur ----- as shown bby Pasteur's famous nineteenth-century experiments refuting the theory of spontaneous generation.