Biological Diversity - Viruses - Literature review Example

VIRUSES Abstract Tortora et al (2013) describes microorganisms as living organisms that are so small the human being cannot see them with naked eye. A special device called a microscope, which magnifies the size of an image to beyond a hundred times its size, is required to view them. Microorganisms, also known as microbes, are responsible for many diseases that plague the human body. However, as Tortora et al (2013) explains, they offer very many benefits to human beings, even huge commercial value. They include bacteria, fungi, protozoa, microscopic algae, and viruses. Tortora et al (2013, 27) defines bacteria as a simple organism made up of a single cell that is not enclosed in any special nuclear membrane. The cell wall comprises of carbohydrates and proteins referred to as ‘peptidoglycan’. Bacteria also go by the name ‘prokaryotes’ i.e. a ‘pronucleus’ microorganism that can easily attach and fuse with another microorganism because of its light outer membrane. Bacteria have the capability of manufacturing their own food through photosynthesis, and can consume inorganic material. Again, Tortora et al (2013, 30) informs different bacteria assume unique spherical, rod-like, spiral, star or square shape. Different species of bacteria form chains, pairs, spirals, clusters, and so on. A bacteria reproduces through binary fission that involves own subdivision into two equal cells. It acquires motion through moving appendages referred to as ‘fragella’. On the other hand, viruses are infectious organisms that are smaller than bacteria. Farabee (2007) describes viruses as sub-microorganisms made up of a nucleic acid core enclosed in a protein coat. Unlike bacteria, viruses cannot perform metabolism, instead they are parasitic to the cells that they infect. Tortora et al (2013, 396) explains that outside the host cell, the virus remain inert. Arguably then, virus are not living microorganisms. However, viruses are intracellular parasites because one inside the host cell, viruses can reproduce, multiply, and even mutate. According to Tortora et al (2013, 396), a clinical view of viruses reveals them as ‘alive’ because once inside the host cell, they multiply using the synthesizing machinery of the cell, and cause infection and disease. Interestingly, viruses can synthesize the transfer of the viral nucleic acid to other cells, thus making them hosts of the viruses and causing them to become diseased. Some of the most salient differences between viruses and bacteria include; 1) Bacteria can contain both deoxyribo nucleic acid (DNA) and retro nucleic acid (RNA) while viruses contain only one. 2) Bacteria reproduce through binary fission while viruses multiply inside the host cell. 3) Bacteria are pronucleus as they have the plasma membrane while viruses have a protein coat. 4) Bacteria can hardly pass through bacteriological filters while viruses can easily pass through because they are much smaller. Tortora et al (2013, 396) further explains that the identity of a virus lies in its simple structural organization and the mechanisms they employ in order to multiply. A virus can have only one type of nucleic acid, either RNA or DNA but not both. According to Farabee (2007), two parts make up a virus. The outer part of a virus is a protein coat known as a ‘capsid’. However, on some occasions carbohydrates, lipids and more proteins may enclose this protein coat. The inner part is the nucleic acid enclosed by the ‘capsid’. Besides the proteins, a viral particle also contains enzymes such as ‘polymerases’, who function is to produce more viral RNA or DNA. Viruses take advantage of the synthesizing machinery of the host cell to perfume two tasks. One is to reproduce many other copies of themselves. This way the virus spreads much faster causing disease more effectively, while weakening or destroying the host cells. Further, Tortora et al (2013, 396) explains that the essence of taking over the cell’s machinery is to sustain the virus since the virus has few or no metabolic enzymes. A virus cannot manufacture or metabolize its own food so it drains the host cell. Two, they cause the synthesis of specialized infection and spread structures that enable the transfer of the viral nucleic acid into other cells. Once this happens, the virus manages to weaken and take over the existing nucleic acid of the infected cell. The infected cell thus becomes a new producer of more viruses and transferable viral nucleic acid. These two factors form a major complication for researchers of anti-retroviral drugs because killing the virus would mean killing the host cell, effectively causing death to the patient. Farabee (2007) further explains that the classification of viruses depends on the shape of the protein coat and the type of nucleic acid that it contains. Interestingly, he states that viruses infect different types of cells, including bacteria. However, specific types of viruses tend to infect specific hosts, for example, HIV affects only certain human or primate cells, rabies virus infects mammals only, and hepatitis virus infects only liver cells, and so on. The following images extracted from Farabee (2007) illustrate the structural organization of viruses. According to Farabee (2007), the replication process of the virus begins with the virus attaching itself to the host cell. Some portions of the ‘capsid’ have elongated spikes which the virus uses to attach itself to and penetrate the plasma membrane of the target host cell. This way the viral RNA or DNA seeps into the host cell where it uses the cell’s synthesizig mechanism to code for the production of proteins similar to the ‘capsid’, thus producing ‘bacteriophages’. Farabee (2007) describes ‘bacteriophages’ as viruses made up of DNA and proteins. The ‘bacteriophages’, designed to attack and kill bacterial cells, take over the host cell and use its metabolism mechanism to multiply viruses by replication. The high number of viruses inside the cell causes it to burst, effectively releasing thousands of viruses into the blood stream and body fluids to infect new cells and thus spread disease and infection. The bursting of the cell spells its death. The body structure of a ‘bacteriophage’ comprises of a head, neck, collar, sheath, base plate, and tail fibre. The head contains the DNA. The following image extracted from Farabee (2007) shows an illustration of a ‘T-bacteriophage’ virus; The process of replication is either ‘lytic’ or ‘lysogenic’. Farabee (2007) informs ‘bacteriophages’ may also insert their viral DNA into the host DNA. There are situations where the viral DNA can detach under certain conditions and simply direct the replication of new viruses until the cell finally dies. Interestingly, viruses sometimes have some genes that enable them to use the host cell’s enzymes, ‘ribosomes’ and transfer RNA (tRNA) to reproduce and then exit the cell, to re-infect other cells. Farabee (2007) explains that the ‘lytic’ cycle occurs when the virus enters the cell, takes over its metabolism mechanism, and produces many more viruses until the cell bursts releasing the replicated virus. The ‘lysogenic’ cycle is more subtle. Upon entering the bacterium, the virus integrates its DNA with the DNA of the bacterium, without destroying the bacterium. When this occurs, the viral DNA becomes a ‘prophage’ and the host bacterium becomes a ‘lysogenic’ cell. When the host cell reproduces by binary fission, the ‘prophage’ replicates; therefore, each new cell carries a copy of the ‘prophage’. The host cells remain intact despite the fact that they are already infected. However with time, when the right conditions emerge, for example ultra-violet radiation, this triggers the ‘prophage’ to proceed into the ‘lytic’ stage through biosynthesis, leading to the release of replicated viruses and the death of the host cell. Several human and animal viruses use this technique to propagate without killing the host cell, instead using it as a ‘viral factory”. The replicated viruses escape the confines of the cell by budding i.e. slowly seeping out of the host cell’s plasma membrane, for example HIV. Animal viruses attach themselves to the host using glycoprotein spikes (Farabee, 2007). Using ‘endocytosis’ the entire virus penetrates the cell, sheds it protein coat, and begins biosynthesis. The new viruses exit the host bacterium through budding, picking their protein coats from the plasma membrane as they exit. On the other hand, retroviruses, according to Farabee (2007), use RNA and not DNA. They contain an enzyme called ‘reverse transcriptase’, which gives them the ability to convert the RNA sequence into a DNA strand inside the host cell. The viral DNA can now integrate with the host DNA and enjoy replication every time the host cell replicates by binary fission. The viral DNA may undergo transcription later releasing more viruses through biosynthesis and budding. Other forms of viruses include ‘Viroids’ and ‘Prions’. Farabee (2007) describes ‘viroids’ as infectious virus particles made up of nucleic acid but lacking the protein coat. On the other hand, ‘Prions’ are infectious proteins, for example the mad cow disease ‘prion’. Virus usually target specific hosts, for example, some viruses can affect plants only, and others can affect fungi only, others vertebrates only, others invertebrates only, others protests, and bacteria. Tortora et al (2013, p.371) explains that the host range is the array of host cells that a specific virus can infect. A virus will usually target specific cells of a specific species. On very rare occasions, viruses affect more than one species, for example the influenza virus H3N2 from human beings also affects pigs, the avian flu H5 and H7 from birds can affect human beings directly or through an intermediary like a pig, and so on. Bacteriophage or phages are viruses that infect bacteria. Tortora et al (2013, p.371) states that viruses target specific cells because of the availability of certain chemical cellular components that support the replication of that virus. The chemical ability to attach the virus to the receptors on the cell plasma membrane provides a strong bond between the virus and the host cell. According to Leland et al (2007), the purpose of viral culture is to identify the presence of a particular type of virus from samples collected from affected people or animals, so that the necessary interventions may apply. As earlier explained, viruses cannot grow outside the host cell. They cannot perform metabolism. This presents a major challenge for growing viral culture, unlike bacteria, which have metabolism capability. Farabee (2007) explains that for viruses to grown in a lab, the scientist must provide living cells for the virus. Some good media for virus culture include live chick embryos, and propagation through cell tissue culture. Tortora et al (2013, p.371) emphasizes that microorganisms will grow only if the right conditions are provided for them such as temperature, PH, pressure, and so on. Leland et al (2007) explains that the first step is to provide several cell cultures such as rhesus monkey, rabbit kidney cells, human lung carcinoma cells, and so on. The scientist then adds the collected samples to each cell culture tube, a process called inoculation. He or she then incubates the inoculated cell tubes under the right temperature of around 36oC. The different cell cultures provide hosts for the virus. The scientist periodically views the cells under a microscope. When the virus propagates on a specific cell culture, the scientist is able to identify the specific type of viral infection. References Leland, D.S., Ginochio, C.C. (2007, January). Role of Cell Culture for Virus Detection in the Age of Technology. Clinical Microbiology Reviews. Retrieved September 6, 2013, from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1797634/ Farabee, M.J. (2007a). Biological Diversity: Viruses. Retrieved September 5, 2013, from http://www2.estrellamountain.edu/faculty/farabee/biobk/BioBookDiversity_1.html Farabee, M.J. (2007b). On-line Biology Book: Glosary. Retrieved September 6, 2013, from http://www2.estrellamountain.edu/faculty/farabee/biobk/BioBookglossB.html #bacteriophages Tortora, G.J., Funke, B.R., Case, C.L. (2013). Microbiology – An Introduction. 11th ed. Pearson Education Inc, Glenview. Read More