What’s going on here?

1. Overview: Microbial Model Systems (This is Ch. 18 in our book, but Ch. 19 according to F & T Guide.) • Viruses called bacteriophages can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli • E. coli and its viruses are called model systems because of their frequent use by researchers in studies that reveal broad biological principles • Beyond their value as model systems, viruses and bacteria have unique genetic mechanisms that are interesting in their own right

2. What’s going on here?

3. Bacteria are prokaryotes with cells much smaller and more simply organized than those of eukaryotes • Viruses are smaller and simpler than bacteria

4. Comparison of animal cell, bacterial cell, and virus Virus Bacterium Animal cell Animal cell nucleus 0.25 µm

5. 1. Concept 18.1: A virus has a genome but can reproduce only within a host cell • Scientists detected viruses indirectly long before they could see them • The story of how viruses were discovered begins in the late 1800s

6. 1—2. 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 • 1883—Adolf Mayer • 1893—Dimitri Ivanowsky • In 1935, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV)

7. Leaf on right is infected with TMV

8. 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

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

10. 4. Capsids and Envelopes • A capsid is the protein shell that encloses the viral genome • Capsomeres are the protein subunits that make up the capsid • A capsid can have various structures

11. 4. Helical capsid Capsomere of capsid RNA 18  250 mm 20 nm Tobacco mosaic virus

12. 4. Polyhedral capsid Capsomere DNA Glycoprotein 70–90 nm (diameter) 50 nm Adenoviruses

13. 4. Polyhedral head and tail apparatus Head DNA Tail sheath Tail fiber 80  225 nm 50 nm Bacteriophage T4

14. 5. Viral envelopes • Some viral structures 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 • They contain host phospholipids and membrane proteins and viral proteins and glycoproteins

15. 5. Enveloped virus Membranous envelope Capsid RNA Glycoprotein 80–200 nm (diameter) 50 nm Influenza viruses

16. 6. Role of the viral envelope • An animal virus with a viral envelope uses it to enter the host cell • Viral glycoproteins bind to specific receptor molecules on the surface of a host cell

17. 6. Capsid Capsid and viral genome enter cell RNA HOST CELL Envelope (with glycoproteins) Viral genome (RNA) Template mRNA Capsid proteins ER Glyco- proteins Copy of genome (RNA) New virus

18. 7. • 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 tailpiece attaches the phage to the host and injects the phage DNA inside

19. 8—9. General Features of Viral Reproductive Cycles • Viruses identify their host cells by a “lock-and-key” fit between proteins on the outside of the virus and specific receptor molecules on the surface of cells • Viruses are obligate intracellular parasites, which means they can reproduce only within a host cell Animation: Simplified Viral Reproductive Cycle

20. 10—11. • Each virus has a host range, a limited number of host cells that it can infect • Broad host range: West Nile virus (mosquitoes, birds, and humans) • Narrow host range: measles virus and poliovirus can only infect humans • Rabies virus has a broad host range (mammals such as dogs, raccoons, bats, humans), but a human cold virus can only infect humans

21. 12. • Viruses use enzymes, ribosomes, and small host molecules (nucleotides, amino acids) to synthesize progeny viruses

22. 12, 13, 15. VIRUS Entry into cell and uncoating of DNA DNA Capsid Transcription Replication HOST CELL Viral DNA mRNA Viral DNA Capsid proteins Self-assembly of new virus particles and their exit from cell

23. 14. Capsid Capsid and viral genome enter cell RNA HOST CELL Envelope (with glycoproteins) Viral genome (RNA) Template mRNA Capsid proteins ER Glyco- proteins Copy of genome (RNA) New virus

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

25. 16, 18. 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

26. The Lytic Cycle Attachment Entry of phage DNA and degradation of host DNA Phage assembly Release Head Tail fibers Tails Synthesis of viral genomes and proteins Assembly

27. 17. • The _______ of the phage enters the cell, leaving the empty _________ outside. • This is accomplished when the sheath of the __________ contracts.

28. 18, 19, 20. Bacterial defenses against phages: • 1. Natural selection favors bacterial mutants with receptor sites that are no longer recognized by phages. • 2. As soon as phage DNA enters the bacterial host cell, it fights back with restriction enzymes, which hydrolyze the viral DNA. (The bacterial cell’s own DNA is chemically modified to protect it from its own restriction enzymes.) • 3. Not all phages kill their hosts—some use the lysogenic cycle

29. 16, 21. The Lysogenic Cycle • The lysogenic cycle replicates the phage genome without destroying the host • The viral DNA molecule is incorporated by genetic recombination 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 • Phages that use both the lytic and lysogenic cycles are called temperate phages Animation: Phage Lambda Lysogenic and Lytic Cycles

30. 23, 24. Phage DNA The phage attaches to a host cell and injects its DNA. Daughter cell with prophage Many cell divisions produce a large 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 normally, copying the prophage and transmitting it to daughter cells. Certain factors determine whether The cell lyses, releasing phages. Lytic cycle is induced Lysogenic cycle is entered or Prophage Phage DNA integrates into the bacterial chromosomes, becoming a prophage. New phage DNA and proteins are synthesized and assembled into phages.

31. 22. • The Trojan horse was full of Greek soldiers who, once they were within the walls of Troy, leapt out and destroyed the city. • Prophages “hide out” within the host cell and are replicated along with the host’s DNA. • They might be triggered to “go lytic” by an environmental signal, such as radiation or the presence of certain chemicals.

32. 25. • Few bacteriophages have an envelope or RNA genome, but… • Animal viruses often have an envelope derived from the host cell’s membrane or nuclear envelope • Animal viruses often have an RNA genome

33. Classifying Animal Viruses • Two key variables in classifying viruses that infect animals: • DNA or RNA? • Single-stranded or double-stranded?

38. 26. 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 is the retrovirus that causes AIDS

39. 27. Viral envelope Glycoprotein Capsid RNA (two identical strands) Reverse transcriptase

40. 28. • 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

41. 29. Membrane of white blood cell HIV HOST CELL Reverse transcription Viral RNA RNA-DNA hybrid 0.25 µm HIV entering a cell DNA NUCLEUS Provirus Chromosomal DNA RNA genome for the next viral generation mRNA New HIV leaving a cell

42. 30. Evolution of Viruses • Viruses do not fit our definition of living organisms • Since viruses can reproduce only within cells, they probably evolved from bits of cellular nucleic acid, such as… • Plasmids, which are small, circular DNA molecules found in bacteria and yeast, or • Transposons, which are DNA segments that can move around within a cell’s genome.

43. Concept 18.2: 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

44. 31. 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 • Some have molecular components that are toxic, such as membrane proteins • Cold viruses attack epithelial cells, which easily replace themselves through cell division. Poliovirus attacks nerve cells, which are irreparable.

45. 32. Fighting back against viruses: • Our immune system provides natural defense against viruses • Modern medicine has developed vaccines and drugs • 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 • Antiviral drugs include -acyclovir, which interferes with viral nucleic acid synthesis -AZT, which interferes with reverse transcriptase

46. 33. 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: --Mutation of existing viruses creating new strains (ex. flu) --Spread of existing viruses from one host species to another (ex. hantavirus, bird flu, swine flu) --Cultural change (ex. people traveling more, blood transfusions, IV drug use, and sexual promiscuity all contributed to the spread of HIV)

47. 33. The SARS-causing agent is a coronavirus like this one (colorized TEM), so named for the “corona” of glyco-protein spikes protruding form the envelope. Young ballet students in Hong Kong wear face masks to protect themselves from the virus causing SARS.

48. 34. H1N1 • H and N stand for the two glycoproteins that are on the surface of the viral envelope of this flu virus (their scientific names are hemagglutinin and neuramidinase) and do its dirty work. There are 16 types of the H protein, numbered H1 through H16, and 9 types of the N protein, numbered N1 through N9. That makes 144 possible combinations of the virus, a constantly changing challenge for prevention or treatment. • A new combination, H2N2, cause a brief swine flu epidemic in 1957. An H3N2 strain was the source of another epidemic in 1968. The bird flu virus that began in Southeast Asia a decade ago and has spread throughout the Old World is an H5N1 combination.

49. Viral Diseases in Plants • More than 2,000 types of viral diseases of plants are known • Some symptoms are spots on leaves and fruits, stunted growth, and damaged flowers or roots