Viruses structure and classification
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Viruses consist of genetic material surrounded by a protein coat. They can have DNA or RNA as their genetic material and come in various shapes. The document discusses the structure and classification of viruses. It describes their components, sizes, morphologies like helical and icosahedral shapes. It provides examples of plant, animal, and bacterial viruses. It specifically examines the structure and replication of Cauliflower Mosaic Virus, a plant virus with circular double-stranded DNA genome.
![ When the new γ strand of DNA reaches the 5′ end of
the new β strand, it displaces the primer and some of
the newly synthesized β strand, resulting in the
recreation of discontinuity 2 (D2).
When the new β strand of DNA reaches the 5′ end of
the new γ strand, it displaces the primer and some of
the newly synthesized γ strand, resulting in the
recreation of discontinuity 3 (D3).
At this point the new viral genome can either be
packaged into capsids and released from the cell or
they can be transported by movement proteins into
an adjacent, uninfected cell.[14]](https://image.slidesharecdn.com/virusesstructureandclassification-210216065402/85/Viruses-structure-and-classification-32-320.jpg)
Viruses structure and classification
- 1. VIRUSES – STRUCTURE AND CLASSIFICATION BY DR.HARINATHA REDDY 7TH CLASS 06/07/2017
- 2. 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 microorganisms, including bacteria. These viral particles, also known as virions, consist of two or three parts: (i) the genetic material made from either DNA or RNA, long molecules that carry genetic information. (ii) a protein coat: called the capsid, which surrounds and protects the genetic material; and in some cases, (iii) an envelope of lipids that surrounds the protein coat when they are outside a cell.
- 3. HISTORY: In 1892 Dimitri Ivanowski published studies showing that leaf extracts from infected plants would induce tobacco mosaic disease even after filtration to remove bacteria. Martinus W. Beijerinck, working independently of Ivanowski, published the results of extensive studies on tobacco mosaic disease in 1898 and 1900. He proposed that the disease was caused by an entity different from bacteria, a filterable virus. He observed that the virus would multiply only in living plant cells, but could survive for long periods in a dried state. At the same time Friedrich Loeffler in Germany found that the hoof-and-mouth disease of cattle was also caused by a filterable virus rather than by a toxin.
- 4. Frederick W. Twort isolated bacterial viruses that could attack and destroy micrococci and intestinal bacilli. Felix d’Herelle noted that when a virus suspension was spread on a layer of bacteria growing on agar, clear circular areas containing viruses and lysed cells developed. A count of these clear zones allowed d’Herelle to estimate the number of viruses present (plaque assay). D’Herelle demonstrated that these viruses could reproduce only in live bacteria; therefore he named them bacteriophages. Wendell M. Stanley announced in 1935 that he had crystallized the tobacco mosaic virus (TMV) and found it to be largely or completely protein.
- 5. GENERAL PROPERTIES OF VIRUSES In summary, viruses differ from living cells in at least three ways: (1) their simple, acellular organization. (2) the presence of either DNA or RNA (3) their inability to reproduce independent of cells and carry out cell division in prokaryotes and eukaryotes cells.
- 6. Virus are particles of nucleoproteins. The nucleocapsid is composed of a nucleic acid, either DNA or RNA, held within a protein coat called the capsid, which protects viral genetic material. Coat: which is protein in nature which called capsid. Capsid made up of number of subunits called capsomers. Capsid is antigenic in nature. Capsid consists of number of receptors which helps in binding of the virus to the host cells.
- 7. VIRUS SIZE: Virus range in size from about 10 to 300 or 400 nm in diameter. The smallest viruses are a little larger than ribosomes, whereas the poxviruses (300 nm) are about the same size as the smallest bacteria and can be seen in the light microscope. Most viruses, however, are too small to be visible in the light microscope and must be viewed with the scanning and transmission electron microscopes
- 8. MORPHOLOGY OF VIRUSES: There are three main viral groups on the basis of electron microscope studies. 1.Helical 2.Icosahedral or cubic 3.Complex
- 9. HELICAL VIRUSES : Best example for Helical viruses is TMV, Bacteriophage, Influenza, mumps virus, Measles, Rabies virus. Some viruses resembles long rods and show helical symmetry. In these viruses the capsomers are arranged in a helix around a single rotational axis.
- 10. The tobacco mosaic virus provides a well- studied example of helical capsid structure. A single type of capsomers associates together in a helical or spiral arrangement to produce a long, rigid tube, 15 to 18 nm in diameter by 300 nm long. The RNA genetic material is wound in a spiral and positioned toward the inside of the capsid. Not all helical capsids are as rigid as the TMV capsid.
- 11. Influenza virus: RNAs are enclosed in thin, flexible helical capsids folded within an envelope. The size of a helical capsid is influenced by both its capsomers and the nucleic acid enclosed within the capsid. The nucleic acid determines helical capsid length because the capsid does not seem to extend much beyond the end of the DNA or RNA.
- 12. ICOSAHEDRAL VIRUSES: Many of the viruses like Herpes virus, Polio virus, Adenovirus have icosahedral symmetry. They resembles small crystals and appear approximately spherical shape in electron microscope. An icosahedron is a regular polyhedron with 20 equilateral triangular faces and12 vertices. Polyhedron Shape
- 13. The capsids are constructed from ring- or knob-shaped units called capsomers, each usually made of five or six protomers. Pentamers (pentons) have five subunits; hexamers (hexons) possess six. Pentamers are at the vertices of the icosahedron, whereas hexamers form its edges and triangular faces. Fig: The Structure of an Icosahedral Capsid: Pentons are located at the 12 vertices. Hexons form the edges and faces of the icosahedron. This capsid contains 42 capsomers; all protomers are identical.
- 14. COMPLEX SYMMETRY: Complex viruses have capsid symmetry that is neither purely icosahedral nor helical. They may possess tails and other structures (e.g., many bacteriophages). or have complex, multilayered walls surrounding the nucleic acid (e.g., poxviruses such as vaccinia).
- 15. 8th class 09/07/2017
- 16. F2 Bacteriophage smallest Bacteriophage 2nm. Smallest plant virus is Satellite tobacco mosaic virus or tobacco mosaic satellivirus is a satellite virus. Foot-and-mouth disease or hoof-and-mouth disease is smallest animal virus. The biggest animal virus is Pox virus (300 nm). Biggest plant virus is Citrus tristeza virus (CTV).
- 17. NUCLEIC ACIDS: There are four possible nucleic acid types: single-stranded DNA, double-stranded DNA, single-stranded RNA, and double-stranded RNA. All four types are found in animal viruses. Plant viruses most often have single- stranded RNA genomes. Bacterial viruses usually contain double- stranded DNA.
- 18. The size of viral genetic material also varies greatly. The smallest genomes of the MS2 are around 1×106 daltons, just large enough to code for three to four proteins. T-even bacteriophages, herpesvirus, and vaccinia virus have genomes of 1.0 to 1.6×108 daltons and may be able to direct the synthesis of over 100 proteins. DNA viruses like ϕX174 and M13 bacteriophages or the parvoviruses possess single-stranded DNA (ssDNA) genomes. Some of these viruses have linear pieces of DNA, whereas others use a single, closed circle of DNA for their genome.
- 19. Most DNA viruses use double-stranded DNA (dsDNA) as their genetic material. The lambda phage has linear dsDNA with cohesive ends—single-stranded. Most RNA viruses employ single-stranded RNA (ssRNA) as their genetic material. The RNA base sequence may be identical with that of viral mRNA, in which case the RNA strand is called the plus strand or positive strand (viral mRNA is defined as plus or positive). However, the viral RNA genome may instead be complementary to viral mRNA, and then it is called a minus or negative strand.
- 20. Many of these RNA genomes are segmented genomes—that is, they are divided into separate parts. It is believed that each fragment or segment codes for one protein. Usually all segments are probably enclosed in the same capsid. Some virus genomes may be composed of as many as 10 to 12 segments. A few viruses have double-stranded RNA (dsRNA) genomes. All appear to be segmented; some, such as the reoviruses, have 10 to 12 segments.
- 21. Plus strand viral RNA often resembles just as eucaryotic mRNA usually has a 5′ cap of 7-methylguanosine, many plant and animal viral RNA genomes are capped. In addition, most or all plus strand RNA animal viruses also have a poly-A stretch at the 3′ end of their genome, and thus closely resemble eucaryotic mRNA.
- 22. DNA Single-Stranded Viruses: Linear single strand: Parvoviruses. Circular single strand: φX174, M13 phages. Double-Stranded DNA viruses: Linear double strand: Herpes simplex viruses, Adenoviruses, T coliphages, Lambda phage, Vaccinia and Smallpox. Closed circular double strand: Polyomaviruses, Papillomaviruses, Cauliflower mosaic.
- 23. Single-Stranded RNA viruses: Linear, single stranded, positive strand: Polio, rhinoviruses, Togaviruses, TMV, Retroviruses (Rous sarcoma virus, human immunodeficiency virus) and most plant viruses. Linear, single stranded, negative strand: Rhabdoviruses (rabies), Paramyxoviruses (mumps, measles), Paramyxoviruses, orthomyxoviruses (influenza). Double-Stranded RNA viruses: Linear, double stranded: Reoviruses, wound-tumor virus of plants, cytoplasmic polyhedrosis virus of insects, many mycoviruses
- 24. PLANT VIRUSES: CAULIFLOWER MOSAIC VIRUS (CAMV): Like all other viruses, plant viruses are obligate intracellular parasites that do not have the molecular machinery to replicate without a host. Plant viruses are pathogenic to higher plants. Cauliflower mosaic virus (CaMV) is a member of the genus Caulimovirus. Caulimoviruses contain circular ds DNA and are spherical in shape CaMV infects mostly plants of the Brassicaceae family (such as cauliflower and turnip) but some CaMV strains are also able to infect Solanaceae species of the genera Datura. CaMV induces a variety of systemic symptoms such as mosaic, necrotic lesions on leaf surfaces, stunted growth, and deformation of the overall plant structure.
- 25. The CaMV particle is an icosahedron with a diameter of 52 nm built from 420 capsid protein (CP) subunits. CaMV contains a closed circular double- stranded DNA molecule of about 8.0 kilobases.
- 26. ORF I - Movement Protein. ORF II - Insect Transmission Factor. ORF III - Structural Protein, DNA- Binding Capabilities. ORF IV - Capsid Protein. ORF V - Protease, Reverse Transcriptase and RNaseH. ORF VI - Translational Activator Viroplasmin, ORF VII - Unknown (Appears to not be required for infection) Genome of CaMV Genome of CaMV contain with six to seven short open reading frames
- 27. The CaMV promoter was named CaMV 35S promoter ("35S promoter") because the coefficient of sedimentation of the viral transcript. The 35S DNA is particularly complex, containing a highly structured 600 nucleotide long leader sequence with six to eight short open reading frames (ORFs). The Cauliflower mosaic virus promoter (CaMV 35S) is used in most transgenic crops to activate foreign genes which have been artificially inserted into the host plant. CaMV replicates by reverse transcription: Viral particles enter a plant cell and are unencapsidated. At this stage the viral DNA consists of three fragments, one on the – strand (α) and two on the + strand (β and γ). Which are imperfectly assembled into a circular genome with three gaps or discontinuities (D1, D2, and D3).
- 29. The viral DNA enters the nucleus where the discontinuities are filled in. The host DNA-dependent RNA polymerase transcribes from the 35S promoter all the way around the viral genome, The viral RNAs pass into the host cytoplasm where they are transcribed.
- 30. The 3′ end of a tRNAfMet anneals to a site corresponding to discontinuity 1 (D1) near the 5′ end of the 35S RNA. The tRNAfMet primes synthesis, by the viral reverse transcriptase (encoded by ORF V), of a new α strand. RNase H removes the RNA from the DNA– RNA duplex, leaving behind the DNA.
- 31. Synthesis of the α strand completes. RNase H activity exposes purine-rich regions at the position of discontinuity 3 (D3), which primes the synthesis of the γ DNA strand. RNase H activity exposes purine-rich regions at the position of discontinuity 2 (D2), which primes the synthesis of the β DNA strand. When the new γ strand of DNA reaches the 5′ end of the new α strand it switches to the 5′ end of the new α strand, recreating discontinuity 1 (D1).
- 32. When the new γ strand of DNA reaches the 5′ end of the new β strand, it displaces the primer and some of the newly synthesized β strand, resulting in the recreation of discontinuity 2 (D2). When the new β strand of DNA reaches the 5′ end of the new γ strand, it displaces the primer and some of the newly synthesized γ strand, resulting in the recreation of discontinuity 3 (D3). At this point the new viral genome can either be packaged into capsids and released from the cell or they can be transported by movement proteins into an adjacent, uninfected cell.[14]
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