Ch. 18: Viruses & Prokaryotes

1. Ch. 18: Viruses & Prokaryotes

2. Main Concepts: • Prokaryotes lack nuclei and membrane-enclosed organelles. • Bacteria occupy every imaginable niche. • Archaebacteria may be the most ancient of all organisms. • Although some prokaryotes cause disease, the vast majority are beneficial and essential. • Some diseases caused by bacteria and viruses are transmitted mainly by sexual contact. • Viruses have profound effects on organisms. • AIDS, caused by HIV, is a worldwide human health problem.

3. 18.1: Studying Viruses and Prokaryotes Virus: an infectious agent that contains a nucleic acid (either DNA or RNA) and a protein coat (capsid). Pathogen: any disease-causing agent, e.g. a virus, a bacteria, etc. Viroid: an infectious particle that causes disease in plants. Prion: an infectious particle consisting of only a protein that can cause other proteins to fold incorrectly. • Contains no genetic material

4. 18.2: Viral Structure & Reproduction • Many diseases are caused by viruses: AIDS, hepatitis, measles, mumps, influenza, colds, polio. • Small enough to pass through biological filters; can only be seen with electron microscope. • Bacteriophage (AKA phage): virus that preys on bacteria (Fig. 18.5, p. 549)

5. 18.2: Viral Structure & Reproduction

6. 18.2: Viral Structure & Reproduction

7. 18.2: Viral Structure & Reproduction • Upon entering a cell, a virus may: • Act as an agent of disease, disrupting normal cell functioning and sometimes killing cell. • Act as an agent of heredity, causing permanent inheritable changes. • Differ from living things in several ways: • Can only reproduce copies of themselves inside a living organism. • Have no metabolism of their own. • Do not have cell parts.

8. 18.2: Viral Structure & Reproduction • Viruses (viroids) can infect plants and bacteria as well as animals.

9. Viral Infection Lytic Infection: • (Fig. 18.6, p. 550)

10. Viral Infection Lysogenic Infection: • (p. 550) • Prophage: the phage DNA inserted into and combined with the host cell’s DNA

11. 18.3: Viral Diseases • Some viruses store genetic material as DNA; others as RNA. • Some of the latter group are known as retroviruses.

12. 18.3: Viral Diseases • Retroviruses go through an extra step before they can reproduce; they make copies of DNA from RNA (reverse of normal flow of genetic information). • The retrovirus binds to the surface of a host cell (like other viruses), and injects its RNA into the cell (p. 553). • Reverse transcriptase, an enzyme associated with RNA, facilitates the formation of a complementary DNA strand, which can then be integrated into the host cell’s DNA, replicated, and transmitted to offspring cells. • When the cell produces RNA from its own DNA, it also produces viral RNA, which becomes the source of new viral particles, thus continuing the infection.

13. 18.3: Viral Diseases • Retroviruses cause cancer in some animals and have been associated with certain types of cancer in humans. • Human oncogenes are in some ways similar to the genes in retroviruses; both can  cancerous growth if disturbed. • AIDS: caused by a retrovirus which attacks and kills certain cells of the immune system.

14. 31.6 HIV & AIDS • AIDS (Acquired Immune Deficiency Syndrome) • Syndrome: a group of conditions that occur together. • First identified in 1983.

15. 31.6 HIV & AIDS • Caused by HIV (Human Immunodeficiency Virus), which primarily infects helper T cells of immune system; also affects brain and nerve cells, all of which have a CD4 molecule on their cell-surfaces where the virus attaches.

16. 31.6 HIV & AIDS

17. HIV: a Retrovirus

18. 31.6 HIV & AIDS

19. 31.6 HIV & AIDS • Inside host cell, the viral RNA is copied into DNA (HIV is a retrovirus). The DNA may then be incorporated into the genome of the host and replicated as the cell divides. • Eventually, the cell ruptures and releases hundreds of viruses, each of which can infect other cells.

20. 31.6 HIV & AIDS • Reproduction of HIV in helper T cells eventually  gradual decline in number of T cells and a weakened immune system. This increases the susceptibility of the individual to opportunistic infections and cancers. • It is these opportunistic diseases that account for 90% of deaths from AIDS. • Incubation period: 2 wks. – 3 mos. • An infected individual may not develop AIDS for months, or even years.

21. 31.6 HIV & AIDS • Symptoms: varied; Pneumocystis pneumonia, Kaposi’s sarcoma (a skin cancer), thrush (candidiasis), dementia, fungal, protozoan and eubacterial infections like tuberculosis, et al. Kaposi’s Sarcoma Oral Thrush

22. 31.6 HIV & AIDS • Periods of illness may alternate with periods of relative good health. • Diagnosis is based on presence of certain infections or cancers (as listed above) in a person who tests positive for HIV antibodies. • Once diagnosed with full-blown AIDS, one may live from 5 – 20 yrs. • Test is for antibodies to the virus

23. 31.6 HIV & AIDS • Test is for antibodies to the virus.

24. 31.6 HIV & AIDS Means of infection: • Sexual contact (blood to blood), contact with contaminated blood by sharing of needles, etc. • Receiving infected blood from a blood transfusion used to be a more serious concern until medical profession began screening blood bank more carefully (~ 1985). • Some health care professionals have become infected by way of cuts on their skin, or accidents with used needles. • From mother to embryo or child: • HIV can cross placental barrier. • In birthing process. • In mother’s milk.

25. 31.6 HIV & AIDS • No evidence that HIV is transmitted by casual contact, e.g. shaking hands, using the same swimming pool.

26. 18.3: Viral Diseases • Vaccines: substances containing antigens which stimulate the production of antibodies in order to ward off disease prior to infection. • Antigen: protein marker on surface of invading cell or virus that allows body to identify it as a “foreign attacker.” • Vaccines contain only enough of the killed or weakened disease-causing agent to bring about the production of antibodies without producing the symptoms of the disease.

27. 18.3: Viral Diseases Antibodies: proteins produced by the body that help protect the body against pathogens • Memory cells keep later infections by the same pathogen from causing disease.

28. 18.3: Viral Diseases • Edward Jenner (1796): first to use vaccines to protect people from smallpox.

29. 31.6 HIV & AIDS • Prospects for a vaccine are improving, though no one has suggested a concrete timeframe for the availability of one. • Studies of pregnant women who are infected and their babies suggest that antibodies may be effective in protecting against the disease. • Research is ongoing to design specific drugs that block the action of the HIV virus: • Introduce CD4 molecules attached to viral toxins as “decoy” attachment sites for virus. • Cytokines: turn off the production of HIV in lab studies, and, thus, delay the onset of the disease • Unfortunately, all these drugs have serious side effects.

30. 31.6 HIV & AIDS • Combination drug therapy, AKA drug “cocktails”: • Proving to be most effective therapy to date. • Inhibitor: prevents enzyme, reverse transcriptase, from copying DNA from RNA. • Nucleotide analogue: ineffective “mimic” nucleotide that would disrupt viral DNA or RNA sequence. • Other drugs, e.g. AZT, that inhibit the virus’ ability to replicate. • In concert, these drugs may block many different steps in viral infection. • May be a new treatment for Kaposi’s sarcoma that blocks the action of tumor growth factors.

31. 31.6 HIV & AIDS • Therapy taking advantage of evolution of virus (as in “Darwin” film): • After virus has “adapted” to/become resistant to drugs, stop drug therapy for a time. • Allows virus to revert back to “wild type” which is susceptible to drugs. • Resume drug therapy after a time in hopes of killing virus before it can evolve resistance once again.

32. 18.4 Prokaryotic Structure • Small; most = microscopic, but range from 0.3 – 0.7 µm (Rickettsia) to 500 µm (a bacteria found in the gut of the Red Sea surgeonfish). • > 20,000 spp. have been described, but most have not yet been identified. • Shapes: 1. Rod 2. Sphere 3. Spiral 4. Filament • Short, curved rod • Fig. 18.12, p. 556 Spirochetes

33. 18.4 Prokaryotic Structure • Some form multicellular structures of colonies or chains. • Many move by means of flagella; others by gliding over surfaces. • Some can cause sickness, or even death (e.g. Rocky Mountain spotted fever) • Most have rigid cell walls containing lipids, carbohydrates and protein (but no cellulose). • Inside the cell wall is a cell membrane.

34. 18.4 Prokaryotic Structure • Fig. 18.13, p. 557

35. 18.4 Prokaryotic Structure • Mesosome: an infolding of the cell membrane that may play roles in: • Secretion: • Movement of particles out of bacterial cell, and/or • Copying chromosome before cell division. • Prokaryotes have one main chromosome (the nucleoid). • No homologous pairs. • No genes in pairs. • Attached to cell membrane.

36. 18.4 Prokaryotic Structure • Usually also contain one or more smaller, circular DNA molecule, a plasmid. • Consist of only a few genes, they confer important abilities on the bacterial cells, e.g. antibiotic resistance. • Important in genetic engineering and gene cloning.

37. 18.4 Bacteria – Not all bad! • Bioremediation: Bacteria are able to digest almost anything, including cellulose and petroleum. • Some have been used in oil spill clean-ups. • Essential in all food webs as decomposers. • Break down organic matter into inorganic nutrients. • Some do opposite and transform inorganic materials into complex organic compounds. • Serve as food for protists and other microorganisms. • Many are photosynthetic. • Some are the source of antibiotics. Thus, they serve many important functions

38. 18.4 Prokaryotes • All reproduce asexually (binary fission). • A form of genetic exchange (conjugation) may occur, in which one prokaryote transfers a portion of its DNA through a tube to another. The genetic contributions of parents, though, are not equal. • Not true reproduction • Viruses can also transfer genetic material between prokaryotes.

39. 18.4 Classification of Prokaryotes • Two fundamentally different groups: • K: Archaebacteria (D: Archaea) • K: Eubacteria (D: Bacteria)

40. Archaebacteria • Archaebacteria: “ancient” bacteria; oldest organisms on Earth??? • Biochemistry and adaptation to extreme environments suggest they were among the earliest life forms on earth.

41. 18.4 Archaebacteria • Prokaryotic fossils date to 3.5 BYA; probably the first living organisms. • Found in stromatolites • ~ 2.5 – 2 BYA some photosynthetic prokaryotes brought about a change in the atmospheric composition, increasing the [O2] to 1% (Plants increased it further to 20%): the “Oxygen Revolution.” • These changes made it possible for multicellular life to evolve.

42. 18.4 Archaebacteria • Some prokaryotes can tolerate extreme environmental conditions, e.g. low temps (even freezing), boiling waters of hot springs, hot acids, high pressure at ocean depths, and low pressure in upper atmosphere. • By forming endospores, complex thick-walled structures containing DNA, they can tolerate nutrient limitations. • Are among the first organisms to invade a new environment, as after volcano or fire. • Help transform such environments, creating more hospitable conditions for other life to take up residence there.

43. Archaebacteria • Different from all other forms of life. • Cell walls lack peptidoglycan (present in cell walls of eubacteria) • Plasma membranes of archaebacteria are different from those of eubacteria and eukaryotes (contains lipids that are unique to this group). • Archaebacterial ribosomes are unique in shape. • RNA nucleotide sequence is different from that of eubacteria. • Using such sequencing can illustrate the degree of relatedness among organisms.

44. 18.4: Aerobes and Anaerobes Obligate anaerobes: poisoned by oxygen • e.g. methanogens Obligate aerobes: require oxygen • Includes many pathogens, e.g. those that cause tuberculosis and leprosy Facultative aerobes: can survive with or without oxygen

45. Several Types of Archaebacteria Exist 1.Thermoacidophiles: live in hot, acidic environments: water temps of 70 - 90C; pH of 2 or less, e.g. sulfer hot springs, coal-mining debris piles; despite the low pH of their environment, thermoacidophiles maintain an internal pH close to neutral (7). • Convert above Celsius values to Fahrenheit (˚F = 9/5˚C + 32). • 158 – 194 ˚F

46. Several Types of Archaebacteria Exist 2. Halophiles: live in environments of high [salt], e.g. salt brine; ocean borders; salty inland water, e.g. Great Salt Lake, Dead Sea; some are photosynthetic but do not contain chlorophyll; use rhodopsin instead; may cause salted fish to become discolored and spoiled.

47. Several Types of Archaebacteria Exist • Methanogens: produce methane (CH4) as a metabolic byproduct from H2 and CO2 • Live only in anaerobic conditions, e.g. stagnant water, sewage treatment plants, ocean bottoms, and hot springs • Commonly found with eubacteria that decompose organic matter and release hydrogen gas • Play role in carbon cycle: remove organic compounds from sediments and release CH4 to atmosphere, where it reacts with O2 to form water and CO2, thus contributing to level of CO2 in atmosphere.

48. Several Types of Archaebacteria Exist Digestion of Cellulose by Methanogens in Ruminants: • Rumen: First of four chambers (stomachs) of the digestive tract of cows, goats, sheep, deer and antelope in which protozoans, eubacteria and archaebacteria (methanogens) live. • Protozoa and eubacteria: contain enzymes that break down cellulose from grass or plant material. • Methanogens: use the CO2 and hydrogen as food and produce methane gas.

49. Several Types of Archaebacteria Exist • Methane can be used as a fuel. • Methanogens may be used in garbage, sewage, agricultural waste and manure to produce methane from these sources. • Methane produced this way could be used to heat homes. • Could help two problems: • Waste and garbage buildup. • Energy crisis.

50. Eubacteria • Classification of eubacteria is still unclear due to great diversity within the group: • Aerobic and anaerobic • Autotrophic and heterotrophic • Photosynthetic and chemosynthetic • Thermophilic and psychrophilic • Metabolically diverse