Monday, March 8th

1. Monday, March 8th • Check Chapter #20 Pretest • Discuss Section 20-1 Notes • 20-1 Review Sheet due next time • Quiz next time: 20-1 Notes • Chapter #20 Vocabulary Quiz Friday, March 12th

2. Chapter #20 Viruses & Bacteria

3. Section 20-1Viruses

4. A. History of Viruses • 1) The first viruses discovered were ones that infected tobacco plants, a disease called TMV (tobacco mosaic virus) • 2) One experiment that attempted to identify the virus involved the use of a fine porcelain filter: • a) The pores in these filters were so small that bacteria could not pass through. • b) However, the infectious agent of TMV passed through the filters without difficulty.

5. c) Scientists concluded that the infectious agent must be smaller than a bacterial cell. • d) They discovered the agent could only reproduce inside living cells. • e) They called the agent a virus, a Latin word for “poison”. • 3) For many years after their discovery, viruses were erroneously regarded as primitive forms of life. Scientists thought viruses were tiny cells that might have been the ancestors of bacteria.

6. 4) The true nature of viruses was discovered in 1933 by biologist Wendell Stanley • a) When Stanley tried to purify an extract of TMV, the extract formed crystals…a property of chemicals. • b) The crystals retained the ability to infect healthy tobacco plants & were therefore the virus itself. • c) Stanley concluded that TMV is chemical matter rather than a living organism.

7. 5) Within a few years of Stanley’s findings, scientists were able to disassemble TMV & confirm Stanley’s conclusion that viruses were in fact chemicals and not living cells. • In fact, each particle of TMV is made of only two kinds of molecules: • a) RNA • b) Protein • 6) Later, scientists were able to separate the RNA from the protein. When they reassembled the two components, the reconstructed TMV particles were fully able to infect healthy tobacco plants.

8. 7) Biologists have concluded that viruses are NOT living organisms. • Instead, a virus is a strand of nucleic acid encased in a protein coat that an infect cells and replicate within them. • B. Structure of Viruses • 1) Most viruses have a protein sheath called a capsid surrounding a core of nucleic acid. • a) Many plant viruses as well as some animal viruses contain RNA. • b) The nucleic acid found in most viruses is DNA.

9. 2) Many viruses found in animals, such as the influenza virus, have a membranous envelope surrounding the capsid. • a) The envelope helps it gain entry into cells. • b) It contains proteins, lipids, & glycoproteins derived from the host cell.

10. 3) Viruses differ greatly in appearance • a) The simplest viruses consist of a single molecule of a nucleic acid & a capsid made of a single protein or a few different protein molecules repeated several times. • b) More complex viruses may consist of several different segments of DNA or RNA contained within a capsid made of several different kinds of protein. • c) TMV is rodlike in appearance, with capsid proteins winding around the core like a helix.

11. d) The capsid of most polyhedral viruses is in the shape of an icosahedron, which is a shape with 20 triangular faces and 12 corners. • The adenovirus, which causes upper respiratory infections in humans, has the icosahedral shape. • e) Bacteriophages are viruses that infect bacteria. They have a very complicated structure shown in Figure 20-5 on p. 457

12. C. Viruses Reproduce Inside Living Cells • 1) Viruses lack the enzymes for metabolism & have no ribosomes or other equipment for protein synthesis. Therefore they must rely on living cells for reproduction. • 2) Before a virus can reproduce, it must first infect a living cell. • a) Bacteriophages punch a hole in the bacterial cell wall & inject their DNA into the cell like a hypodermic needle. • b) Plant viruses, like TMV, enter plant cells through tiny rips in the cell wall at points of injury. • c) Animal viruses enter the host cell by endocytosis.

13. 3) Once they are inside a cell, many viruses are pathogens….agents that cause disease. • 4) Before any virus can be engulfed by a cell, it must first bind to the cell membrane. • a) The envelope of an animal virus has spikes of glycoproteins & lipids that are able to bind to specific receptor molecules on the cell membrane. • b) An animal virus is able to infect only cells with surface receptor proteins to which the virus’ envelope molecules can attach. • c) This is why viruses affect very specific cells in their host, the spikes of the virus will only attach to specific receptor proteins.

14. 5) Mammals protect themselves from viral infection by producing antibodies to the virus envelope’s glycoprotein. • a) An antibody is a protein secreted by cells in the immune system in response to a foreign substance in the body. • b) However, mutations in viruses often change their glycoproteins & make it difficult for the antibodies to recognize the virus. • c) Mutations in the virus genes that encode the structure of its glycoproteins may also enable the virus to bind to a receptor protein it failed to recognize earlier.

15. D. How HIV Infects Human WBC’s • 1) HIV gains access to a WBC by first binding to the cell membrane. • 2) The binding triggers endocytosis. • 3) The virus enters the cell within a membrane-bound vesicle, which soon releases the virus into the cell cytoplasm. • 4) Once within the host, HIV sheds its envelope & capsid, leaving two strands of the virus’ RNA floating in the cytoplasm. • 5) HIV utilizes the enzyme reverse transcriptase, which manufactures DNA from its RNA. This ability makes HIV a retrovirus.

16. 6) After the viral RNA is transcribed into DNA, the genes are translated into HIV proteins. • 7) The host cell’s machinery is then used to produce & assemble many copies of the HIV virus. • 8) Some of the newly assembled virus particles leave the cell by exocytosis, and eventually the host cell ruptures, releasing thousands of additional virus particles. • 9) These newly released virus particles are then free to infect other WBC’s and continue the cycle of infection.

17. E. Origins of Viruses • 1) Viruses are considered to be escaped fragments of host genomes. • 2) This is why viruses are almost always highly specific to the host they infect.

18. Tuesday, March 16th • Check Directed & Active Reading • Discuss Sections 20-2 & 20-3 Notes • Review Sheet due Thursday • Quiz Thursday: 20-2 & 20-3 Notes

19. Section 20-2Bacteria

20. A. Bacteria are small, single cells and are the only ones characterized by prokaryotic organization. • B. A bacterial cell is usually one of three basic shapes: • 1) Bacillus: Rod-shaped cells • 2) Coccus: Spherical cells • 3) Spirillum: Spiral cells

21. C. Bacteria differ in cell wall structure. • 1) A bacterium’s plasma membrane is encased within a cell wall. • 2) Some eubacteria have a cell wall covered with an outer membrane layer made of large molecules called lipopolysaccharides. • 3) Outside of the cell wall & membrane, many bacteria have a gelatinous layer called a capsule.

22. D. Bacteria are commonly classified by differences in their cell walls. • 1) Gram Positive bacteria have a cell wall containing a large amount of peptidoglycan. • 2) Gram Negative bacteria have a cell wall containing a thin layer of peptidoglycan covered by an outer membrane. • E. Danish microbiologist Hans Gram developed a staining procedure in 1884 that allows scientists to distinguish between Gram+ and Gram- bacteria.

23. F. The staining method is based on the fact that the thick cell wall of Gram+ bacteria will hold on to a stain, while the stain will be washed out of the thin cell wall of Gram- bacteria. • G. Steps in the Gram Stain • 1) Cover the smear with crystal violet stain and let it react for 60 seconds. • 2) Pour off the excess stain and gently rinse with water. • 3) Cover the smear with Gram’s iodine and let it react for 60 seconds. • 4) Gently rinse the smear with ethanol until no stain rinses off.

24. 5) Rinse off the alcohol with water. • 6) Cover the smear with Safranin & let it react for 15 seconds. • 7) Rinse the slide with water. • 8) Gently blot the slide dry. • H. Medical Importance of Gram Staining • 1) Gram+ bacteria tend to be killed by penicillin. • 2) Gram- bacteria tend to be resistant to penicillin, but are much more susceptible to tetracycline. • 3) This is why doctors try to identify the type of bacteria causing a disease before prescribing a certain antibiotic.

25. I. Some bacteria form thick-walled endospores around their chromosomes and a small bit of cytoplasm when they are exposed to harsh conditions. • 1) These endospores are highly resistant to environmental stress & may germinate after years to form new, active bacteria. • 2) Endospores of Clostridium botulinum are responsible for botulism, a very serious form of food poisoning.

26. J. Bacteria vs. Eukaryotes • 1) Bacteria are prokaryotic. They lack a nucleus & membrane-bound organelles. • 2) Eukaryotic cells are about 10x larger than bacterial cells. • 3) All bacteria are single-celled. Eukaryotes can be unicellular or multicellular. • 4) Bacterial chromosomes consist of a single circular strand of DNA. Eukaryotes have more complex DNA.

27. 5) Bacteria divide by binary fission. Most eukaryotes divide by mitosis & meiosis. • 6) Eukaryotic flagella are more complex than bacterial flagella. Some bacteria have shorter, thicker outgrowths called pili. • 7) Bacteria have many different types of metabolism. All eukaryotes have the same general metabolic requirements.

28. K. Bacterial Metabolism • 1) Photosynthetic bacteria utilize sunlight for their energy source. Examples include cyanobacteria, green sulfur bacteria, purple sulfur bacteria, and purple nonsulfur bacteria • 2) Chemoautotrophic bacteria utilize electrons in inorganic molecules. Examples include Nitrosomonas and Nitrobacter. • 3) Heterotrophic bacteria utilize organic material from other organisms. Most bacteria are heterotrophic.

29. Section 20-3:Bacteria & Virusesas Pathogens Diseases caused by Bacteria: Table 20-1 Diseases caused by Viruses: Table 20-2

30. B. Treating Bacterial Diseases • 1) In 1928, the British bacteriologist Alexander Fleming observed a Pennicillium mold growing on a bacterial culture • 2) He noticed the bacteria did not grow near the mold. • 3) He isolated the substance that killed the bacteria and named it penicillin. • 4) Penicillin is an antibiotic, which is a drug used to fight pathogenic microorganisms. • a) Antibiotics work by interfering with the microorganism’s cellular processes. • b) Antibiotics can’t be used to fight viral diseases. • 5) Scientists are concerned because many microorganisms are becoming resistant to antibiotics.