Module: Biochemical Processes______
All viruses are pathogens. Some viruses are deadly, for example, HIV and Ebola virus. Unlike bacteria, fungi and protists, viruses do not fit into the framework of cellular organization that is the hallmark of organisms. Bacteria, fungi and protists are cells but not virus. Viruses have unique structural and functional properties and cannot reproduce unless they are inside the cell. In fact, they can neither generate energy nor make proteins. Virus are extremely small particles comprise mainly of protein and nucleic acid. The range of virions in size is from 10 to 400nm in diameter. Most viruses are too small to be visible in the light microscope and must be viewed with electron microscope. A complete virus particle is called a virion. All virions have nucleic acid contained in a protective coat of protein called capsid. Together, the nucleic acid and capsid is known as nucleocapsid. This capsid is made of subunits called capsomeres. It is used to protect the viral genome and helps in it transfer between host cells. Some other viruses such as HIV and Ebola virus have their nucleocapsid contained within a membranous envelope that usually is covered with projections or spikes. Although all viruses contain nucleic acids, the state of their nucleic acid varies. Firstly, in contrast to all cells, the virions of all but a single type of virus contain only one type of nucleic acid. They might have DNA, or only RNA. For example, rhinoviruses that causes common cold has only single stranded RNA while virus that causes chickenpox has only double stranded DNA. Not only that, the nucleic acid may exist as one large molecule or it may be divided into a set of smaller molecules. For instance, the nucleic acid in the viruses that causes influenza consists of eight separate segments of single-stranded RNA. Besides that, the nucleic acid may be linear or by forming a closed circle. For example, the DNA of chicken pox virus is linear while the DNA of the viruses that causes warts is circulized. Furthermore, viral nucleic acids range in molecular weight from less than 2million to more than 200million. The smallest virus is estimated to have fewer than five genes, whereas the largest possess several hundred. Lastly, the nucleic acid of some viruses is packed into the capsid similar to a box, while for others, the nucleic acid is assembled together with the capsomeres and is integrated into the structure of the nucleocapsid. The shapes of capsomeres in nucleocapsid are different depending on its location within the capsid structure. Capsomeres are assembled from copies of only a single protein or complexes of several different proteins. Depending on the particular virus, a nucleocapsid may have one of three different morphological forms which consist of helical, icosahedral or complex form. In helical form, the nucleocapsids are shaped like hollow tubes with protein walls. While for icosahedral form, it is constructed from ring-to-knob-shaped capsomeres usually made up of five or six protomeres. The nucleic acid is packaged inside a capsid that forms a shell, usually with the geometry of an icosahedron. For complex form, it is not readily categorized as either helical or cubic as the nucleocapsid might have both helical and polyhedral components which possess an exceptionally complex internal structure. The nucleocapsid of many viruses is surrounded by a membrane called the envelope. Viral envelopes are derived from cellular membranes and are acquired when nucleocapsids leave a cell by the process of budding. Many enveloped viruses display some of their unique envelope proteins on the envelop surfaces as spikes. The spike plays a crucial role during the interaction of a virus with a cell. The spikes on different virus carry out different functions. Virus cannot reproduce themselves. They can only reproduce inside a cell, after infecting the host cell. There is a few key phase of...
References: 1. Joanne M.Willey, Christopher M.Sherwood, Christopher J.Woolverton,2011. Prescott 's Microbiology. 8th ed. New York: McGraw-Hill.
2. Barry, L.Batzing, 2002. Microbiology. 1st ed. United States of America: Wadsworth Group, Thomson Learning, Inc.
Please join StudyMode to read the full document