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Viruses
Viruses, viroids, and prions are infectious agents that can replicate only inside the cells of living hosts. Viruses are nonliving particles that contain genetic material and infect all types of life forms. They have a protein coat and nucleic acid core. Viruses replicate by injecting their genetic material into host cells and using the cell's machinery to produce new virus particles. Some viruses become dormant after infection and can reactivate later. Prions are misfolded protein particles that cause neurodegenerative diseases by converting normal proteins into the abnormal prion state. Prions have no nucleic acid and cause diseases like mad cow disease and kuru.
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Viruses
- 1. Viruses, Viroids, and Prions Dr.MohammedAzim bagban Assistant Professor & Head, C. U. SHAH INSTIUTE OF SCIENCE 1
- 2. Are Viruses Livingor Nonliving? • Viruses are both and neither • They have some properties of life but not others • For example, viruses can be killed, even crystallized like table salt • However, they can’t maintain a constant internal state (homeostasis). 2
- 3. What are Viruses? •A virus is a noncellular particle made up of genetic material (DNA) and protein that can invade living cells. 3
- 4. 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 4
- 5. Tobacco Mosaic Virus •WendellStanley (1935) crystallized sap from sick tobacco plants •He discovered viruses were made of nucleic acid and protein 5
- 6. Smallpox •Edward Jenner (1796) developeda smallpox vaccine using milder cowpox viruses •Deadly viruses are said to be virulent •Smallpox had been eradicated in the world today, but is now coming back 6
- 7. Viewing Viruses •Viruses aresmaller than the smallest cell •Measured in nanometers •Viruses couldn’t be seen until the electron microscope was invented in the 20th century 7
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- 10. Characteristics • Non livingstructures • Noncellular • Have a protein coat called the capsid • Have a nucleic acid core containing DNA or RNA • Capable of reproducing only when inside a HOST cell 10
- 11. Characteristics • Some virusesare enclosed in a protective envelope • Some viruses may have spikes to help attach to the host cell • Most viruses infect only SPECIFIC host cells 11 CAPSID ENVELOPE DNA SPIKES
- 12. Characteristics •Capsids (coats) are madeof individual protein subunits called capsomeres 12 CAPSOMERES
- 13. Characteristics •Outside of hostcells, 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 13 EBOLA VIRUS HIV VIRUS
- 14. Characteristics •Some viruses cause diseasesuch as: •Smallpox, measles, mononucleosis, influenza, colds, warts, AIDS, Ebola •Some viruses may cause some cancers like leukemia •Virus-free cells are rare 14 MEASLES
- 15. Viral Shapes •Viruses comein 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 15
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- 25. Used for Virus Identification •RNA or DNA Virus • Do or do NOT have an envelope • Capsid shape • HOST they infect 25
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- 27. Phages •Viruses that attack bacteriaare a bacteriophage (phage) •T-phages are a specific class of bacteriophages with icosahedral heads, double- stranded DNA, and tails 27
- 28. T-phages •The most commonly studiedT-phages are T4 and T7 •They infect E. coli , an intestinal bacteria •Six small spikes at the base of a contractile tail are used to attach to the host cell •Inject viral DNA into cell 28
- 29. Escherichia coli Bacterium 29 T -EVEN PHAGES ATTACK THIS BACTERIUM
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- 31. Diagram of T-4 Bacteriophage •Headwith 20 triangular surfaces •Capsid contains DNA •Head & tail fibers made of protein 31
- 32. 32 Clicker Question -Phages •A Bacteriophage or a phage is: A virus that infects a __________ cell. A. Plant B. Animal C. Bacteria D. Protista E. Fungi
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- 34. Characteristics of Retroviruses •ContainRNA, not DNA •Family Retroviridae •Contain enzyme called Reverse Transcriptase •When a retrovirus infects a cell, it injects its RNA and reverse transcriptase enzyme into the cytoplasm of that cell 34
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- 36. Retroviruses •The enzyme reverse transcriptase(or RTase), which causes synthesis of a complementary DNA molecule (cDNA) using virus RNA as a template 36 RTase
- 37. Retroviruses •HIV, the AIDS virus,is a retrovirus •Feline Leukemia Virus is also a retrovirus 37
- 38. Clicker Question! • Retrovirusescontain: A. tRNA B. DNA C. RNA D. rRNA E. mRNA
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- 40. Viral Attack • HOSTspecific • Humans rarely share viral diseases with other animals • Eukaryotic viruses usually have protective envelopes made from the host cell membrane 40
- 41. 5 Steps ofLytic Cycle • 1. Attachment to the cell • 2. Penetration (injection) of viral DNA or RNA • 3. Replication (Biosynthesis) of new viral proteins and nucleic acids • 4. Assembly (Maturation) of the new viruses • 5. Release of the new viruses into the environment (cell lyses) 41
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- 43. Viral Latency •Some viruseshave the ability to become dormant inside the cell •Called latent viruses •They may remain inactive for long periods of time (years) •Later, they activate to produce new viruses in response to some external signal •HIV and Herpes viruses are examples 43
- 44. Viral Latency •Once aprophage cell is activated, host cell enters the lytic cell •New viruses form & the cell lyses (bursts) 44 INACTIVE STAGE ACTIVE STAGE
- 45. Latency in Eukaryotes •Someeukaryotic viruses remain dormant for many years in the nervous system tissues • Chickenpox (caused by the virus Varicella zoster) is a childhood infection •It can reappear later in life as shingles, a painful itching rash limited to small areas of the body 45 SHINGLES
- 46. Latency in Eukaryotes •Herpesviruses also becomes latent in the nervous system •A herpes infection lasts for a person’s lifetime •Genital herpes (Herpes Simplex 2) •Cold sores or fever blisters (Herpes Simplex1) 46 SKIN TO SKIN CONTACT PASSED AT BIRTH TO BABY
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- 49. Clicker Question! • Virusesreplicate: A. On their own B. With other viruses by joining pili C. When ever they want to D. In a host cell only
- 50. 50 • All VirusesONLY contain RNA. A. True B. False Clicker Question!
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- 52. Vaccines • An attenuatedvirus is a weakened, less vigorous virus • Attenuated virus is capable of stimulating an immune response and creating immunity, but not causing illness 52
- 53. Other Viral Treatments •Interferonare naturally occurring proteins made by cells to fight viruses •Antiviral drugs (AZT) •Protease inhibitors – prevents capsid formation 53
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- 55. Viroids •Small, circular RNA molecules withouta protein coat •Infect plants •Potato famine in Ireland •Resemble introns 55
- 56. Prions •Prions are “infectious proteins” •They are normal body proteins that get converted into an alternate configuration by contact with other prion proteins • They have no DNA or RNA •The main protein involved in human and mammalian prion diseases is called “PrP” 56
- 57. Prion Diseases •Causes neuronsto rapidly degenerate. •Examples: •Mad cow disease (bovine spongiform encephalitis: BSE) •People in New Guinea used to suffer from kuru, which they got from eating the brains of their enemies 57
- 58. Clicker Question Prions canbe contracted by… A: Eating leftovers from the fridge. B: Eating the brain of an infected animal. C: Eating meat that has come into contact with the brain or spinal cord of an infected animal. D: Both B and C
- 59. Clicker Question! • Whatinfectious element causes Mad Cow Disease? A: Phages B: Prions C: Viroids D: Plasma E: Virus