Dahlia – Group 3 | Apa-ap, Narida & RiveraWHERE DID VIRUSES

COME FROM?

A Few Facts about Viruses

Viruses are much smaller and simpler in comparison to the smallest of cells.

Viruses cannot live without their host cell because they depend on it for growth and reproduction.

The first viruses may have evolved from the genetic material of living cells, and have continued to evolve with them for billions of years.

Viruses are found wherever there is life.

Viruses do not form fossils.

Problems in looking for the Origin of Viruses

The origin of viruses is unclear because they do not form fossils, so  molecular techniques have been used to compare the DNA or RNA of viruses and are a useful means of investigating how they arose.

Viruses may have arose after living cells were formed because viruses cannot survive without its host cell.

They may have evolved from genetic material of cells they infect.

The Three Main Hypotheses The three main hypotheses on the Origin of Viruses:

The Progressive HypothesisThe Regressive HypothesisThe Virus-First Hypothesis

The Progressive HypothesisStates that pieces of genetic material gained the ability to exit one cell and enter another.

This can happen to retroviruses by the conversion of the single-stranded RNA Genome to a double-stranded DNA genome in the host-cell by the use of a viral enzyme, reverse transcriptase.The DNA then transfers to the nucleus of the host cell. Integrase, another viral enzyme, inserts the new DNA into the cell’s genome.Viral genes can then be transcribed and translated. RNA polymerase can produce new copies of the single-stranded RNA genome.Pogeny viruses exit the cell and do the process again.

The Regressive HypothesisViruses may have evolved from more complex, possibly free living organisms that lost genetic information over time, as they adapted a parasitic approach to replication. Because of the parasitic relationship, the gene’s not used in parasitism were lost. This resulted to the loss of their ability to survive outside the host cell.Summary: Viruses were once more complex organisms but lost a lot of genes over time as they got used to parasitism and lost the ability to survive outside a cell.

The Virus-First HypothesisVillarreal and DeFlippis (2000) & Bell (2001)

Theorized that the nucleus found in the present organisms were complex enveloped DNA virus through an endosymbiotic-like event 

The Virus-First Hypothesis

Koonin Martin (2005)Stated that viruses existed as self-replicating units before the existence of cellsTheorized that over time they became more complex and organized which helped to the formation of cells.Since self-replicating RNA molecules were theorized that hey came before DNA & cells, it is speculated whether hat the single strand RNA viruses today came from those molecules. 

Summary: The Hypothesis states that viruses may have existed long before cells and had the ability to self-replicate and to form membranes and cells walls, leading to the evolution to the three domains of life.

No Hypothesis May be Correct

When viruses was first discovered, many questions have arisen which aren't simple to answer.One can argue quite convincingly that certain viruses arose through a progressive process.One can also argue that large DNA viruses arose through a regressive process.

The idea that viruses gave rise to life as we know it presents very intriguing possibilities.Perhaps today's viruses arose multiple times, via multiple mechanisms.Today's basic research in fields like microbiology, genomics, and structural biology may provide us with answers to this basic question.

SourcesMiller, K. & Levine, J. (1991). Biology. Location: New Jersey.Bubanovic, I., Najman, S., & Andjelkovic, Z. (2009, February 16). Origin and Evolution of Viruses: Escaped DNA/RNA Sequences as Evolutionary Accelerators and Natural Biological Weapons. Retrieved from: http://www.biology-online.org/articles/origin-evolution-viruses-escapeddna-rna.html Virus. (n.d.) Retrieved October 10, 2011 from Wiki:http://en.wikipedia.org/wiki/Virus?vm=rWessner, D. (2010). The Origin of Viruses. Retrieved from: http://www.nature.com/scitable/topicpage/the-origins-of-viruses-14398218Slide 2 Image – Retrieved from:http://www.google.com.ph/imgres?=Virus&um=1&hl=tl&sa=N&tbm=isch&tbnid=EgH5S9acRPZ4IM:&imgrefurl=http://www.sharenator.com/Viruses/&docid=JKdz-7eysSlide 5 Image – Retrieved from: http://www.nature.com/scitable/content/ne0000/ne0000/ne0000/ne0000/14465196/f2_pommier_nrd1660-f2.jpgSlide 7 Image – Retrieved from: http://www.nature.com/scitable/content/ne0000/ne0000/ne0000/ne0000/14828326/f4_prang_nrmicro1527-i1.jpg: