2. Virus Structure and Method of Invasion

3. http://videos.howstuffworks.com/tlc/29852- understanding-viruses-video.htmhttp://videos.howstuffworks.com/tlc/29852- understanding-viruses-video.htm

4. Viruses called bacteriophages –Can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli Figure 18.1 0.5  m

5. Bacteriophages, also called phages –Have the most complex capsids found among viruses Figure 18.4d 80  225 nm 50 nm (d) Bacteriophage T4 DNA Head Tail fiber Tail sheath

6. Recall that bacteria are prokaryotes –With cells much smaller and more simply organized than those of eukaryotes Viruses Are smaller and simpler still Figure 18.2 0.25  m Virus Animal cell Bacterium Animal cell nucleus

7. Obligate parasites -- A virus has a genome but can reproduce only within a host cell Reproduction is their only true characteristic of being alive Specific to type of cells they target -poliomylelitis virus attacks nerve cells -hepatitis virus attacks liver cells

8. Viral Structure Not a cell! Hijacks biochemical machinery of host cell to carry out processes necessary to reproduce Obligate intracellular parasite

9. Viral Genomes Viral genomes may consist of –Double- or single-stranded DNA –Double- or single-stranded RNA

10. Classes of animal viruses Table 18.1

11. Figure 18.4a, b 18  250 mm 70–90 nm (diameter) 20 nm 50 nm (a) Tobacco mosaic virus (b) Adenoviruses RNA DNA Capsomere Glycoprotein Capsomere of capsid Capsids and Envelopes A capsid –Is the protein shell that encloses the viral genome –Can have various structures

12. Some viruses have envelopes –Which are membranous coverings derived from the membrane of the host cell Figure 18.4c 80–200 nm (diameter) 50 nm (c) Influenza viruses RNA Glycoprotein Membranous envelope Capsid

13. Bacteriophage

14. Invasion of a cell by a virus Virus can lie dormant for many years until it comes into contact with a suitable host cell Binds with molecules on surface of host cell Herpes -whole virus enters cell Bacteriophage -viral DNA injected via hollow tail

15. Alteration of Cell Instruction Virus takes control of cell machinery Depends on host for -ATP -supply of free nucleotides Virus suppresses cell’s normal nucleic acid replication and protein synthesis Manufactures many identical copies of viral nucleic acid and protein coats

16. The Lytic Cycle The lytic cycle –Is a phage reproductive cycle that culminates in the death of the host –Produces new phages and digests the host’s cell wall, releasing the progeny viruses

17. The lytic cycle of phage T4, a virulent phage Phage assembly Head Tails Tail fibers Figure 18.6 Attachment. The T4 phage uses its tail fibers to bind to specific receptor sites on the outer surface of an E. coli cell. 1 Entry of phage DNA and degradation of host DNA. The sheath of the tail contracts, injecting the phage DNA into the cell and leaving an empty capsid outside. The cell’s DNA is hydrolyzed. 2 Synthesis of viral genomes and proteins. The phage DNA directs production of phage proteins and copies of the phage genome by host enzymes, using components within the cell. 3 Assembly. Three separate sets of proteins self-assemble to form phage heads, tails, and tail fibers. The phage genome is packaged inside the capsid as the head forms. 4 Release. The phage directs production of an enzyme that damages the bacterial cell wall, allowing fluid to enter. The cell swells and finally bursts, releasing 100 to 200 phage particles. 5

18. The Lysogenic Cycle The lysogenic cycle –Replicates the phage genome without destroying the host Temperate phages –Are capable of using both the lytic and lysogenic cycles of reproduction

19. The lytic and lysogenic cycles of phage, a temperate phage Many cell divisions produce a large population of bacteria infected with the prophage. The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a prophage. New phage DNA and proteins are synthesized and assembled into phages. Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Certain factors determine whether The phage attaches to a host cell and injects its DNA. Phage DNA circularizes The cell lyses, releasing phages. Lytic cycle is induced Lysogenic cycle is entered Lysogenic cycleLytic cycle or Prophage Bacterial chromosome Phage DNA Figure 18.7

20. Retrovirus Contains RNA No DNA to transcribe into mRNA Reverse transcriptase injected into cell by virus with RNA -enzyme reverses normal transcription -Produces viral DNA from RNA -Virus uses DNA to replicate

21. Some video clips Bacteriophage entering a cell HIV replication http://www.youtube.com/watch?v=HhhRQ 4t95OIhttp://www.youtube.com/watch?v=HhhRQ 4t95OI

22. Emerging Viruses Emerging viruses –Are those that appear suddenly or suddenly come to the attention of medical scientists

23. Severe acute respiratory syndrome (SARS) –Recently appeared in China Figure 18.11 A, B (a) Young ballet students in Hong Kong wear face masks to protect themselves from the virus causing SARS. (b) The SARS-causing agent is a coronavirus like this one (colorized TEM), so named for the “corona” of glycoprotein spikes protruding from the envelope.

24. Outbreaks of “new” viral diseases in humans –Are usually caused by existing viruses that expand their host territory

25. Viral Diseases in Plants More than 2,000 types of viral diseases of plants are known Common symptoms of viral infection include –Spots on leaves and fruits, stunted growth, and damaged flowers or roots Figure 18.12

26. Plant viruses spread disease in two major modes –Horizontal transmission, entering through damaged cell walls –Vertical transmission, inheriting the virus from a parent

27. Viroids and Prions: The Simplest Infectious Agents Viroids –Are circular RNA molecules that infect plants and disrupt their growth

28. Prions –Are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals –Propagate by converting normal proteins into the prion version Figure 18.13 Prion Normal protein Original prion New prion Many prions

29. Concept 18.3: Rapid reproduction, mutation, and genetic recombination contribute to the genetic diversity of bacteria Bacteria allow researchers –To investigate molecular genetics in the simplest true organisms

30. The Bacterial Genome and Its Replication The bacterial chromosome –Is usually a circular DNA molecule with few associated proteins In addition to the chromosome –Many bacteria have plasmids, smaller circular DNA molecules that can replicate independently of the bacterial chromosome

31. Operons: The Basic Concept In bacteria, genes are often clustered into operons, composed of –An operator, an “on-off” switch –A promoter –Genes for metabolic enzymes

32. Bacterial cells divide by binary fission –Which is preceded by replication of the bacterial chromosome Replication fork Origin of replication Termination of replication Figure 18.14

33. Mutation and Genetic Recombination as Sources of Genetic Variation Since bacteria can reproduce rapidly –New mutations can quickly increase a population’s genetic diversity

34. An operon –Is usually turned “on” –Can be switched off by a protein called a repressor

35. The trp operon: regulated synthesis of repressible enzymes Figure 18.21a (a) Tryptophan absent, repressor inactive, operon on. RNA polymerase attaches to the DNA at the promoter and transcribes the operon’s genes. Genes of operon Inactive repressor Protein Operator Polypeptides that make up enzymes for tryptophan synthesis Promoter Regulatory gene RNA polymerase Start codon Stop codon Promoter trp operon 5 3 mRNA 5 trpD trpE trpCtrpB trpA trpR DNA mRNA ED C BA

36. DNA mRNA Protein Tryptophan (corepressor) Active repressor No RNA made Tryptophan present, repressor active, operon off. As tryptophan accumulates, it inhibits its own production by activating the repressor protein. (b) Figure 18.21b

37. Repressible and Inducible Operons: Two Types of Negative Gene Regulation In a repressible operon –Binding of a specific repressor protein to the operator shuts off transcription In an inducible operon –Binding of an inducer to an innately inactive repressor inactivates the repressor and turns on transcription

38. The lac operon: regulated synthesis of inducible enzymes Figure 18.22a DNA mRNA Protein Active repressor RNA polymerase No RNA made lacZ lacl Regulatory gene Operator Promoter Lactose absent, repressor active, operon off. The lac repressor is innately active, and in the absence of lactose it switches off the operon by binding to the operator. (a) 5 3

39. mRNA 5' DNA mRNA Protein Allolactose (inducer) Inactive repressor lacl lacz lacYlacA RNA polymerase PermeaseTransacetylase  -Galactosidase 5 3 (b) Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced. mRNA 5 lac operon Figure 18.22b

40. Inducible enzymes –Usually function in catabolic pathways Repressible enzymes –Usually function in anabolic pathways

41. Regulation of both the trp and lac operons –Involves the negative control of genes, because the operons are switched off by the active form of the repressor protein

42. Positive Gene Regulation Some operons are also subject to positive control –Via a stimulatory activator protein, such as catabolite activator protein (CAP)

43. Promoter Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized. If glucose is scarce, the high level of cAMP activates CAP, and the lac operon produces large amounts of mRNA for the lactose pathway. (a) CAP-binding site Operator RNA polymerase can bind and transcribe Inactive CAP Active CAP cAMP DNA Inactive lac repressor lacl lacZ Figure 18.23a In E. coli, when glucose, a preferred food source, is scarce –The lac operon is activated by the binding of a regulatory protein, catabolite activator protein (CAP)

44. When glucose levels in an E. coli cell increase –CAP detaches from the lac operon, turning it off Figure 18.23b (b) Lactose present, glucose present (cAMP level low): little lac mRNA synthesized. When glucose is present, cAMP is scarce, and CAP is unable to stimulate transcription. Inactive lac repressor Inactive CAP DNA RNA polymerase can’t bind Operator lacl lacZ CAP-binding site Promoter