Viruses, structure, classification and characteristics
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The document discusses viruses, defining them as infectious agents that can only multiply within host cells and consist of nucleic acids and protein coats. It covers virus structure, classification, life cycles, methods of transmission between hosts, discovery, theories of origin, and techniques for studying and cultivating viruses. The document provides an overview of the key aspects of virology, including what viruses are, how they spread and reproduce, approaches to classifying them, and historical developments in the field.
Viruses, structure, classification and characteristics
- 1. The Virus
- 2. Viruses: • Defination: • An infective agent that typically consists of a nucleic acid molecule in a protein coat, is too small to be seen by light microscopy, and is able to multiply only within the living cells of a host.
- 3. Introduction to viruses Viruses do not have cells that divide; new viruses are assembled in the infected host cell But unlike still simpler infectious agents, viruses contain genes, which gives them the ability to mutate and evolve. Evolved from plasmids : pieces of DNA that can move between cells while others may have evolved from bacteria. Over 5,000 species of viruses have been discovered.
- 4. Introduction to viruses A virus consists of two or three parts: genes, made from either DNA or RNA, long molecules that carry genetic information protein coat that protects the genes; and in some viruses, an envelope of fat Viruses vary in shape from the simple helical and icosahedral to more complex structures. Viruses range in size from 20 to 300 nanometres; it would take 30,000 to 750,000 of them, side by side, to stretch to 1 centimeter.
- 5. Spreading , Vectors: Viruses spread in many ways. Just as many viruses are very specific as to which host species or tissue they attack, Plant viruses are often spread from plant to plant by insects and other organisms, known as vectors. Some viruses of animals, including humans, are spread by exposure to infected bodily fluids Viruses such as influenza are spread through the air by droplets of moisture when people cough or sneeze. Viruses such as norovirus are transmitted by the faecal–oral route, which involves the contamination of hands, food and water.
- 6. The human immunodeficiency virus, HIV, is transmitted by bodily fluids transferred during sex. Dengue virus, are spread by blood-sucking insects. Rotavirus is often spread by direct contact with infected children. Antibiotics have no effect on viruses, but antiviral drugs have been developed to treat life-threatening infections. Vaccines that produce lifelong immunity can prevent some viral infections.
- 7. Discovery: In 1884 the French microbiologist Charles Chamberland invented a filter, known today as the Chamberland filter or Chamberland–Pasteur filter, that has pores smaller than bacteria. Thus he could pass a solution containing bacteria through the filter and completely remove them from the solution. In the early 1890s the Russian biologist Dmitri Ivanovsky used this filter to study what became known as the tobacco mosaic virus. His experiments showed that extracts from the crushed leaves of infected tobacco-plants remain infectious after filtration.
- 8. Discovery: In 1899 the Dutch microbiologist Martinus Beijerinck observed that the agent multiplied only in dividing cells. Having failed to demonstrate its particulate nature he called it a "contagium vivum fluidum", a "soluble living germ". In the early 20th century the English bacteriologist Frederick Twort discovered viruses that infect bacteria With the invention of the electron microscope in 1931 by the German engineers Ernst Ruska and Max Knoll came the first images of viruses.
- 9. Discovery: In 1935 American biochemist and virologist Wendell Meredith Stanley examined the tobacco mosaic virus and found it to be mostly made from protein. A short time later, this virus was separated into protein and RNA parts. The breakthrough came in 1931, when the American pathologist Ernest William Goodpasture grew influenza and several other viruses in fertilised chickens' eggs.
- 10. Origins Theories: Viruses co-exist with life wherever it occurs. They have probably existed since living cells first evolved. Viruses do not form fossils so molecular techniques have been the most useful means of hypothesising how they arose. Three main theories speculate on the origins of viruses: Regressive theory Viruses may have once been small cells that parasitised larger cells. Over time, genes not required by their parasitism were lost. The bacteria rickettsia and chlamydia are living cells that, like viruses, can reproduce only inside host cells.
- 11. Origins Theories: They lend credence to this theory, as their dependence on parasitism is likely to have caused the loss of genes that enabled them to survive outside a cell Cellular origin theory Some viruses may have evolved from bits of DNA or RNA that "escaped" from the genes of a larger organism. The escaped DNA could have come from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria.
- 12. Origins Theories: Coevolution theory Viruses may have evolved from complex molecules of protein and DNA at the same time as cells first appeared on earth and would have depended on cellular life for many millions of years Problems with these theories: the regressive hypothesis does not explain why even the smallest of cellular parasites do not resemble viruses in any way. The escape hypothesis does not explain the structures of virus particles. The coevolution, or virus-first hypothesis, contravenes the definition viruses, in that they are dependent on host cells.
- 13. Characteristics Obligate intracellular parasites of bacteria, protozoa, fungi, algae, plants, and animals. Ultramicroscopic size, ranging from 20 nm up to 450 nm (diameter). Not cellular in nature; structure is very compact and economical. Do not independently fulfill the characteristics of life. Inactive macromolecules outside the host cell and active only inside host cells. Basic structure consists of protein shell (capsid) surrounding nucleic acid core. • Nucleic acid can be either DNA or RNA but not both
- 14. • Nucleic acid can be double-stranded DNA, single- stranded DNA single-stranded RNA, or double-stranded RNA. • Molecules on virus surface impart high specificity for attachment to host cell. • Multiply by taking control of host cell’s genetic material and regulating the synthesis and assembly of new viruses. • Lack enzymes for most metabolic processes. • Lack machinery for synthesizing proteins. • Most RNA viruses multiply in & are released from the cytoplasm. • l Viral infections range from very mild to life threatening.
- 15. A comparison with cell
- 16. • Viruses have no nucleus, no organelles, no cytoplasm or cell membrane—Non-cellular
- 17. Size of virus ? • Smallest infectious agents • Most are so small, they can only be seen with an electron microscope • Animal viruses • Proviruses- around 20 nm in diameter • Mimi viruses- up to 450 nm in length • Viewing viruses • Special stains and an electron microscope • Negative staining outlines the shape • Positive staining shows internal details • Shadow casting technique
- 19. Structure Of virus • Questions Relating to Structure • Is it rigid? • How big is it? • Is it flexible? • Structure Must Serve Virus • It should provide protection for genome • It should allow virus to move from one host to next • It should allow for attachment of virus on to new host
- 20. Size Of Virus : Virion size range is ~10-400 nm All virions contain a nucleocapsid which is composed of nucleic acid (DNA or RNA) and a protein coat (capsid) Some viruses consist only of a nucleocapsid, others have additional components
- 22. Capsids: nucleocapsid The capsid and the nucleic acid together are called the nucleocapsid virion Fully formed virus that is able to establish an infection in a host cell
- 24. Generalized Structure: Viral components • Nucleic acids • Capsid • Envelope
- 25. Nucleic Acids: • Genome: the sum total of the genetic information carried by an organism • Number of viral genes compared with a cell- quite small • They only have the genes necessary to invade host cells and redirect their activity
- 26. DNA Viruses : • ssDND (single stranded DNA) • dsDNA (double stranded DNA)
- 28. RNA Viruses: • Mostly single-stranded • Positive-sense RNA: genomes that are ready for immediate translation into proteins • Negative-sense RNA: genomes have to be converted into the proper form to be made into protein
- 29. RNA viruses
- 32. Tools For Studying Structure : • Electron Microscopy • Excellent tool with some limitations • High resolution • Image can be a distortion due to specimen processing • X-ray Diffraction • Good for naked virions (no envelope) • Cryoelectron Microscopy
- 33. Structural Symmetries: • Icosahedral Symmetry • 20 triangular faces • It is a common capsid structure • Examples of viruses with icosahedral symmetry • Parvoviruses • These are simple viruses • ssDNA genome • Capsid is formed with 60 copies of single protein • Polio virus • Uses 180 copies of 3 subunit proteins • Much bigger virus
- 34. Capsid: • Constructed from identical subunits called capsomers • Made up of protein molecules Two different types • Helical • Rod-shaped capsomers • Assemble in to helical nucleocapsid
- 37. Icosahedral: • Three-dimensional • 20-sided figure • 12 evenly spaced corners • Although they all display this symmetry, there are wide variations
- 40. Functions of the Viral Capsid • Protects nucleic acids • Help introduce the viral DNA or RNA into a suitable host cell • Stimulate the immune system to produce antibodies that can protect the host cells against future infections
- 43. Ebola Virus: • Morphology under electron microscope • filamentous, enveloped RNA virus • approx. 19 kb in length (1 kb = 1000 RNA bases/nucleotides) or 60-80 nm in diameter • single-stranded, linear, non-segmented • negative-sense RNA (encoded in a 3’ to 5’ direction) • appears to have “spikes” due to glycoprotein on outside membrane
- 44. Poxvirus: • Brick shaped complex symmetry • 240nm x 300 nm • Enveloped • antibody neutralization sites • Core contains enzymes for virus uncoating dsDNA • Inner membrane
- 45. Viral reproduction • Viruses can reproduce only when they enter cells and utilize the cellular machinery of their hosts. Viruses’ code their genes on a single type of nucleic acid, either DNA or RNA, but viruses lack ribosomes and the enzymes necessary for protein synthesis. Viruses are able to reproduce because their genes are translated into proteins by the cell’s genetic machinery. These proteins lead to the production of more viruses.
- 46. Viral multiplication proceeds as following manner. • Adsorption, • Penetration, • Uncoating, • Synthesis, • Assembly and Release • Adsorption.
- 47. Adsorption/attachment • Virus encounters susceptible host cells • Adsorbs specifically to receptor sites on the cell membrane • Because of the exact fit required, viruses have a limited host range
- 48. Penetration • Flexible cell membrane of the host is penetrated by the whole virus or its nucleic acid • Endocytosis: entire virus engulfed by the cell and enclosed in a vacuole or vesicle • The viral envelope can also directly fuse with the host cell membrane
- 49. Uncoating • Enzymes in the vacuole dissolve the envelope and capsid • The virus is now uncoated
- 51. synthesis • Free viral nucleic acid exerts control over the host’s synthetic and metabolic machinery • DNA viruses- enter host cell’s nucleus where they are replicated and assembled • DNA enters the nucleus and is transcribed into RNA • The RNA becomes a message for synthesizing viral proteins (translation) • New DNA is synthesized using host nucleotides • RNA viruses- replicated and assembled in the cytoplasm
- 52. Assembly • Mature virus particles are constructed from the growing pool of parts
- 53. Release • Nonenveloped and complex viruses are released when the cell lyses or ruptures • Enveloped viruses are liberated by budding or exocytosis • Anywhere from 3,000 to 100,000 virions may be released, depending on the virus • Entire length of cycle- anywhere from 8 to 36 hours
- 55. Cultivation of viruses • Primary purposes of viral cultivation • To isolate and identify viruses in clinical specimens • To prepare viruses for vaccines • To do detailed research on viral structure, multiplication cycles, genetics, and effects on host cells • Using Live Animal Inoculation • Specially bred strains of white mice, rats, hamsters, guinea pigs, and rabbits • Occasionally invertebrates or nonhuman primates are used • Animal is exposed to the virus by injection
- 57. Tissue culture technique • Most viruses are propagated in some sort of cell culture • The cultures must be developed and maintained • Animal cell cultures are grown in sterile chambers with special media • Cultured cells grow in the form of a monolayer • Primary or continuous
- 58. Classification • Viruses are classified on the basis of habitat (host).which is trivial system beside this Viruses are classified on following criteria. • Structure • Chemical composition • Similarities in genetic makeup • International Committee on the Taxonomy of Viruses, which includes • 3 orders • 63 families “-viridae” • 263 genera “-virus”
- 60. Types of Classification: • 3 Types of systems were proposed to classify the viruses: • Baltimore Classification. • Classical System Classification. • Genetic Classification.
- 61. Baltimore Classification: 7 groups were made. Its principles are fundamental to an understanding of virus classification and genome replication. The Baltimore classification has + RNA as its central point. I: dsDNA viruses (e.g. Adenoviruses, Herpesviruses, Poxviruses) II: ssDNA viruses (+ strand or "sense") DNA (e.g. Parvoviruses) III: dsRNA viruses (e.g. Reoviruses) IV: (+)ssRNA viruses (+ strand or sense) RNA (e.g. Picornaviruses, Togaviruses) V: (−)ssRNA viruses (− strand or antisense) RNA (e.g. Orthomyxoviruses, Rhabdoviruses) VI: ssRNA viruses (+ strand or sense) RNA with DNA intermediate in life-cycle (e.g. Retroviruses) VII: dsDNA viruses (e.g. Heptadnaviruses)
- 63. Classification on basis of host • Animal viruses: • Viruses of animal host • Rabies, Polio, Mumps, Chicken pox, Small pox, and Influenza. • Plant Viruses: • viruses which show their live characteristics when attached to plants. • Tobacco mosaic virus (TMV), Banana streak virus, Carrot thin leaf virus • Bacterial Virus: Bacteriophages ( T1, T2, T3, and T4.)
- 65. Classification on Genetics basis • According to genetic consequences viruses are classified as. DNA Viruses and RNA Viruses • Genes may be linear or circular • The smallest have only 4 genes and largest have several hundred. • DNA Viruses • DNA Viruses are the viruses which consist of DNA genome . They complete their activities by transcription and most of them attack on organisms of similar genome. • RNA Viruses • RNA Viruses are the viruses which consist of RNA genome. They complete their activities by reverse transcription.
- 66. Classification on structural basis • With relevant to morphology of viral structure viruses are organized as Enveloped and Nonenveloped viruses. • However they are also arranges subclasses of DNA and RNA viruses
- 67. DNA viruses
- 68. Benefits In Genetic Engineering harmless virus are used as genetic vectors which carry good genes into cells. Viral envelop Stimulate the immune system to produce antibodies that can protect the host cells against future infections Viral genome contain enzymes for specific operations within the host cell Antiviral drugs block virus replication by targeting one of the steps in the viral life cycle
- 69. Interferon shows potential for treating and preventing viral infections Some recently-developed drugs do combat some viruses, mostly by interfering with viral nucleic acid synthesis. AZT interferes with reverse transcriptase of HIV. Acyclovir inhibits herpes virus DNA synthesis
- 70. Uses of viruses • The first vaccine was developed in the late 1700s by Edward Jenner to fight smallpox. • Vaccines can help prevent viral infections, but they can do little to cure most viral infection once they occur. • Both plasmids and transposons are mobile genetic elements.
- 71. • Human Diseases: Warts, common cold, Influenza (flu), Smallpox, Ebola, Herpes, AIDS, Chicken pox, Rabies are due to virus actions. • Viruses can be prevented with vaccines, but NOT treated with antibiotics. • Cytopathic effects- virus-induced damage to the cell that alters its microscopic appearance • Inclusion bodies- compacted masses of viruses or damaged cell organelles • Oncoviruses- mammalian viruses capable of initiating tumors
- 72. WartsHPV
- 74. Smallpox Variola
- 75. Ebola
- 77. HIV/AIDS
- 78. Viral disease