ORIGIN AND EVOLUTION OF VIRUS .pptx

Origin of virus have always been a concern of debate and confusion among the virologist owing to its ambiguous nature of existence.
Viruses have played a major role in cellular evolution yet they have not been placed in the Tree of Life (TOF) that consists of the major 3 domains- Bacteria, Archaea and Eukarya.

1. Origin, Evolution And Interrelationship Between Plant and Animal Virus Presented By: Kavita Kushwaha

2. Origin of virus have always been a concern of debate and confusion among the virologist owing to its ambiguous nature of existence. Viruses have played a major role in cellular evolution yet they have not been placed in the Tree of Life (TOF) that consists of the major 3 domains- Bacteria, Archaea and Eukarya. HOW?

3. Life first emerged at least 3.8 billion years ago, approximately 750 million years after Earth was formed At the time life arose, the prebiotic-atmosphere of Earth is thought to have contained little or no free oxygen, instead consisting principally of CO 2 , N 2  and traces of H 2 , H 2 S, and CO. These constituents provided a reducing atmosphere in which organic molecules were formed spontaneously given a source of energy such as sunlight or electrical discharge. This spontaneous formation of organic molecules was first demonstrated experimentally in the 1950s, by Stanley Miller. These monomeric building blocks later polymerized to form the macromolecules such as proteins carbohydrates and nucleic acids. Among all the macromolecules only nucleic acids have the ability to self replicate. They can serve as templates for their own synthesis as a result of specific base pairing between complementary nucleotides. Consequently, RNA is generally believed to have been the initial genetic system, and an early stage of chemical evolution ( RNA world) . Ordered interactions between RNA and amino acids then evolved into the present-day genetic code, and DNA eventually replaced RNA as the genetic material.

4. Virus – first hypothesis  This theory states that viruses first evolved from the complex molecules of nucleic acid and proteins before cells first appeared on earth. Hence, according to this theory, viruses contribute to cellular life. Contrastingly, all known viruses need a cellular host to replicate, thus necessitating the existence of cells before virus survival. Therefore, the virus-first hypothesis has been challenged and the existence of an ancient and independent viral world critiqued.

5. T he regressive hypothesis or Degeneracy theory An alternative general hypothesis associates the origin of viruses to cells and considers viruses to be the reduced forms of parasitic organisms . This hypothesis, better known as the “reduction hypothesis,” is supported by the recent discovery of giant viruses (e.g., mimiviruses and megaviruses ) with genomic and physical features that overlap those of numerous parasitic bacteria.

6. The progressive or escape hypothesis:   This states that viruses were once part of the genetic material of host cells but escaped cell control. They originated from genetic elements that gain the ability to move between cells . This theory states that some viruses evolved from bits of DNA and RNA that escaped from the genes of larger organisms and species. HGT is believed by some scientists to be the predominant force shaping many viral genomes. This hypothesis, however, fails to explain the presence of structures that are unique to viruses and are not present in cells

7. Plant and animal virus interrelationship Comparison of the genome organizations of related plant and animal viruses reveals a recurrent theme with an apparent simple “evolutionary logic”. Virus genes can be roughly partitioned into the “ housekeeping (replication) module ” and the “ interactive module ”. The replication modules are conserved between viruses of plants and animals whereas the interactive modules are host-specific

8. Overall, while there are significant differences between animal and plant viruses in terms of their hosts and transmission dynamics, there are also notable similarities and potential points of interaction. Studying the interrelationship between animal and plant viruses can provide insights into viral evolution, host-pathogen interactions, and strategies for viral control and management in both agricultural and medical contexts.

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