Pharmaceutical Microbiology-I PHR 110 Chapter 5: Virus

Virus  A virus is a small infectious agent that replicates only inside the living cells of other organisms. Viruses can infect all types of life forms, from animals and plants to bacteria and archaea.  Viruses are non-cellular entities whose properties have been identified through technological advances in microscopy and tissue culture. Viruses are infectious particles that invade every known type of cell. They are not alive, yet they are able to redirect the metabolism of living cells to reproduce virus particles.  Viral replication inside a cell usually causes death or loss of function of that cell.  A virus is a small infectious agent that replicates only inside the living cells of other organisms. Viruses can infect all types of life forms, from animals and plants to bacteria and archaea.  Viruses are non-cellular entities whose properties have been identified through technological advances in microscopy and tissue culture. Viruses are infectious particles that invade every known type of cell. They are not alive, yet they are able to redirect the metabolism of living cells to reproduce virus particles.  Viral replication inside a cell usually causes death or loss of function of that cell.

Discovery of virus Beijerinck (1897) coined the Latin name “virus” meaning poison. He studied filtered plant juices & found they caused healthy plants to become sick.

Virus Vs Human battle! Tobacco Mosaic Virus Wendell Stanley (1935) crystallized sap from sick tobacco plants. He discovered viruses were made of nucleic acid and protein. Smallpox Edward Jenner (1796) developed a smallpox vaccine using milder cowpox viruses. Deadly viruses are said to be virulent. Smallpox has been eradicated in the world today virion virion, an entire virus particle, consisting of an outer protein shell called a capsid and an inner core of nucleic acid (either ribonucleic or deoxyribonucleic acid—RNA or DNA). The core confers infectivity, and the capsid provides specificity to the virus. Tobacco Mosaic Virus Wendell Stanley (1935) crystallized sap from sick tobacco plants. He discovered viruses were made of nucleic acid and protein. Smallpox Edward Jenner (1796) developed a smallpox vaccine using milder cowpox viruses. Deadly viruses are said to be virulent. Smallpox has been eradicated in the world today virion virion, an entire virus particle, consisting of an outer protein shell called a capsid and an inner core of nucleic acid (either ribonucleic or deoxyribonucleic acid—RNA or DNA). The core confers infectivity, and the capsid provides specificity to the virus.

Virion

Origin of viruses How did these become independent genetic entities? The only absolute requirement is an origin of replication in the nucleic acid.  Regressive theory  Progressive theory  Coevolution theory No compelling reason to think that RNA viruses have evolved in the same way as DNA viruses How did these become independent genetic entities? The only absolute requirement is an origin of replication in the nucleic acid.  Regressive theory  Progressive theory  Coevolution theory No compelling reason to think that RNA viruses have evolved in the same way as DNA viruses

Origin of viruses Regressive theory: viruses are degenerate forms of intracellular parasites. The leprosy bacillus, has evolved in this direction. Mitochondria and chloroplasts are often suggested to have been derived from intracellular parasites. However viruses do not have their own rRNAs or protein synthesis machinery. Also begs the question of RNA virus evolution ? Progressive theory: Cellular RNA and DNA components: Normal cellular nucleic acids that gained the ability to replicate autonomously and therefore to evolve. DNA viruses came from plasmids or transposable elements. They then evolved coat proteins and transmissibility. Retroviruses derived from retrotransposons and RNA virus from mRNA. Regressive theory: viruses are degenerate forms of intracellular parasites. The leprosy bacillus, has evolved in this direction. Mitochondria and chloroplasts are often suggested to have been derived from intracellular parasites. However viruses do not have their own rRNAs or protein synthesis machinery. Also begs the question of RNA virus evolution ? Progressive theory: Cellular RNA and DNA components: Normal cellular nucleic acids that gained the ability to replicate autonomously and therefore to evolve. DNA viruses came from plasmids or transposable elements. They then evolved coat proteins and transmissibility. Retroviruses derived from retrotransposons and RNA virus from mRNA.

Origin of viruses  Coevolution theory: Viruses coevolved with life – their evolotion might go all the way back to RNA world!

Characteristics of virus  Non living structures  Noncellular  Contain a protein coat called the capsid  Have a nucleic acid core containing DNA or RNA  Capable of reproducing only when inside a HOST cell  Some viruses are enclosed in an protective envelope  Some viruses may have spikes to help attach to the host cell  Most viruses infect only SPECIFIC host cells  Some viruses cause disease-Smallpox, measles, mononucleosis, influenza, colds, warts, AIDS, Ebola  Some viruses may cause some cancers like leukemia  Virus-free cells are rare  Non living structures  Noncellular  Contain a protein coat called the capsid  Have a nucleic acid core containing DNA or RNA  Capable of reproducing only when inside a HOST cell  Some viruses are enclosed in an protective envelope  Some viruses may have spikes to help attach to the host cell  Most viruses infect only SPECIFIC host cells  Some viruses cause disease-Smallpox, measles, mononucleosis, influenza, colds, warts, AIDS, Ebola  Some viruses may cause some cancers like leukemia  Virus-free cells are rare

Viral shapes  Viruses come in a variety of shapes  Some may be helical shape like the Ebola virus  Some may be polyhedral shapes like the influenza virus  Others have more complex shapes like bacteriophages  Viruses come in a variety of shapes  Some may be helical shape like the Ebola virus  Some may be polyhedral shapes like the influenza virus  Others have more complex shapes like bacteriophages Helical Viruses

Viral shapes Polyhedral viruses Complex viruses

Virus classification  It is the process of naming viruses and placing them into a taxonomic system.  Viruses are mainly classified by phenotypic characteristics, such as morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause.  Currently there are two main schemes used for the classification of viruses: the ICTV system; Baltimore classification system, which places viruses into one of seven groups. Accompanying this broad method of classification are specific naming conventions and further classification guidelines set out by the International Committee on Taxonomy of Viruses.  It is the process of naming viruses and placing them into a taxonomic system.  Viruses are mainly classified by phenotypic characteristics, such as morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause.  Currently there are two main schemes used for the classification of viruses: the ICTV system; Baltimore classification system, which places viruses into one of seven groups. Accompanying this broad method of classification are specific naming conventions and further classification guidelines set out by the International Committee on Taxonomy of Viruses.

Virus classification The International Committee on Taxonomy of Viruses began to devise and implement rules for the naming and classification of viruses early in the 1970s. The system shares many features with the classification system of cellular organisms, such as taxon structure. Viral classification starts at the level of order and continues as follows, with the taxon suffixes given in italics: Order ( -virales ) Family ( -viridae ) Subfamily ( -virinae ) Genus ( -virus ) Species Species names generally take the form of [Disease] virus. The International Committee on Taxonomy of Viruses began to devise and implement rules for the naming and classification of viruses early in the 1970s. The system shares many features with the classification system of cellular organisms, such as taxon structure. Viral classification starts at the level of order and continues as follows, with the taxon suffixes given in italics: Order ( -virales ) Family ( -viridae ) Subfamily ( -virinae ) Genus ( -virus ) Species Species names generally take the form of [Disease] virus.