1. 18.1 Studying Viruses and Prokaryotes KEY CONCEPT Infections can be caused in several ways.

2. 18.1 Studying Viruses and Prokaryotes Viruses, bacteria, viroids, and prions can all cause infection. Any disease-causing agent is called a pathogen. viruses 50-200 nm prokaryotics cells 200-10,000 nm prion 2-10 nm viroids 5-150 nm eukaryotics cells 10,000-100,000 nm 100 nm 1 nanometer (nm) = one billionth of a meter

3. 18.1 Studying Viruses and Prokaryotes A virus is made of DNA or RNA and a protein coat. –non-living pathogen –can infect many organisms A viroid is made only of single-stranded RNA. –causes disease in plants –passed through seeds or pollen

4. 18.1 Studying Viruses and Prokaryotes A prion is made only of proteins. –causes misfolding of other proteins –results in diseases of the brain

5. 18.1 Studying Viruses and Prokaryotes KEY CONCEPT Viruses exist in a variety of shapes and sizes.

6. 18.1 Studying Viruses and Prokaryotes Viruses differ in shape and in ways of entering host cells. Viruses have a simple structure. –genetic material –capsid, a protein shell –maybe a lipid envelope, a protective outer coat 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)

7. 18.1 Studying Viruses and Prokaryotes Bacteriophages infect bacteria. capsid DNA tail sheath tail fiber

8. 18.1 Studying Viruses and Prokaryotes Viruses enter cells in various ways. colored SEM; magnifications: large photo 25,000; inset 38,000x –bacteriophages pierce host cells

9. 18.1 Studying Viruses and Prokaryotes –viruses of eukaryotes enter by endocytosis Viruses enter cells in various ways.

10. 18.1 Studying Viruses and Prokaryotes –viruses of eukaryotes also fuse with membrane Viruses enter cells in various ways.

11. 18.1 Studying Viruses and Prokaryotes host bacterium The bacterophage attaches and injects it DNA into a host bacterium. The host bacterium breaks apart, or lyses. Bacteriophages are able to infect new host cells. The viral DNA directs the host cell to produce new viral parts. The parts assemble into new bacteriophages. The viral DNA forms a circle. Viruses cause two types of infections. A lytic infection causes the host cell to burst. The virus may enter the lysogenic cycle, in which the host cell is not destroyed.

12. 18.1 Studying Viruses and Prokaryotes A lysogenic infection does no immediate harm. The viral DNA is called a prophage when it combines with the host cell’s DNA. Although the prophage is not active, it replicates along with the host cell’s DNA. Many cell divisions produce a colony of bacteria infected with prophage. The prophage may leave the host’s DNA and enter the lytic cycle.

13. 18.1 Studying Viruses and Prokaryotes KEY CONCEPT Some viral diseases can be prevented with vaccines.

14. 18.1 Studying Viruses and Prokaryotes Viruses cause many infectious diseases There are many examples of viral infections. –common cold

15. 18.1 Studying Viruses and Prokaryotes –influenza Viruses cause many infectious diseases There are many examples of viral infections. –common cold

16. 18.1 Studying Viruses and Prokaryotes –influenza Viruses cause many infectious diseases There are many examples of viral infections. –common cold –SARS

17. 18.1 Studying Viruses and Prokaryotes –HIV HIV-infected white blood cell Viruses cause many infectious diseases There are many examples of viral infections. The body has natural defenses against viruses.

18. 18.1 Studying Viruses and Prokaryotes Vaccines are made from weakened pathogens. A vaccine stimulates the body’s own immune response. Vaccines prepare the immune system for a future attack. Vaccines are the only way to control the spread of viral disease.

19. 18.1 Studying Viruses and Prokaryotes KEY CONCEPT Bacteria and archaea are both single-celled prokaryotes.

20. 18.1 Studying Viruses and Prokaryotes Prokaryotes are widespread on Earth. Prokaryotes can be grouped by their need for oxygen. –obligate anaerobes are poisoned by oxygen –obligate aerobes need oxygen –facultative aerobes can live with or without oxygen

21. 18.1 Studying Viruses and Prokaryotes Bacteria and archaea are structurally similar but have different molecular characteristics. Bacteria commonly come in three forms. –rod-shaped, called bacilli Lactobacilli: rod-shapedSpirochaeta: spiralEnterococci: spherical Archaea have many shapes. – spiral, called spirilla or spirochetes – spherical, called cocci

22. 18.1 Studying Viruses and Prokaryotes Bacteria and archaea have similar structures. flagellum pili plasmid cell wall chromosome plasma membrance This diagram shows the typical structure of a prokaryote. Archaea and bacteria look very similar, although they have important molecular differences. –plasmid –flagellum –pili

23. 18.1 Studying Viruses and Prokaryotes Bacteria and archaea have molecular differences. GRAM NEGATIVEGRAM POSITIVE –Archaea have different lipids entirely –The amount of peptidoglycan within the cell wall can differ between bacteria

24. 18.1 Studying Viruses and Prokaryotes Gram staining identifies bacteria. Gram-negative bacteria have a thin layer of peptidoglycan and stain red. Gram-positive bacteria have a thicker peptidoglycan layer and stain purple. –stains polymer peptidoglycan –gram-positive stains purple, more peptidoglycan –gram-negative stains pink, less peptidoglycan

25. 18.1 Studying Viruses and Prokaryotes Bacteria have various strategies for survival. Prokaryotes exchange genes during conjugation. conjugation bridge TEM; magnification 6000x Bacteria may survive by forming endospores.