Viruses, Viroids, and Prions

Transcription

1 11/21/2017 PowerPoint Lecture Presentations prepared by Bradley W. Christian, McLennan Community College CHAPTER 13 Viruses, Viroids, and Prions General Characteristics of Viruses Learning Objective Differentiate a virus from a bacterium. 1

2 General Characteristics of Viruses Obligatory intracellular parasites Require living host cells to multiply Contain DNA or RNA Contain a protein coat No ribosomes No ATP-generating mechanism Table 13.1 Viruses and Bacteria Compared 2

3 Host Range The spectrum of host cells a virus can infect Most viruses infect only specific types of cells in one host Determined by specific host attachment sites and cellular factors Bacteriophages viruses that infect bacteria Range from 20 nm to 1000 nm in length Figure 13.1 Virus sizes. Bacteriophage M nm Bacteriophages f2, MS2 Poliovirus 24 nm 30 nm Ebola virus 970 nm Rhinovirus 30 nm Adenovirus Rabies virus Prion Bacteriophage T4 90 nm nm nm 225 nm E. coli bacterium nm 300 nm Chlamydia bacterium elementary body Human red blood cell 10,000 nm in diameter Tobacco mosaic virus Viroid nm nm Plasma membrane of red blood cell 10 nm thick Vaccinia virus nm 3

4 Viral Structure Virion complete, fully developed viral particle Nucleic acid DNA or RNA can be single- or doublestranded; linear or circular Capsid (kapsidi) protein coat made of capsomere subunits (kapsomeeri) Envelope (vaippa) lipid, protein, and carbohydrate coating on some viruses Spikes projections from outer surface Figure 13.2 Morphology of a nonenveloped polyhedral virus. Nucleic acid Capsomere Capsid 4

5 Figure 13.3 Morphology of an enveloped helical virus. Nucleic acid Capsomere Spikes Envelope General Morphology Helical viruses hollow, cylindrical capsid Polyhedral viruses many-sided Enveloped viruses Complex viruses complicated structures 5

6 Figure 13.4 Morphology of a helical virus. Nucleic acid Capsomere Capsid Figure 13.5 Morphology of complex viruses. 65 nm Capsid (head) DNA Sheath Tail fiber Pin Baseplate A T-even bacteriophage Orthopoxvirus 6

7 Solunetti: Isolation, Cultivation, and Identification of Viruses Learning Objectives Describe how bacteriophages are cultured. 7

8 Growing Bacteriophages in the Laboratory Viruses must be grown in living cells Bacteriophages are grown in bacteria Bacteriophages form plaques, which are clearings on a lawn of bacteria on the surface of agar Each plaque corresponds to a single virus; can be expressed as plaque-forming units (PFU) Figure 13.6 Viral plaques formed by bacteriophages. Plaques 8

9 Viral Multiplication Learning Objectives Describe the lytic cycle of bacteriophages. Describe the lysogenic cycle of bacteriophages. Compare and contrast the multiplication cycle of DNA-containing animal viruses to bacteriophages. Multiplication of Bacteriophages For a virus to multiply: It must invade a host cell It must take over the host's metabolic machinery Lytic cycle Phage causes lysis and death of the host cell Lysogenic cycle Phage DNA is incorporated in the host DNA Phage conversion Specialized transduction 9

10 T-Even Bacteriophages: The Lytic Cycle Attachment: phage attaches by the tail fibers to the host cell Penetration: phage lysozyme opens the cell wall; tail sheath contracts to force the tail core and DNA into the cell Biosynthesis: production of phage DNA and proteins Maturation: assembly of phage particles Release: phage lysozyme breaks the cell wall Figure The lytic cycle of a T-even bacteriophage. Attachment: Phage attaches to host cell. Bacterial cell wall Bacterial chromosome Capsid DNA Capsid (head) Sheath Tail fiber Tail Baseplate Pin Cell wall Plasma membrane Penetration: Phage penetrates host cell and injects its DNA. Sheath contracted Biosynthesis: Phage DNA directs synthesis of viral components by the host cell. Tail core Tail DNA Maturation: Viral components are assembled into virions. Capsid Release: Host cell lyses, and new virions are released. Tail fibers 10

11 Bacteriophage Lambda (λ): The Lysogenic Cycle Lysogeny: phage remains latent Phage DNA incorporates into host cell DNA Inserted phage DNA is known as a prophage When the host cell replicates its chromosome, it also replicates prophage DNA Results in phage conversion the host cell exhibits new properties Figure The lysogenic cycle of bacteriophage λ in E. coli. Phage DNA (double-stranded) Phage attaches to host cell and injects DNA. Bacterial chromosome Occasionally, the prophage may excise from the bacterial chromosome by another recombination event, initiating a lytic cycle. Many cell divisions Lytic cycle Lysogenic cycle Cell lyses, releasing phage virions. Phage DNA circularizes and enters lytic cycle or lysogenic cycle. OR Prophage Lysogenic bacterium reproduces normally. New phage DNA and proteins are synthesized and assembled into virions. Phage DNA integrates within the bacterial chromosome by recombination, becoming a prophage. 11

12 Bacteriophage Lambda (λ): The Lysogenic Cycle Specialized transduction Specific bacterial genes transferred to another bacterium via a phage Changes genetic properties of the bacteria Figure Specialized transduction. Prophage gal gene Bacterial DNA Prophage exists in galactose-using host (containing the gal gene). Galactosepositive donor cell gal gene Phage genome excises, carrying with it the adjacent gal gene from the host. gal gene Phage matures and cell lyses, releasing phage carrying gal gene. Phage infects a cell that cannot utilize galactose (lacking gal gene). Galactosenegative recipient cell Along with the prophage, the bacterial gal gene becomes integrated into the new host's DNA. Lysogenic cell can now metabolize galactose. Galactose-positive recombinant cell 12

13 Restriction enzymes A restriction enzyme or restriction endonuclease is an enzyme that cuts DNA at or near specific recognition nucleotide sequences known as restriction sites. These enzymes are found in bacteria and archaea and provide a defense mechanism against invading viruses. Over 3000 restriction enzymes have been studied in detail, and more than 600 of these are available commercially. Essential tools in recombinant DNA technology! Multiplication of Animal Viruses Attachment: viruses attach to the cell membrane Entry by receptor-mediated endocytosis or fusion Uncoating by viral or host enzymes Biosynthesis: production of nucleic acid and proteins Maturation: nucleic acid and capsid proteins assemble Release by budding (enveloped viruses) or rupture 13

14 Solunetti: 1. Tarttuminen isäntäsolun pinnalle (attachment) 2. Tunkeutuminen isäntäsoluun (penetration) 3. Kapsidin hajoaminen ja genomin vapautuminen (uncoating) 4. Replikaatio (replication) 5. Geenien ilmentyminen (gene expression) 6. Uusien virusten kokoaminen (assembly) 7. Kypsyminen (maturation) 8. Vapautuminen solusta (release) Figure The entry of viruses into host cells. Host plasma membrane proteins at site of receptor-mediated endocytosis Fusion of viral envelope and plasma membrane Entry of pig retrovirus by receptor-mediated endocytosis. Entry of herpesvirus by fusion. 14

15 Figure Budding of an enveloped virus. Viral capsid Host cell plasma membrane Viral protein Bud Bud Envelope Release by budding Lentivirus The Biosynthesis of DNA Viruses DNA viruses replicate their DNA in the nucleus of the host using viral enzymes Synthesize capsid in the cytoplasm using host cell enzymes 15

16 Figure Replication of a DNA-Containing Animal Virus. RELEASE Virions are released. MATURATION Virions mature. Papovavirus DNA ATTACHMENT Virion attaches to host cell. Capsid Nucleus Cytoplasm Host cell A papovavirus is a typical DNA-containing virus that attacks animal cells. ENTRY and UNCOATING Virion enters cell, and its DNA is uncoated. Capsid proteins Viral DNA BIOSYNTHESIS Viral DNA is replicated, and some viral proteins are made. mrna Capsid proteins Late translation; capsid proteins are synthesized. A portion of viral DNA is transcribed, producing mrna that encodes "early" viral proteins. KEY CONCEPTS Viral replication in animals generally follows these steps: attachment, entry, uncoating, biosynthesis of nucleic acids and proteins, maturation, and release. Knowledge of viral replication phases is important for drug development strategies, and for understanding disease pathology. Check Your Understanding Describe the principal events of attachment, entry, uncoating, biosynthesis, maturation, and release of an enveloped DNA-containing virus. 16