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Information about VIRUS

Published on September 18, 2010

Author: aditya0291


Slide 1: VIRUS (Latin : poison) In 1892 Dmitry Ivanovsky first discovered virus. Slide 2: Viruses Are non cellular organisms made up of genetic material and protein that can invade living cells. Size 20-200 nm Always have at least two parts: --an outer capsid composed of protein subunits --an inner core of nucleic acid DNA or RNA May be surrounded by membranous envelope May also contain various proteins, especially enzymes such as polymerases Slide 3: Each kind of virus usually infects one kind of organism and/or one kind of tissue, Viruses are called obligate intracellular parasites, which means they cannot live outside of a living cell e.g. hepatitis virus only infects liver cells Believed that viruses are derived from the kind of cell they infect and must have evolved after cells They can mutate and, therefore, they evolve; e.g. flu viruses are constantly mutating Slide 4: GENERAL PROPERTIES OF VIRUSES EXTRACELLULAR STATE Minute particle containing nucleic acid surrounded by protein and occasionally other macromolecules. In this state virus particle(virion) is metabolically inert. Virion is the structure by which the viral genome is transferred from the host cell in which it was made to another host to another host cell INTRACELLULAR STATE Once inside a host cell viral nucleic acid is released. Viral replication occurs New copies of genome produced. Viral coat components synthesized. Infection – process that occurs when a virus enters and replicates. ORIGIN : ORIGIN Viruses are found wherever there is life and have probably existed since living cells first evolved. The origin of viruses is unclear because they do not form fossils, so molecular techniques have been the most useful means of investigating how they arose. These techniques rely on the availability of ancient viral DNA or RNA, but, unfortunately, most of the viruses that have been preserved and stored in laboratories are less than 90 years old. There are three main hypotheses that try to explain the origins of viruses: Regressive hypothesis  Cellular origin hypothesis  Co evolution hypothesis Virus Categories : Virus Categories DNA viruses – stable, do not mutate rapidly Single-stranded or double-stranded Smallpox, Hepatitis B RNA viruses – mutate rapidly, unstable Single-stranded or double-stranded HIV, Rhinovirus :  Genomic diversity among viruses Nucleic acid DNA RNA Both DNA and RNA (at different stages in the lifecycle) Shape Linear Circular Segmented Strandedness Single - stranded Double - stranded Double –stranded with regions of single – starndedness All viruses use the host cell translation machinery i.e. viral mRNA must be generated that can be translated on the host cell ribosome Sense Positive sense (+) Negative sense(−) Ambisense (+/−) Slide 8: Structure of viruses Core of genetic material (nucleic acid). Protein coat or capsid Protects viral genes from inactivation by adverse environmental factors. Core + capsid = Nucleocapsid In many viruses important in attachment of viruses specific receptors on host cells. Composed of a large number of subunits – capsomers Many animal virus particles are surrounded by a lipoprotein envelope. Slide 9: Capsomers Subunit structure is important Economy of genetic information – reduces the number of different proteins the genome has to encode if the viral coat is made up of repeating units of a single protein. Allows for construction the virus particles by a process of self assembly into structures held together by the non – covalent bonds as occurs in the process of crystallization – so no need for enzyme – catalyzed reaction for coat assembly. Intracellular release if the viral genome only requires dissociation of non – covalent bonds rather than degradation of a protein coat . Geometry of Capsomers : Geometry of Capsomers Helical symmetry Icosahedral symmetry Complex structure Virus Shapes : Virus Shapes Helical — spirals of many protein units called capsomers Icosahedrons — has 20 triangular subunits Bacteriophages — with polyhedral head, tail sheath, and tail fibers May also have membranous envelope and glycoprotein spikes. A generalized viral reproduction cycle : A generalized viral reproduction cycle 1) Entry into the host cell -injection -membrane fusion 2) Replication and Translation of the genetic material -using the host cells genetic machinery 3) Assembly and release of the new viral particles -lysis of host cell -budding from the host cell 4) Reproductive cycles are classified as follows -LYTIC CYCLE -LYSOGENIC CYCLE VIRAL REPRODUCTION : VIRAL REPRODUCTION LYTIC REPRODUCTION LYSOGENIC REPRODUCTION Lytic cycle may be divided into five stages: Attachment, Penetration, Biosynthesis, Maturation, Release. Phage becomes a prophage that is integrated into the host genome. Becomes latent, and later may reenter the lytic cycle. Slide 14: Protein represses most of the other phage genome Environmental trigger Slide 15: Replication of different viruses types DNA viruses The genome replication of most DNA viruses takes place in the cell's nucleus. If the cell has the appropriate receptor on its surface, these viruses enter the cell by fusion with the cell membrane or by endocytosis. Most DNA viruses are entirely dependent on the host cell's DNA and RNA synthesizing machinery, and RNA processing machinery. The viral genome must cross the cell's nuclear membrane To access this machinery. RNA viruses Replication usually takes place in the cytoplasm. RNA viruses can be placed into about four different groups depending on their modes of replication. The polarity (whether or not it can be used directly to make proteins) of the RNA largely determines the replicative mechanism, and whether the genetic material is single-stranded or double-stranded. RNA viruses use their own RNA replicase enzymes to create copies of their genomes. Slide 16: Reverse Transcriptase Virus These replicate using reverse transcription, which is the formation of DNA from an RNA template. Reverse transcribing viruses containing RNA genomes use a DNA intermediate to replicate, whereas those containing DNA genomes use an RNA intermediate during genome replication. DNA genomes use an RNA intermediate during genome replication. Both types use the  reverse transcriptase enzyme to carry out the nucleic acid conversion. Effects on the host cell : Effects on the host cell The range of structural and biochemical effects that viruses have on the host cell is extensive. These are called cytopathic effects. Most virus infections eventually result in the death of the host cell. The causes of death include cell lysis, alterations to the cell's surface membrane and apoptosis. Often cell death is caused by cessation of its normal activities because of suppression by virus-specific proteins, not all of which are components of the virus particle. Some viruses cause no apparent changes to the infected cell. Cells in which the virus is latent and inactive show few signs of infection and often function normally. This causes persistent infections and the virus is often dormant for many months or years. This is often the case with herpes viruses Some viruses, such as Epstein-Barr virus, can cause cells to proliferate without causing malignancy, while others, such as papillo maviruses, are established causes of cancer. Slide 18: Epidemiology Viral epidemiology is the branch of medical science that deals with the transmission and control of virus infections in humans. The rate or speed of transmission of viral infections depends on factors that include population density, the number of susceptible individuals, (i.e. those who are not immune), the quality of health care and the weather. Transmission of viruses can be vertical, that is from mother to child, or horizontal, which means from person to person. Examples of vertical transmission include hepatitis B virus HIV where the baby is born already infected with the virus Another, more rare, example is the varicella zoster virus, which although causing relatively mild infections in humans, can be fatal to the fetus and newly born baby. Vaccines : Vaccines Vaccination is a cheap and effective way of preventing infections by viruses. Vaccines were used to prevent viral infections long before the discovery of the actual viruses. Their use has resulted in a dramatic decline in morbidity (illness) and mortality (death) associated with viral infections such as polio, measles, mumps & rubella. Vaccines are available to prevent many viral infections of humans, and more are used to prevent viral infections of animals.  Vaccines can consist of live-attenuated or killed viruses, or viral proteins (antigens). Live vaccines contain weakened forms of the virus, which do not cause the disease but nonetheless confer immunity. Such viruses are called attenuated. Live vaccines can be dangerous when given to people with a weak immunity, because in these people, the weakened virus can cause the original disease. These vaccines use only the capsid proteins of the virus. Hepatitis B vaccine is an example of this type of vaccine. Subunit vaccines are safe for immuno –compromised patients because they cannot cause the disease. The yellow fever virus vaccine, a live-attenuated strain called 17D, is probably the safest and most effective vaccine ever generated. Antiviral drugs : Antiviral drugs Antiviral drugs are often nucleoside analogues, (fake DNA building blocks), which viruses mistakenly incorporate into their genomes during replication. The life-cycle of the virus is then halted because the newly synthesized DNA is inactive. This is because these analogues lack the hydroxyl groups, which, along with phosphorus atoms, link together to form the strong "backbone" of the DNA molecule. This is called DNA chain termination. Examples of nucleoside analogues are Aciclovir for Herpes simplex virus infections and lamivudine for HIV and Hepatitis B virus infections.  Aciclovir is one of the oldest and most frequently prescribed antiviral drugs. Other antiviral drugs in use target different stages of the viral life cycle. HIV is dependent on a proteolytic enzyme called the HIV-1 protease for it to become fully infectious. There is a large class of drugs called protease inhibitors that inactivate this enzyme. Slide 21: Applications Life sciences and medicine Material science and nanotechnology Synthetic viruses BioWeapons Slide 22: Presented by Aditya Ramamurthy Sanket Karandikar Smit Mehta Sapna Mishra Nikhil Mohanan Ankita Panchal Shivkumar Krishnan Priyank Dhakan MTI 09001 MTI 09023 MTI 09031 MTI 09032 MTI 09033 MTI 09034 MTI 09041 MTI 09055

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