Virus PDF

Viruses History of virus discovery In late 1800s,botanists had been trying to find the cause of tobacco mosaic disease In1892, D. Iwanoski tried to filter the sap of infected tobacco plants  The filtrate was fully capable of producing the original disease in new hosts. Iwanoski con...

Viruses History of virus discovery In late 1800s,botanists had been trying to find the cause of tobacco mosaic disease In1892, D. Iwanoski tried to filter the sap of infected tobacco plants  The filtrate was fully capable of producing the original disease in new hosts. Iwanoski concluded that the bacteria was so small or they made a filterable toxin. Tobacco Mosaic Virus Definition An infective agent that typically consist of a nucleic acid molecule in a protein coat, is too small to be seen by light microscope, and is able to multiply only within the living cells of a host.  Introduction to Viruses Virus are not classified into any of the biological classification system. They lie in the threshold of life and nonlife Non-Life characteristics- Can't carry out metabolic activities independently because of the absence of the cell nucleus/organelles/cytoplasm (Acellular), Can't move and reproduce on their own Life characteristics- Can only reproduce within the living cells that they infect and use their genetic information to force the host cell to replicate themselves Viruses range in size from 20nm-300nm.Can't see by a light microscope.  Structure A virus consist of; 1. Core - Nucleic acid (DNA or RNA and single/ double stranded) 2. Capsid - Protein coat which functions in protecting the genetic material during the viral infection process ( Nucleic acid genome and protein capsid together called the nucleocapsid) 3. Some virus has an outer Envelope – encloses the coat and is made of parts of the previously infected cells A complete virus that consist of the genetic material (Core), protein coat (Capsid) ,an envelope along with some additional parts such as the neck, tail sheath, tail fibers, pins, and endplate is called the virion  Nucleic Acid Genome The genetic material of most viruses is double-stranded DNA (Deoxyribonucleic acid), with some exceptions where the genetic material is either single-stranded DNA ----- RNA (Ribonucleic acid). Functions Providing genetic information to make it possible to synthesize all viral proteins Helping in the synthesis of new viruses within the host organism  Protein Capsid It is the protein shell that encloses the viral genome. Capsids are made up of individual subunits called capsomeres, which form the basic structure of a virus. Based on structure, capsid can be of following types: a) Icosahedral: Capsid with a hollow, quasi-spherical structure. Twenty equilateral triangles arranged around the surface of a sphere forms an icosahedron. A particular type of icosahedral shape called prolate has an elongated symmetry found in bacteriophages. b) Filamentous: Capsid with a linear, thin, thread-like appearance. They may also be called rod-shaped or helical. c) Complex: Capsid having a combination of icosahedral and helical shape and may have a complex outer wall or head-tail morphology. Functions Enabling the virion to penetrate the host cell membrane Protecting the nucleic acid from digestion by host enzymes Containing unique sites on its surface that allow the virion to attach to a host cell  Lipid Envelope In some viruses found in animals, the nucleocapsid is surrounded by a membrane, also called an envelope. The envelope is composed of lipid-bilayer with proteins attached to them. Many viruses also develop special structures called spikes made of glycoprotein on their envelopes. Functions Helping the entry of viruses into the host organism Helping the virion to attach to specific host-cell surfaces using spikes Protecting the genetic material of virus from destruction by host enzymes Escaping identification by the host immune system  Virus Replication It works using a set of steps in which a virus recognizes and enters the host cell to modify and synthesize more virus particles. A typical virus lifecycle include following main steps: Attachment: The virus recognizes and binds to a host cell with the help of receptor molecules Entry: The entire virus or its genetic material enters the host cell cytoplasm and then into the nucleus Replication: The viral genome is copied, and its genes are expressed to make viral proteins Assembly: New viruses are formed from the copies of the genome and proteins inside the host Release: Complete viral particles leave the cell and infect other host cells. Bacteriophage Bacteriophage possess double stranded DNA inside their capsid (Protein head) The capsid functions as protection of their genetic material Their tail fibers are the base used to attach themselves to bacteria host cell The tail is the channel for their genetic material to be injected to the host cell Bacteriophage Replication There are two types of bacteriophage replication, LYTIC and LYSOGENIC cycles In a Lytic cycle, the virus destroys the host cell. It is a rapid process where the host cell undergoes lysis. In a Lysogenic cycle, the viral genome usually becomes integrated into the host cell. Lytic cycle Lysogenic Cycle Viroids  In 1961, an infective agent in potatoes has been discovered. The agent is called viroid and it is smaller than viruses with no ------------- coats Viroids are infectious -------- particle that may cause plant disease by interfering with mRNA processing  Viroids are non-coding circular RNA molecules with rod-like or branched structures. They are often ribozymes, characterized by catalytic RNA. Ribozymes are the essential elements for protein synthesis of cellular organisms as parts of ribosomes They can perform many basic functions of life and may have played a role in evolution since the beginning of life on Earth. They can cleave, join, replicate, and undergo Darwinian evolution. Characteristics of Viroids Obligate intracellular parasites Smaller than viruses Single stranded covalently closed circular RNA molecules 246-400 nucleotides long RNA does not code for any protein Do not have capsid/protein coat Prions  Pronounced "Pree-on" Shortened term for " Proteinaceous Infections Particle" Causes TSE ( Transmissible Spongiform encephalopathies) which attacks the Central nervous system -Sickness kuru (trembling in fear) -Creutzfeldt-Jakob disease (CJD) -Scrapie -Mad cow disease -Encephalopathy Structure of Prions Normal prions contain about 200-250 amino acids twisted into three telephone chord -like coils known as helices, with tails of more amino acids Prions do not contain genetic material which only contain proteins They can occur in two forms called PrP- sen and PrP- res Both PrP- sen and PrP- res are made up of the exact same string of amino acids but they have two different shapes  PrP- sen is produced by normal healthy cells. The sen stands for "Sensitive" because the protein is sensitive to being broken down. It is mainly in neurons in the brain but also found in other cell types PrP- res is the disease- causing form. Organisms with it develop spongiform disease.The res stands for "Resistant" because the protein is resistant to being broken down.  Prions are proteins that are unique in their ability to reproduce on their own and become infectious. 100 times smaller than the smallest known virus They are extremely resistant to heat and chemicals Prions are very difficult to decompose biologically. They survive in soil for many years Question 01. Fill the table accordingly and find more characteristics Virus Viroid Prion Nucleic Acid (DNA/RNA) Single strand RNA Capsid Absence Protein Present Temperature sensitivity Sensitive to high temperature Pathogenicity Infected to Kingdom Plantae Protein denaturation 02. Explain the virus structure 03. Briefly describe the virus replication process with the aid of a diagram 04. Describe the bacteriophage replication 05. Write the differences between lytic and lysogenic cycles 06. Compare the cell structure and nucleic acid of eukaryotic cells, bacteria ,virus, viroid and prions Thank you