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Viruses Ch. 19.

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Presentation on theme: "Viruses Ch. 19."— Presentation transcript:

1 Viruses Ch. 19

2 Discovery – Tobacco Mosaic Virus
1883 – Discovered that diseases were contagious 1890’s – Tried to filter out the contagious agent from the sap 1897 – Proposal that pathogen was something even smaller than a bacteria Reproduction only occurred in the host Couldn’t be cultured Couldn’t be killed by alcohol

3 Discovery – Tobacco Mosaic Virus
1935 – Stanley crystallized the tobacco mosaic virus


5 Structure of a Virus Infectious particle consisting of a DNA and a protein coat Viral genomes Linear or circular DNA (double or single strand) or RNA (double or single strand) Capsid protein shell around virus Rod shaped, polyhedral, or more complex Envelope covers capsid; derived from host cell Contain host cell phospholipids and membrane proteins

6 Bacteriophages Viruses that infect bacteria
First seven were named accordingly T2, T4, T6 very similar in structure 6

7 Characteristics Obligate intracellular parasites Host range
limited # or range of host cells that a parasite can infect Tissue specific Cold virus and HIV Virulent causes disease immediately Only utilizes the lytic cycle Temperate remains dormant for right conditions Uses both the lysogenic and lytic cycles

8 How viruses work


10 Lytic Cycle Attachment
phage attaches to the host cell using its tail fibers to bind to receptor sites Entry DNA/RNA injected Cell’s DNA is hydrolyzed Replication of viral genomes and proteins phage components made – proteins and genome Assembly phage components assembled Release cell lyses and releases phage

11 Bacterial Defenses Why haven’t viruses exterminated bacteria?
Evolution favors bacterial mutants with receptors no longer recognizable by the virus Restriction enzymes naturally occurring bacterial enzymes that protect bacteria against intruding DNA by cutting up the DNA Blunt cuts or sticky end cuts Coevolving In constant evolutionary “flux” bacterial hosts and viral parasite

12 Figure 18.5 The lysogenic and lytic reproductive cycles of phage , a temperate phage

13 Lysogenic Cycle Temperate v. virulent
cycle utilized by temperate viruses Prophage DNA incorporated into a bacterial chromosome and leaves the host’s genome during the lytic cycle Provirus once incorporated into the genome – never leaves the cell HIV typically in animals Reproduces when bacteria reproduces Eventually switches to lytic cycle environmental triggers

14 Replication Cycle of an enveloped RNA virus


16 RNA as Genetic Material
Classified according to strandedness Retroviruses genetic info. flows backwards Reverse transcriptase enzyme that forms DNA from RNA Difficult to make long term vaccines due to mutations (HIV, influenza)

17 Figure 19.8 Membrane of white blood cell Glycoprotein Viral envelope
HIV Capsid RNA (two identical strands) HOST CELL Reverse transcriptase HIV Reverse transcriptase Viral RNA RNA-DNA hybrid DNA 0.25 m HIV entering a cell NUCLEUS Provirus Chromosomal DNA RNA genome for the next viral generation Figure 19.8 The replicative cycle of HIV, the retrovirus that causes AIDS. mRNA New virus New HIV leaving a cell

18 Figure 18.7x1 HIV infection

19 Viruses may… Damage or kill cells
Be toxic or cause infected cells to produce toxins Cause varying degrees of cell damage Colds vs. polio

20 Figure 18.x1 Smallpox

21 Figure 18.x2 Measles

22 Figure 18.x6 Herpes

23 Vaccines Harmless variants or derivatives of pathogenic microbes that mobilize a host’s immune mechanism against the pathogen Antibiotics are powerless Viruses don’t have enzymes to interrupt the metabolism Some exceptions…drugs that interfere with viral processes Acyclovir Inhibits herpes virus the viral polymerase AZT Curbs HIV replication by interfering with reverse transcriptase Tama flu Interferes with the enzyme that cuts the virus loose from the cell membrane so the virus remains attached and doesn’t enter body fluids Must take within 48 hours




















43 What happened????





48 Emerging Diseases Viruses that suddenly become apparent Epidemic
General outbreak April 2009 H1N1 influenza A virus Pandemic Global epidemic What causes the emergence? Mutation of existing RNA viruses Dissemination of a viral disease from a small, isolated human population Spread of existing viruses from other animals 48

49 Viroids Smaller than viruses Small, circular molecules of naked RNA
Originated as escaped introns Infect plants Do not encode proteins Disrupt plant metabolism errors in systems that control growth 49

50 Figure 18.9 Viral infection of plants

51 Prions Infectious proteins Cannot self replicate
Act slowly (incubation period of 10 years) Virtually indestructible Cause degenerative brain diseases Misfolded protein (theory) that converts normal proteins into the incorrectly folded version Mad cow disease Scrapie in sheep Creutzfelt-Jakob disease and Kuru - humans

52 Bacteria – supplement to Ch. 19 pgs. 559-564 in textbook

53 Figure 18.x7 E. coli

54 Bacterial Chromosome One-double stranded, circular molecule of DNA
Simpler and fewer associated proteins than a eukaryotic chromosome Many contain plasmids Rapidly reproduce Binary fission Mutation is key cause of genetic variation

55 Figure 18.11 Replication of the bacterial chromosome

56 Figure 18.x8 E. coli dividing

57 Figure 18.12 Detecting genetic recombination in bacteria

58 Three Natural Processes of Genetic Recombination
Transformation Process of gene transfer during which a bacterial cell assimilates foreign DNA from the surroundings Transduction Gene transfer from one bacterium to another by a bacteriophage Conjugation Bacterial “sex”

59 Genetic Engineering

60 Transformation Alteration of bacterial DNA
naked foreign DNA is taken in by bacterial genotype Genetic recombination New combination of DNA exchange of DNA segments through crossing over (in eukaryotes) Transformation, transduction, and conjugation in prokaryotes Use of E. coli Make cell competent by heat shock and altering cell wall with Ca+ insulin & growth hormone

61 Transduction DNA transfer process
carry bacterial DNA from one host to another Generalized transduction random pieces of bacterial chromosome packaged in phage Specialized transduction only certain genes are transferred temperate phages

62 Figure 18.13 Transduction (Layer 1)

63 Figure 18.13 Transduction (Layer 2)

64 Figure 18.13 Transduction (Layer 3)

65 Figure 18.13 Transduction (Layer 4)

66 Conjugation and Plasmids
direct transfer of genetic material b/w bacteria while being temporarily joined; DNA donor and recipient bacterial version of sexual reproduction F factor used allow conjugation Plasmid circular DNA that is separate from bacterial chromosomes; self replicating

67 Figure 18.15 Conjugation and recombination in E. coli (Layer 1)

68 Figure 18.15 Conjugation and recombination in E. coli (Layer 2)

69 Figure 18.15 Conjugation and recombination in E. coli (Layer 3)

70 Figure 18.15 Conjugation and recombination in E. coli (Layer 4)

71 Conjugation and Plasmids
Episome replicates by itself or as part of a bacterial chromosome F Plasmid and conjugation F for fertility allows genetic recombination to occur F+ has plasmid; F- plasmid is absent

72 Antibiotic Resistance
Shigella 1st strain to show resistance R plasmids (R = resistance) genes code for enzymes that destroy certain antibiotics MRSA Methicillin resistant staphylococcus areus

73 Transposons (jumping genes)
piece of DNA that can move from one location to another in the cell’s genome

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