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Chapter 19

Chapter 19. Viruses. Overview: A Borrowed Life. Viruses called bacteriophages can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli Viruses lead “a kind of borrowed life” between life-forms and chemicals

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Chapter 19

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  1. Chapter 19 Viruses

  2. Overview: A Borrowed Life • Viruses called bacteriophages can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli • Viruses lead “a kind of borrowed life” between life-forms and chemicals • The origins of molecular biology lie in early studies of viruses that infect bacteria

  3. Fig. 19-1 • 0.5 µm

  4. Concept 19.1: A virus consists of a nucleic acid surrounded by a protein coat • Viruses were detected indirectly long before they were actually seen

  5. The Discovery of Viruses: Scientific Inquiry • Tobacco mosaic disease stunts growth of tobacco plants and gives their leaves a mosaic coloration • In the late 1800s, researchers hypothesized that a particle smaller than bacteria caused the disease • In 1935, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV)

  6. Fig. 19-2 • RESULTS • Extracted sap • from tobacco • plant with • tobacco • mosaic • disease • Passed sap • through a • porcelain • filter known • to trap • bacteria • Rubbed filtered • sap on healthy • tobacco plants • 3 • 1 • 2 • Healthy plants • became infected • 4

  7. Structure of Viruses • Viruses are not cells • Viruses are very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope

  8. Viral Genomes • Viral genomes may consist of either • Double- or single-stranded DNA, or • Double- or single-stranded RNA • Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus

  9. Capsids and Envelopes • A capsid is the protein shell that encloses the viral genome • Capsids are built from protein subunits called capsomeres • A capsid can have various structures

  10. Fig. 19-3a • RNA • Capsomere • of capsid • 18  250 nm • 20 nm • (a) Tobacco mosaic • virus

  11. Fig. 19-3b • DNA • Capsomere • Glycoprotein • 70–90 nm (diameter) • 50 nm • (b) Adenoviruses

  12. Fig. 19-3c • Membranous • envelope • RNA • Capsid • Glycoproteins • 80–200 nm (diameter) • 50 nm • (c) Influenza viruses

  13. Fig. 19-3d • Head • DNA • Tail • sheath • Tail • fiber • 80  225 nm • 50 nm • (d) Bacteriophage T4

  14. Some viruses have membranous envelopes that help them infect hosts • These viral envelopes surround the capsids of influenza viruses and many other viruses found in animals • Viral envelopes, which are derived from the host cell’s membrane, contain a combination of viral and host cell molecules

  15. Bacteriophages, also called phages, are viruses that infect bacteria • They have the most complex capsids found among viruses • Phages have an elongated capsid head that encloses their DNA • A protein tail piece attaches the phage to the host and injects the phage DNA inside

  16. Concept 19.2: Viruses reproduce only in host cells • Viruses are obligate intracellular parasites, which means they can reproduce only within a host cell • Each virus has a host range, a limited number of host cells that it can infect

  17. General Features of Viral Reproductive 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 Animation: Simplified Viral Reproductive Cycle

  18. Fig. 19-4 • VIRUS • Entry and • uncoating • 1 • DNA • Capsid • Transcription • and manufacture • of capsid proteins • 3 • Replication • 2 • HOST CELL • Viral DNA • mRNA • Capsid • proteins • Viral DNA • Self-assembly of • new virus particles • and their exit from • the cell • 4

  19. Reproductive Cycles of Phages • Phages are the best understood of all viruses • Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle

  20. The Lytic Cycle • The lytic cycle is a phage reproductive cycle that culminates in the death of the host cell • The lytic cycle produces new phages and digests 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 Animation: Phage T4 Lytic Cycle

  21. Fig. 19-5-1 • Attachment • 1

  22. Fig. 19-5-2 • Attachment • 1 • 2 • Entry of phage • DNA and • degradation of • host DNA

  23. Fig. 19-5-3 • Attachment • 1 • 2 • Entry of phage • DNA and • degradation of • host DNA • 3 • Synthesis of viral • genomes and • proteins

  24. Fig. 19-5-4 • Attachment • 1 • 2 • Entry of phage • DNA and • degradation of • host DNA • Phage assembly • 4 • Assembly • 3 • Synthesis of viral • genomes and • proteins • Head • Tail • Tail fibers

  25. Fig. 19-5-5 • Attachment • 1 • 2 • Entry of phage • DNA and • degradation of • host DNA • 5 • Release • Phage assembly • 4 • Assembly • 3 • Synthesis of viral • genomes and • proteins • Head • Tail • Tail fibers

  26. 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 Animation: Phage Lambda Lysogenic and Lytic Cycles

  27. 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

  28. Fig. 19-6 • Daughter cell • with prophage • Phage • DNA • The phage injects its DNA. • Cell divisions • produce • population of • bacteria infected • with the prophage. • Phage DNA • circularizes. • Phage • Bacterial • chromosome • Occasionally, a prophage • exits the bacterial • chromosome, • initiating a lytic cycle. • Lytic cycle • Lysogenic cycle • The bacterium reproduces, • copying the prophage and • transmitting it to daughter cells. • The cell lyses, releasing phages. • Lytic cycle • is induced • Lysogenic cycle • is entered • or • Prophage • Phage DNA integrates into • the bacterial chromosome, • becoming a prophage. • New phage DNA and proteins • are synthesized and • assembled into phages.

  29. Reproductive Cycles of Animal Viruses • There are two key variables used to classify viruses that infect animals: • DNA or RNA? • Single-stranded or double-stranded?

  30. Table 19-1a

  31. Table 19-1b

  32. Viral Envelopes • Many viruses that infect animals have a membranous envelope • Viral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host cell • Some viral envelopes are formed from the host cell’s plasma membrane as the viral capsids exit

  33. Other viral membranes form from the host’s nuclear envelope and are then replaced by an envelope made from Golgi apparatus membrane

  34. AIDS

  35. RNA as Viral Genetic Material • The broadest variety of RNA genomes is found in viruses that infect animals • Retroviruses use reverse transcriptase to copy their RNA genome into DNA • HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome)

  36. Transmission of HIV requires the transfer of body fluids containing infected cells, such as semen or blood, from person to person. • Unprotected sex • Nonsterile needles • HIV transmission among heterosexuals is rapidly increasing as a result of unprotected sex with infected partners. • HIV in Africa and Asia- primarily by heterosexual sex

  37. AIDS In 1983, a retrovirus, now called human immunodeficiency virus (HIV), had been identified as the causative agent of AIDS. HIV budding

  38. HIV/AIDS in Hawaii Hawaii State Department of Health

  39. HIV/AIDS in Hawaii Hawaii State Department of Health

  40. With the AIDS mortality close to 100%, HIV is the most lethal pathogen ever encountered. • Molecular studies reveal that the virus probably evolved from another HIV-like virus in chimpanzees in central Africa and appeared in humans sometimes between 1915 and 1940. • These first rare cases of infection and AIDS went unrecognized.

  41. AIDS

  42. HIV Testing: • The HIV antibody test has be used to screen all blood supplies in the U.S. since 1985. • May take weeks or months before anti-HIV antibodies become detectable. • Drug treatment available • Best prevention is education and protected sex HIV/AIDS patients “cured” by bone marrow transplants http://www.npr.org/blogs/health/2012/07/26/157444649/two-more-nearing-aids-cure-after-bone-marrow-transplants-doctors-say Scientists Report First Cure Of HIV In A Child, Say It's A Game-Changer http://www.npr.org/blogs/health/2013/03/04/173258954/scientists-report-first-cure-of-hiv-in-a-child-say-its-a-game-changer

  43. Fig. 19-8a • Viral envelope • Glycoprotein • Capsid • RNA (two • identical • strands) • HOST CELL • Reverse • transcriptase • HIV • Reverse • transcriptase • Viral RNA • RNA-DNA • hybrid • DNA • NUCLEUS • Provirus • Chromosomal • DNA • RNA genome • for the • next viral • generation • mRNA • New virus

  44. The viral DNA that is integrated into the host genome is called a provirus • Unlike a prophage, a provirus remains a permanent resident of the host cell • The host’s RNA polymerase transcribes the proviral DNA into RNA molecules • The RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new virus particles released from the cell Animation: HIV Reproductive Cycle

  45. Evolution of Viruses • Viruses do not fit our definition of living organisms • Since viruses can reproduce only within cells, they probably evolved as bits of cellular nucleic acid • Candidates for the source of viral genomes are plasmids, circular DNA in bacteria and yeasts, and transposons, small mobile DNA segments • Plasmids, transposons, and viruses are all mobile genetic elements

  46. Mimivirus, a double-stranded DNA virus, is the largest virus yet discovered • There is controversy about whether this virus evolved before or after cells

  47. Concept 19.3: Viruses, viroids, and prions are formidable pathogens in animals and plants • Diseases caused by viral infections affect humans, agricultural crops, and livestock worldwide • Smaller, less complex entities called viroids and prions also cause disease in plants and animals, respectively

  48. Viral Diseases in Animals • Viruses may damage or kill cells by causing the release of hydrolytic enzymes from lysosomes • Some viruses cause infected cells to produce toxins that lead to disease symptoms • Others have envelope proteins that are toxic

  49. Vaccines are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen • Vaccines can prevent certain viral illnesses • Viral infections cannot be treated by antibiotics • Antiviral drugs can help to treat, though not cure, viral infections

  50. Emerging Viruses • Emerging viruses are those that appear suddenly or suddenly come to the attention of scientists • Severe acute respiratory syndrome (SARS) recently appeared in China • Outbreaks of “new” viral diseases in humans are usually caused by existing viruses that expand their host territory

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