Viruses Notes PDF

The Viruses  VIRUSES Definition: Viruses are ultra-microscopic, non-cellular living particles, composed solely of a nucleic acid (DNA or RNA) core, surrounded by a protein envelope called capsid. Characteristics of Viruses: The major distinguishing char...

The Viruses  VIRUSES Definition: Viruses are ultra-microscopic, non-cellular living particles, composed solely of a nucleic acid (DNA or RNA) core, surrounded by a protein envelope called capsid. Characteristics of Viruses: The major distinguishing characteristics of viruses are given below. 1. They are non-cellular and very simple in structure, consisting mainly of a nucleic acid surrounded by a protein envelope called capsid. Therefore, a unit of virus is referred to as ‘a virus particle’ rather than ‘a virus cell’. 2. They are devoid of the sophisticated enzymatic and biosynthetic machinery essential for independent activities of cellular life. Therefore, they can grow only inside suitable living cells. That is why; they are cultivated in the laboratory only inside living cells, unlike bacteria and fungi, which can be cultivated in the laboratory on non-living matter like nutrient agar. 3. They are ultra-microscopic and can only be visualized under electron microscope. 4. They do not increase in size. 5. They can pass through filters, through which bacteria cannot pass. 6. A virus is called either ‘DNA virus’ or ‘RNA virus’ depending on whether it contains the nucleic acid DNA or RNA. A virus cannot have both DNA and RNA  Structure of Viruses: Much of our knowledge about viruses has been gathered from the study of few viruses, which infect bacteria. These viruses, which infect bacteria, are called ‘bacteriophages’ or ‘phages’. The bacteriophages were first described in 1915 almost simultaneously by Twort and d’Herelle. The name bacteriophage, which in Greek means to eat bacteria, was coined by d’Herelle because of the ability of these viruses to destroy the infected bacteria cells through lysis. Bacteriophages exhibit notable variability in their size, shape and complexity of structure. The T- even (T2, T4 and T6) phages demonstrate the greatest morphological complexity (Figure 8.1).  The functions of its structural components are as follows: 1. Capsid (Protein Coat): Protection of nucleic acid from destruction by DNase. 2. Nucleic Acid Core: Phage genome carrying genetic information necessary for replication of new phage. 3. Spiral Protein Sheath: Retracts, so that nucleic acid can pass from capsid into host cell’s cytoplasm. 4. End Plate and Tail Fibers: Attachment of phage to specific receptor sites on a susceptible host’s cell wall.  Viral Infection Phage replication depends on the ability of the phage particle to infect a suitable bacterial host cell. Infection consists of the following sequential events: 1. Attachment Tail portion of the phage particle binds to receptor sites on host’s cell wall. 2. Injection (penetration) Spiral protein sheath retracts and an enzyme, early muramidase, perforates the bacterial cell wall enabling the phage nucleic acid to pass through the hollow core into the host cell’s cytoplasm. The empty protein shell remains attached to the cell wall and is called the protein ghost. 3. Synthesis The phage genome subverts the synthetic machinery, which is then used for the production of new phage components. 4. Assembly The new phage components are assembled and form complete, mature virulent phage particles. 5. Release Late muramidase lyses the cell wall, liberating infectious phage particles that are now capable of infecting new susceptible host cells, thereby starting the cycle over again.  Shape of Viruses The shape of some of the important viruses has been revealed by electron microscope (Figure 8.2). All of them cause different diseases in plants and animals. The bacteriophages infect bacteria cells. Introduction Introduction  Introduction The term “viroid” applies to infective virus particles that do not have protein capsids  Introduction Viral diseases greatly reduce the productivity of many agricultural crops  Introduction (of worldwide distribution) which is currently prevalent in Liberia, Nigeria, Sierra Leone and Congo DR Nature of Viruses  Nature of Viruses The protein coat is in form of a which is known as the Nature of Viruses Nature of Viruses Nature of Viruses Classification of Viruses  Mechanism of Viral Infection The virus operates in the same way as a computer programme When a virus is inside the host cell, the virion sheds off its protein coat to release its nucleic acid Within the cell, the RNA or DNA of the virus multiplies by taking over the cell’s genetic machinery This then gives rise to the production of more nucleic acid and protein to assemble additional viral particles Viral particles are thus capable of producing high numbers of genome copies in infected cells  Mechanism of Viral Infection This means that when the virus infects a host cell, the viral genome induces enzymes of the host cell to make several copies of a particular virus This concept has been extended to the phenomenon of computer viruses In computing systems, the computer viruses perform similar actions by taking over the programmed functions of a computer so as to direct its operations  Mechanism of Viral Infection But when viruses are outside the cells, they are metabolically inert (or inactive)  Mechanism of Viral Infection But when viruses get inside a host cell, they will hijack the to produce their own Such viral actions cause disorders to the host and such disruption to normal physiological function of the animal or plant is referred to as a Important Plant Viral Diseases Cassava Mosaic Virus Maize Streak Virus (MSV) Maize Streak Virus Important Human Viral Diseases Ebola Viral Disease (EVD) Ebola Virus Disease  Mode of Infection a. The Ebola virus may be transmitted to humans by contact with an infected animal’s body b. Human to human transmission can also occur from an infected person (this can include the process of embalming dead human bodies) c. By contact with contaminated medical equipment such as needles and syringes d. By contact via semen which is infectious for a period of up to 50 days in survivors  Mode of Infection e. Transmission through oral exposure by way of mouth to mouth contacts with an infected person f. Transmission through conjuctiva exposure (i.e. conjuctiva is a delicate mucous membrane that covers the front of the eye and lines the inside of the eyelids – and this has been confirmed in primates (baboons, chimpanzees, gorillas, monkeys) g. The virus has also been reported to be transmitted without contact from pigs to wild primates  Mode of Infection h. Another route has been reported where bats drop partially eaten fruits, where land animals such as gorillas and duikers feed on these fallen fruits, thus creating a chain of indirect means of transmission from the natural host to animal populations i. Human consumption of bush meat has also been linked to the transmission of virus caused diseases j. But transmission between natural reservoir and humans is rare k. Hence, research has been embarked on viral shedding in the saliva of bats  Symptoms Symptoms of EVD start 2 – 21 days after contracting the virus Indications of infection are a fever, sore throat, muscle pains and headaches These symptoms are followed by nausea (a feeling of a sensation to vomit), onset of actual vomiting punctuated with diarrhea, followed with decreased function of the liver and kidneys The virus also attacks connective tissues leading to extensive hemorrhage and death – peak mortality of 50 – 90 % if not treated Rabies Acquired Immune Deficiency Syndrome (AIDS) Acquired Immune Deficiency Syndrome (AIDS) Acquired Immune Deficiency Syndrome (AIDS) Mechanism of HIV Infection Mechanism of HIV Infection Mechanism of HIV Infection  Mechanism of HIV Infection Contact with HIV-positive persons bring about the risk of viral infection LYTIC AND LYSOGENIC CYCLES  Lytic Cycle With lytic phages, bacterial cells are broken open (lysed) and destroyed after immediate replication of the virion. As soon as the cell is destroyed, the phage progeny can find new hosts to infect. An example of a lytic bacteriophage is T4, which infects E. coli found in the human intestinal tract.  Lysogenic Cycle In contrast, the lysogenic cycle does not result in immediate lysing of the host cell. Those phages able to undergo lysogeny are known as temperate phages. Their viral genome will integrate with host DNA and replicate along with it fairly harmlessly, or may even become established as a plasmid. The virus remains dormant until host conditions deteriorate, perhaps due to depletion of nutrients; then, the endogenous phages (known as prophages) become active. At this point they initiate the reproductive cycle, resulting in lysis of the host cell. As the lysogenic cycle allows the host cell to continue to survive and reproduce, the virus is reproduced in all of the cell’s offspring.