introduction to virus, virology, types, lytic and lysogenicpptx

A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism.[1] Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.[2][3] Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity.[4][5] Since Dmitri Ivanovsky's 1892 article describing a non-bacterial pathogen infecting tobacco plants and the discovery of the tobacco mosaic virus by Martinus Beijerinck in 1898,[6]: 4  more than 11,000 of the millions of virus species have been described in detail.[7][8] The study of viruses is known as virology, a subspeciality of microbiology.When infected, a host cell is often forced to rapidly produce thousands of copies of the original virus. When not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent viral particles, or virions, consisting of (i) genetic material, i.e., long molecules of DNA or RNA that encode the structure of the proteins by which the virus acts; (ii) a protein coat, the capsid, which surrounds and protects the genetic material; and in some cases (iii) an outside envelope of lipids. The shapes of these virus particles range from simple helical and icosahedral forms to more complex structures. Most virus species have virions too small to be seen with an optical microscope and are one-hundredth the size of most bacteria.

The origins of viruses in the evolutionary history of life are still unclear. Some viruses may have evolved from plasmids, which are pieces of DNA that can move between cells. Other viruses may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity in a way analogous to sexual reproduction.[9] Viruses are considered by some biologists to be a life form, because they carry genetic material, reproduce, and evolve through natural selection, although they lack some key characteristics, such as cell structure, that are generally considered necessary criteria for defining life. Because they possess some but not all such qualities, viruses have been described as "organisms at the edge of life"[10] and as replicators.[11]

Viruses spread in many ways. One transmission pathway is through disease-bearing organisms known as vectors: for example, viruses are often transmitted from plant to plant by insects that feed on plant sap, such as aphids; and viruses in animals can be carried by blood-sucking insects. Many viruses spread in the air by coughing and sneezing, including influenza viruses, SARS-CoV-2, chickenpox, smallpox, and measles. Norovirus and rotavirus, common causes of viral gastroenteritis, are transmitted by the faecal–oral route, passed by hand-to-mouth contact or in food or water. The infectious dose of norovirus required to produce infection in humans is fewer than 100 particles.[12] HIV is one of several viruses transmitted.

1. Contents  Definition of viruses  Discovery of viruses  Concept of viruses  Characteristics of viruses  Structure of viruses  Symmetry of viruses  Classification of viruses

2. What are viruses ? A virus is a non-cellular particle made up of genetic material and protein that can invade living cells.

3. Discovery of viruses Filtration of a mixture of bacteria and viruses . If a mixture of viruses and bacteria are filtered through a bacterial-proof filter (red), the viruses will pass through into the filtrate in the flask. Filtered beer is produced by a similar process.

4.  W h e n re p e a t e d , f i ltr a ti o n s p rod u c e d t h e same results and nothing could be seen in the filtrates using the most powerful microscopes, nor could anything be cultivated from the filtrates, Iwanowski and associates concluded that they had discovered a new pathogenic life-form which they called un i m a g i n a ti v e , b u t f u n cti o n a l , b y t h e n a m e of "FILTERABLE VIRUS".

5. The concept of virus  E dwar d Jenner ( 1 7 9 8 ) , introduce d t h e term virus in microbiology.  Virus in Greek means poison.  E dwar d Jenner notice d tha t milk maids w h o infected with cowpox develop immunity against smallpox.  He inoculated a boy with the vesicle fluid taken from the hand of infected maid.  The boy developed sustained immunity against smallpox.

6.  Edward Jenner assumed that the vesicle fluid that has been taken from the hand of the milk maid contained a poison ( virus ), that was responsible for immunity.

7. General characteristics of viruses  Viruses are smaller than bacteria, they range in size between 20-300 nanometer (nm)  Viruses contain only one type of nucleic acid, either DNA or RNA, but never both .  Viruses consist of nucleic acid surrounded by a protein coat. Some viruses have additional lipoprotein envelope .  Viruses lack cellular organelles, such as mitochondria and ribosomes.

8. General characteristics of viruses  Viruses are obligate cellular parasites. They replicate only inside living cells.  Viruses replicate through replication of their nucleic acid and synthesis of the viral protein.  Viruses do not multiply in chemically defined media.  Viruses do not undergo binary fission .

9. General structure of viruses  Viruses composed of nucleic acid either DNA or RNA, surrounded by a protein coat called the capsid.  The capsid is composed of small structural units called capsomeres.  The capsid protects nucleic acid from inactivation by the outer physical conditions. CAPSOMERES

10. General structure of viruses  Some viruses have additional lipoprotein envelope , composed of virally coded protein and host lipid. The viral envelope is covered with glycoprotein spikes  Some viruses have enzymes inside the virion. All ss- RNA viruses with negative polarity have the enzyme transcriptase ( RNA dependent RNA polymerase) inside virions.  Retroviruses and hepatitis B virus contain the enzyme reverse transcriptase.

11. Symmetry of viruses Viruses are divided into three groups, based on the morphology of the nucleocapsid and the arrangement of capsomeres .  Cubic symmetry : The virus particle is icosahedral in shape (almost spherical particle ) and the nucleic acid contained inside the capsid. The icosahedrons particle is composed of 20 equilateral triangles , 12 vertices and has 2,3,5 rotational symmetry.

12. Helical symmetry : The virus particle is elongated or pleomorphic (not spherical), and the nucleic acid is spiral. Caposomeres are arranged round the nucleic acid

13. complex symmetry: The virus particle does not confirm either cubic or helical symmetry.

14. Baltimore classification Viruses were divided into six groups based on the their nucleic acid and m-RNA production.  1- ds-DNA viruses.  2- ss-DNA viruses.  3- ds- RNA viruses.  4- ss-RNA viruses with positive strands( positive polarity).  5 - s s - R N A v i r use s w it h n ega t i v e s t r a nds ( n e ga t i v e polarity).  6- ss-RNA viruses associated with the enzyme reverse transcriptase .