Viruses

VIRUSES General Information About Viruses:  Noncellular entities that are parasites of cells (acellular)       Consist of Nucleic acid core (DNA or RNA) surrounded by a protein coat called capsid. An envelope (lipid membrane) may or may not surround the coat. Capsid is made of proteins called capsomeres. Envelope has proteins embedded in it called spikes. They reproduce only by using the cellular machinery of host organism. Virionis a complete fully developed viral particle composed of nucleic acid surrounded by a coat.    Viruses are very small relative to the size of a living cell. Could only be seen with Electron microscope. Size is a range from 20 to 14,000 nm (nanometers) in length. The information the virus can carry is not enough to allow it to make copies (replicate). General Morphology: 1. Helical viruses (for example, Ebola virus) resemble long rods and their capsids are hollow cylinders surrounding the nucleic acid. 2. Polyhedral viruses (for example, adenovirus) are many-sided. Usually the capsid is icosahedrons. 3. Envelope viruses are covered by an envelope and are roughly spherical but highly pleomorphic. There are also enveloped helical viruses (for example, Influenza virus) and enveloped polyhedral viruses (for example, Herpevirus). 4. Complex viruses have complex structures. For example, many bacteriophages have a polyhedral capsid with a helical tail attached. Naming Viruses: 1. Virus family names end in –viridae; genus names end in –virus; specific epithets have not been assigned. Ex. Family---------Herpesviridae & genus---------Human Herpes Virus 2. A viral species is a group of viruses sharing the same genetic information and ecological niche. What things can become infected by a virus? So far, there is not a living thing identified that doesn’t have some sort of susceptibility to a particular virus. Plants, animals, bacteria-every living thing, whether multicellular or single-celled, can be infected with a virus specific for the organism. And, within a species, there may be 100 or more different viruses which can infect that species alone. So, whenever viruses are discussed, they are discussed as being either plant, animal or bacteria viruses-which means that an animal virus only infects a certain animal, and a plant virus only infects particular plant. We say that a virus is specific for a particular thing if the virus infects only that thing. So, there are viruses which infect only humans (smallpox), some which infect humans and one or two additional kinds of animals (influenza), some which infect only a particular kind of plant (tobacco mosaic virus), and some which infect only a particular species of bacteria (lambda bacteriophage which infects E. coli). *Multiplication of Bacteriophages:  Phages multiply by 2 mechanisms: a. Lytic Cycle: phage causes the lysis and death of the host cell. b. Lysogenic Cycle: phages have their DNA incorporated as a prophage into the DNA of the host cellhere the host cell remains alive. A. Lytic Cycle (T-even Phage) o Attachment phase: Phages attach to host cell. o In penetration: phage penetrates host cell and injects its DNA. o In biosynthesis: phage DNA directs synthesis of viral components by the host cell. o During maturation: Viral components are assembled into virions. o During release: Host cell lyses and new virions are released. B. Lysogenic Cycle (lambda phage) o Phage attaches to host cell and injects DNA. The originally linear phage DNA forms a circle. (Enters either lytic cycle or lysogenic cycle. o The circle can recombine with and become a part of the circular bacterial DNA (this is called a prophage). o Every time host cells machinery replicates the bacterial chromosome it also replicates the prophage DNA. o Occasionally, (because of a mutagen) leads to the excision (pooping-out) of the phage DNA and to initiation of lytic cycle. DNA-Containing Viruses:  Adenoviridae: Cause of some common colds; named after adenoids.   Poxviridae: Infections such as smallpox. Poxes are pus-filled sacs on skin. Herpesviridae: Named after the spreading (herpetic) appearance of cold sores. Official names are human herpesviruses (HHV) numbered for identification. o HHV-1 (Herpes simplex 1) o HHV-2 (Herpes simplex 2) o HHV-3 (Varicella, or chickenpox virus) o HHV-4 (Epstein-Barr virus) o HHV-5 (Cytomegalovirus) o HHV-6 (Roseolovirus) o HHV-7 (mostly infecting infants) o HHV-8 (Probable cause of Kaposi’s sarcoma) Papovaviridae: Named for papillomas (warts), polymas (tumors). Hepadnaviridae: Named because they cause hepatitis.   RNA Viruses:  Picornavidae (from pico, meaning small, and RNA): single-stranded RNA viruses such as poliovirus.    Togaviridae (from toga, or covering): differ from picornaviruses in that two types of mRNA. One codes for capsid proteins and the other for envelope proteins. Rhabdoviridae (from rhabdo-, rod): bullet-shaped (such as the rabies virus). Reoviridae (from reverse transcriptase): Some retroviruses cause cancers, and one type has been implicated as the cause of acquired immune deficiency syndrome (AIDS). These viruses carry a polymerase (reverse transcriptase) that uses the RNA of the virus to make a complementary strand of DNA. This DNA becomes integrated into the DNA of a host cell (provirus), and transcription into mRNA may then take place normally.