Are Viruses Alive and Should They Be Classified among Micro-Organisms - Literature review Example

Student Name Tutor Title: Are Viruses Alive? Should They Be Classified Among Micro-Organisms? Institution Date Are Viruses Alive? Should They Be Classified Among Micro-Organisms? Introduction Viruses are the biological objects that have sparked renewed interest following the recent discovery of Mimivirus (Claverie, et al, 2006; Helen Pearson, 2008). At one end, there are those who still hold the traditional view that although they possess some of the properties of living systems such as having a genome, they are actually nonliving infectious entities and should not be considered microorganisms. On the other hand, there are those who argue that they fall in the boundary between life and non-life, organism and non-organism, and biology and chemistry. A clear distinction in the conceptualization is thus needed. This paper provides insight in the debate; are viruses alive. This will be discussed under the following heading: What is life? There is no precise scientific definition of life, but most observers would agree that life includes certain qualities in addition to an ability to replicate. For some, life is often defined in terms of a list of distinctive properties that distinguish living systems from dead organisms and inanimate matter. Mickle and Aune (2011) list seven features common in living things described as description of life. These include homeostasis, complex and cellular organism, ability to reproduce, metabolism, growth and development, ability to adopt and evolve and, ability to respond to stimulus. This is closely related to the earlier definition by Macklem and Seely(2010) who described life as self-contained, self-regulating, self-organizing, self-reproducing, interconnected, open thermodynamic network of component parts which performs work, existing in a complex regime which combines stability and adaptability in the phase transition between order and chaos, as a plant, animal, fungus, or microbe. Traditional concepts of viruses as non-living (the differences between a virus and a cell) The traditional view conceptualized viruses as quite distinct from cells. Cells are generally described as membrane-enclosed compartments that contain genomic DNA (chromosomes), molecular machinery for genome replication and expression, a translation system that makes proteins, metabolic and transport systems that supply monomers for these processes, and various regulatory systems (Koonin, 2010). On the other hand, Ahlquist (2006) defines Viruses as generally very small packages of single or double stranded DNA or RNA often just a few genes, wrapped up in a coating of protein and sometimes an additional lipid envelope. This is contrary to the earlier definition given by Marc van Regenmortel and Mahyt as any number of concrete objects that possess various relational properties (for instance, its host, vector, and infectivity) that arise by virtue of a relation with other objects (Marc van Regenmortel and Mahyt, 2004). The traditional view of Viruses revolved around the premise that they did not meet all the criteria of the generally accepted definition of life thus excluding them from organismal status. They were initially thought to be not compost of cell structure or membranes not even cells as bacteria and other living things do, yet this is regarded as the basic unit of life. In addition, they were thought to be unable to metabolize independently and could not replicate and synthesize new products without a host. Instead they only seemed most lifelike by invading and co-opting the machinery of living cells in order to make more of themselves, often killing their hosts in the process. They use their hosts, which probably include every organism past and present, or occasionally work in collaboration with other viruses to make necessary enzymes. According to Dupré and O’Malley (2009) viruses can carry out such biologically impressive activities as entering cells, co-opting the transcription and translation machinery of the cell, and picking up and moving about DNA from the host organisms with which they interact. In view of the two scholars, by exploiting or collaborating with cellular organisms in these ways, viruses very effectively reproduce themselves and have no need of autonomous metabolism (Dupré and O’Malley, 2009). Viruses were initially believed to possess small genomes, ranging in size from between about 1,000 and 1,000,000 nucleotides. This was not until the discovery of the giant Mimivirus Viruses which has currently blurred the division between viruses and cells in terms of particle and genome size, leading to a renewed interest and definition of virus as indicated by various scholars like (Jean-Michel Claverie, 2006; Dupré and O’Malley, 2009). Virus was also considered non-living due to its inability to independently reproduce. Until recently, it was generally believed that although viruses could synthesize some of their own proteins, they couldn’t reproduce independently (Van Regenmortel 2007). Instead of reproducing by division they however do so by self-assembly of the components that they manufacture with the help of the host cell. Dupré and O’Malley, (2009) assert that some viruses facilitates or influence host behaviour quite significantly by, for instance, conferring either protection against other viruses or virulence properties as the case of diphtheria or cholera toxins. The major weakness of this is that some living organisms exhibited characteristics of both living and non-living particles similar to viruses. Discovery of the Mimivirus- why Mimivirus destroys traditional ideas of viruses The recently discovered Mimivirus has destroyed the previously held notion of viruses as a non-living. Unlike the view that viruses are generally microscopic, ranging from about 1,000 and 1,000,000 nucleotides, by Closer inspection Claverie et al (2006) revealed that the Mimivirus was a huge virus. In the words of La Scola et al. (2003), it is one of the largest and most complex viruses known. Observation revealed that both the particle size and the genome size of mimivirus is larger than that of some small bacteria. Pearson (2008) in her later work revealed, a genome harbouring approximately more than 900 protein-coding genes, at least three times more than that of the biggest previously known viruses and bigger than that of some bacteria like Rickettsia conorii and Tropheryma whipplei. The similarities between Mimivirus and the smallest microorganisms are not restricted only to size of both the particle and the genetic material. According to Claverie and his colleagues, the external structure of Mimivirus is quite comparable or alike to that of small prokaryotic bacteria. In addition, these scholars revealed that the icosohedral capsid of Mimivirus is coated in a layer of proteins that in some ways mimics the cell wall of Gram-positive bacteria. The protein coating acts as a disguise designed to deceive the ameboid host of Mimivirus into recognizing the virion as a bacteria and compel the host to engulf the virus (Claverie et al, 2009). Furthermore, the 1.2 Mbp genome that harbor 911 protein coding genes gives sufficient information to allow the virus to perform most of the functions of living cells for instance encoding some genetic products previously not known to be possessed by any virus. Mimivirus particularly contains genes coding for nucleotide and amino acid synthesis which conspicuously lack even in some small obligate intracellular bacteria, implying that unlike these bacteria, mimivirus is not dependent on the host cell genome for coding the metabolic pathways for these products. It does however, lack genes for ribosomal proteins, making mimivirus dependent on a host cell for protein synthesis and energy metabolism (Dupré and O’Malley, 2009). Therefore, the complexity and magnitude of the Mimivirus genome, in combination with the large size of the virus, is all but sufficient evidence that it is a living cell. They are also associated with metabolism. Raoult and his collogues argue that the Mimiviruses carry genes that are known to encode translation, DNA repair and metabolic activities (Raoult et al. 2004). Although it may be argued that some other large viruses also do carry translations and metabolic genes, Mimivirus is quite unique in that it profoundly or greatly extends the known repertoire of these genes in viruses (Koonin 2005). According to Dupré and O’Malley, (2009) Mimiviruses do not seem to have picked some of these genes up from their host. Even though. Mimiviruses cannot synthesize their own ribosome and do not metabolize unaided since their metabolic pathways are not fully coded, they can easily be conceived of as entities in transition from viruses to free-living organisms (Claverie et al. 2006). This is enough evidence that Mimiviruses certainly exhibit more independence than organelles and, moreover, seem to be in an ‘evolutionary steady state’ with no apparent signs of genome reduction (Claverie et al. 2006). It is However worth to note that the fact that, mimivirus particles, like all viruses do not reproduce by division, but are replicated by the self-assembly of preformed components. This differentiates it from cellular living organisms such as bacteria. Mimivirus as alive- is the Virocell idea true or false? (Claverie’s work) The Virocell of Mimivirus is true owing to available evidence. Patients with pneumonia have previously shown positive serological tests for mimivirus, and a laboratory technician working with the virus developed pneumonia and seroconverted. However, neither of these observations was definitive proof that mimivirus can cause disease, so experimental infections have been carried out in mice, which also developed pneumonia (Claverie et al, 2006). Claveria and his group also argue that the fact that other smaller viruses can infect Mimivirus is not only a true testimony that is alive but also to the facts that its relative size to other viruses. Likewise, the dependencies of the virophage lifecycle on a successful infection of an amoeboid host by Mimivirus only strengthens or justify the view of a long-standing parasitic relationship between Mimivirus and its host. The replication cycle of Mimivirus within its amoeboid host is assumed to be a long-lasting and conserved interaction; otherwise it would be very unlikely for a virophage to adapt to the specific conditions caused by Mimivirus infection (Claverie et al, 2006). What roles could viruses have played in the evolution of cells? Viruses can play a significant role in the evolution of cells. One way is through impacts of infectious disease. Viral parasites have played a major role in human evolution (de Souza Leal and Zanotto, 2000) for instance through maintaining degree of polymorphism. Linda M. Van Blerkom (2003) has indicated that the need for genetic diversity in a “host-pathogen arms race” may have even significantly contributed to the evolution of sexual reproduction. The astonishing degree of polymorphism in the major histocompatibility complex (MHC), which codes for membrane glycoproteins also known as human leukocyte antigens (HLA) in humans know to be responsible for the immune system by binding fragments of infectious origin and presenting them to T-cells is closely linked to Disease-driven selective pressure (Linda M. Van Blerkom, 2003). Lastly, viral pathogens can also affect the evolution of their hosts through direct interaction with host DNA. By virtue of their simplicity and their need to use host-cell replication and transcription machinery, viruses act as “molecular genetic parasites” consequently altering host genomes through mechanisms like recombination, retro transposition, and gene conversion (Sverdlov, 2000). Conclusion It is clear that definition of virus has come a long way. One cannot conclusively say that it is a nonliving organism. Evidence is sufficient to indicate that a virus may take characteristics and probably fall in between the two extremes, living and non-living organism as illustrated by the recently discovered Mimivirus. It is evidence that though virus has been viewed with negative connotation, it is has played a great role in the evolution of cells. References Claverie, JM, H Ogata, S Audic, C Abergel, K Shure and PE Fournier(2006) Mimivirus and the emerging concept of ‘giant’ virus. Virus Research 117: 133-144. Helen Pearson (2008) Virophage suggests viruses are alive Nature 454, 677 7 August 2008) James E. Mickle and Patricia M. Aune(2011)A Simple, Inexpensive, Dynamic, & Hands-on Exercise for Prompting Discussion of the Characteristics of Living Things The American Biology Teacher Vol. 73, No. 3 (March 2011), pp. 164-166 Jean-Michel Claverie (2006) Viruses take center stage in cellular evolution, Genome Biology, June 2006, 7:110 John Dupré and Maureen A. O’Malley (2009) Varieties of Living Things: Life at the Intersection of Lineage and Metabolism Philos Theor Biol (2009) 1:e003 Koonin, E. V. (2010) The Two Empires and Three Domains of Life in the Postgenomic Age. Nature Education 3(9):27 Marc H. van Regenmortel and Brian W.J. Mahyt (2004) Emerging Issues in Virus Taxonomy Emerging Infectious Diseases Vol. 10, No. 1, January 2004 Peter T. Macklem and Andrew Seely(2010) Towards a Definition of Life: Perspectives in Biology and Medicine, The Johns Hopkins University Press Volume 53, Number 3, Summer 2010, pp. 330-340 Read More