Viruses-size,shape & types

Viruses-size,shape & types

1. Viruses colored SEM; magnifications: large photo 25,000; inset 38,000x

2. Viruses of Prokaryotes: T4 phage M13 (General properties and structure,classification,reproduction ) Viruses of Eukaryotes: Retrovirus Herpes simplex virus (Classification, reproduction ) Cytocidal infection and cell damage Plant viruses: TMV (Morphology, taxonomy, and reproduction) V iroids and prions

3. What are Viruses? A virus is a non-cellular particle made up of genetic material and protein that can invade living cells. 3

4. Viral History copyright cmassengale 4

5. Discovery of Viruses Beijerinck (1897) coined the Latin name “virus” meaning poison He studied filtered plant juices & found they caused healthy plants to become sick copyright cmassengale 5

6. Tobacco Mosaic Virus Wendell Stanley (1935) crystallized sap from sick tobacco plants He discovered viruses were made of nucleic acid and protein copyright cmassengale 6

7. 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 copyright cmassengale 7

8. Viewing Viruses Viruses are smaller than the smallest cell Measured in nanometers Viruses couldn’t be seen until the electron microscope was invented in the 20 th century copyright cmassengale 8

9. Size of Viruses copyright cmassengale 9

10. Viral Structure copyright cmassengale 10

11. Characteristics 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 copyright cmassengale 11

12. Characteristics 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 copyright cmassengale 12 CAPSID ENVELOPE DNA SPIKES

13. Characteristics Viral capsids (coats) are made of individual protein subunits Individual subunits are called capsomeres copyright cmassengale 13 CAPSOMERES

14. Characteristics Outside of host cells, viruses are inactive Lack ribosomes and enzymes needed for metabolism Use the raw materials and enzymes of the host cell to be able to reproduce copyright cmassengale 14 EBOLA VIRUS HIV VIRUS

15. capsid nucleic acid lipid envelope surface proteins capsid nucleic acid lipid envelope Surface proteins capsid surface proteins nucleic acid helical (rabies) polyhedral (foot-and-mouth disease) enveloped (influenza)

16. Characteristics 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 copyright cmassengale 16 MEASLES

17. 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 copyright cmassengale 17

18. Helical Viruses 18

19. 19 Icosahedral 20-sided with 12 corners Vary in the number of capsomers Each capsomer may be made of 1 or several proteins Some are enveloped

20. Complex Viruses 20

21. 21 Virus penetration Viral entry into host cells occurs through one of the following methods: Endocytosis Direct fusion Nucleic acid translocation

22. 22 Endocytosis All of the virus is engulfed and enclosed in a vacuole

23. 23 Direct Fusion Host cell membrane fuses with the virus

24. 24 Nucleic acid translocation Non enveloped virus injecting its nucleic acid to the host cell

25. Viruses replicate only in host cells Viruses are obligate intracellular parasites, which means they can replicate only within a host cell Each virus has a host range , a limited number of host cells that it can infect © 2011 Pearson Education, Inc.

26. General Features of Viral Replicative Cycles Once a viral genome has entered a cell, the cell begins to manufacture viral proteins The virus makes use of host enzymes, ribosomes, tRNAs , amino acids, ATP, and other molecules Viral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses © 2011 Pearson Education, Inc.

27. VIRUS 2 1 3 4 Entry and uncoating Replication Transcription and manufacture of capsid proteins Self-assembly of new virus particles and their exit from the cell DNA Capsid HOST CELL Viral DNA Viral DNA mRNA Capsid proteins Figure 19.4

28. Replicative Cycles of Phages Phages are the best understood of all viruses Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle © 2011 Pearson Education, Inc.

29. The Lytic Cycle The lytic cycle is a phage replicative cycle that culminates in the death of the host cell The lytic cycle produces new phages and lyses (breaks open) the host’s cell wall, releasing the progeny viruses A phage that reproduces only by the lytic cycle is called a virulent phage Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA © 2011 Pearson Education, Inc. Animation: Phage T4 Lytic Cycle

30. Figure 19.5-1 Attachment 1

31. Figure 19.5-2 Attachment 2 1 Entry of phage DNA and degradation of host DNA

32. Figure 19.5-3 Attachment 2 1 3 Entry of phage DNA and degradation of host DNA Synthesis of viral genomes and proteins

33. Figure 19.5-4 Attachment 2 1 4 3 Entry of phage DNA and degradation of host DNA Synthesis of viral genomes and proteins Assembly Phage assembly Head Tail Tail fibers

34. Figure 19.5-5 Attachment 2 1 5 4 3 Entry of phage DNA and degradation of host DNA Release Synthesis of viral genomes and proteins Assembly Phage assembly Head Tail Tail fibers

35. The Lysogenic Cycle The lysogenic cycle replicates the phage genome without destroying the host The viral DNA molecule is incorporated into the host cell’s chromosome This integrated viral DNA is known as a prophage Every time the host divides, it copies the phage DNA and passes the copies to daughter cells © 2011 Pearson Education, Inc. Animation: Phage Lambda Lysogenic and Lytic Cycles

36. An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode Phages that use both the lytic and lysogenic cycles are called temperate phages © 2011 Pearson Education, Inc.

37. lytic cycle is induced or Phage DNA circularizes. Certain factors determine whether lysogenic cycle is entered Lysogenic cycle Prophage Daughter cell with prophage Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Cell divisions produce a population of bacteria infected with the prophage. The bacterium reproduces, copying the prophage and transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a prophage. Figure 19.6b

38. Figure 19.6 New phage DNA and proteins are synthesized and assembled into phages. The cell lyses, releasing phages. Phage Phage DNA The phage injects its DNA. Bacterial chromosome Lytic cycle lytic cycle is induced or Phage DNA circularizes. Certain factors determine whether lysogenic cycle is entered Lysogenic cycle Prophage Daughter cell with prophage Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Cell divisions produce a population of bacteria infected with the prophage. The bacterium reproduces, copying the prophage and transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a prophage.

39. The lytic cycle of the bacteriophage : a lunar analogy Lytic Cycle Lysogenic Cycle The lytic cycle causes the host bacterium to undergo cell lysis , that is, cell destruction. The lysogenic cycle does not cause cell lysis or cell destruction. The lytic cycle can lead to production of 100 to 200 progeny phages. The DNA of the phage gets integrated into the bacterial chromosome and no progeny are produced mostly. Lytic cycle cannot be converted into the lysogenic cycle. Lysogenic cycle can be converted into lysogenic cycle when the host cell is exposed to chemical or physical agents.

40. Taxonomy of Viruses 40

41. Viral Taxonomy Family names end in -viridae Genus names end in -virus Viral species : A group of viruses sharing the same genetic information and ecological niche (host). Common names are used for species Subspecies are designated by a number copyright cmassengale 41

42. Viral Taxonomy Examples Herpesviridae Herpesvirus Human herpes virus 1, HHV 2, HHV 3 Retroviridae Lentivirus Human Immunodeficiency Virus 1, HIV 2 copyright cmassengale 42

43. Adenovirus copyright cmassengale 43 COMMON COLD

44. Viroids Smaller than viruses Strands or circles of RNA No protein-coding genes No protein coat Cause many plant diseases Viroid Small, single-stranded, covalently closed circular RNA molecules existing as highly base-paired rod-like structures; they do not possess capsids They range in size from 246 to 375 nucleotides in length. The extracellular form of the viroid is naked RNA—there is no capsid of any kind The RNA molecule contains no protein-encoding genes, and the viroid is therefore totally dependent on host functions for its replication The RNAs of viroids have been shown to contain inverted repeated base sequences at their 3' and 5' ends, a characteristic of transposable elements and retroviruses. Thus, it is likely that they have evolved from transposable elements or retroviruses by the deletion of internal sequences 44

45. Prion A kind of infectious protein that can resist the digestion of proteinase The cellular form of the prion protein (PrPc) is encoded by the host’s chromosomal DNA An abnormal isoform of this protein (PrPres) is the only known component of the prion and is associated with transmissibility. Kuru, Creutzfeldt-Jakob disease (CJD), Gerstmann- Sträussler-Scheinker disease, fatal familial insomnia , and Bovine spongiform encephalopathy (BSE) Dr.T.V.Rao MD 45

46. Used for Virus Identification RNA or DNA Virus Do or do NOT have an envelope Capsid shape HOST they infect copyright cmassengale 46