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T4 bacteriophage infecting an E. coli cell 0.5 m Science as a Process Research into TMV led to the conclusion that the pathogen was smaller than a bacterial cell The pathogen was named virus Comparing the size of a virus, a bacterium, and an animal cell Virus Bacterium Animal cell Animal cell nucleus 0.25 m Infection by Tobacco Mosaic Virus Figure 18.4 Viral Structure Capsomere of capsid RNA Capsomere Membranous envelope DNA Head Capsid Tail sheath RNA DNA Tail fiber Glycoprotein 18 250 mm 20 nm (a) Tobacco mosaic virus Glycoprotein 70–90 nm (diameter) 80–200 nm (diameter) 50 nm 50 nm (b) Adenoviruses (c) Influenza viruses 80 225 nm 50 nm (d) Bacteriophage T4 A virus has either DNA or RNA for its genome, but not both and has no mechanism for protein synthesis (ribosomes) or metabolism (mitochondria) It also has a protein coat called a capsid. Some have a coat made of glycoproteins. A virus can reproduce only within a host and not on its own. Capsids and Envelopes Capsid = protein coat that surrounds the viral genome viral envelope = derived from host cell or nuclear membranes; it helps the virus invade the host cell Figure 18.02x2 Phages Figure 18.9 Viral infection of plants Viral Genome Double stranded DNA Single Stranded DNA Double stranded RNA Single stranded RNA A virus has only one of these types of nucleic acids Table 18.1 Classes of Animal Viruses, Grouped by Type of Nucleic Acid Figure 18.6 The reproductive cycle of an enveloped virus Viral Replication What are the possible patterns of viral replication? DNA --> DNA RNA --> RNA, where viral genes code for RNA replicase RNA --> DNA --> RNA; where viral gene uses reverse transcriptase Bacterial Viruses Which scientists used bacteriophages to prove that DNA was the hereditary material? Hershey and Chase What are the two mechanisms of phage infection? Lytic and Lysogenic cycles Figure 18.4 The lytic cycle of phage T4 Figure 18.5 The lysogenic and lytic reproductive cycles of phage , a temperate phage Bacterial Defense What defense do bacteria have against phage infection? Restriction enzymes What do restriction enzymes do? They cut up DNA The bacterial DNA is modified to protect it from the restriction endonucleases. Animal Viruses What is the viral envelope? An outer membrane that helps the virus to invade the animal cell. The invasion of the virus has the following stages ... 1. Attachment 2. Entry 3. Uncoating 4. RNA and protein synthesis 5. Assembly and release Herpesvirus Consists of double stranded DNA Envelope derived from host cell nuclear envelope not from plasma membrane It, therefore, reproduces within the nucleus May integrate its DNA as a provirus Tends to recur throughout lifetime of infected individual. Figure 18.x6 Herpes The structure of HIV, the retrovirus that causes AIDS Glycoprotein Viral envelope Capsid Reverse transcriptase RNA (two identical strands) Figure 18.7x1 HIV infection HIV The reproductive cycle of HIV, a retrovirus Membrane of white blood cell 1 The virus fuses with the cell’s plasma membrane. The capsid proteins are removed, releasing the viral proteins and RNA. 2 Reverse transcriptase catalyzes the synthesis of a DNA strand complementary to the viral RNA. HOST CELL 3 Reverse transcriptase catalyzes the synthesis of a second DNA strand complementary to the first. Reverse transcriptase Viral RNA RNA-DNA hybrid 4 The double-stranded DNA is incorporated as a provirus into the cell’s DNA. 0.25 µm HIV entering a cell DNA NUCLEUS Chromosomal DNA Provirus 5 Proviral genes are transcribed into RNA molecules, which serve as genomes for the next viral generation and as mRNAs for translation into viral proteins. RNA genome for the next viral generation mRNA 6 The viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelope glycoproteins (made in the ER). New HIV leaving a cell 9 New viruses bud off from the host cell. 8 Capsids are assembled around viral genomes and reverse transcriptase molecules. 7 Vesicles transport the glycoproteins from the ER to the cell’s plasma membrane. Viral Evolution What is the current hypothesis concerning how viruses evolved? Viral Disease Some viruses have toxic components and cause infected cells to release enzymes from lysosomes Recovery involves ability to repair damaged region of the body. Ex: polio may permanently damage nerve cells. Figure 18.x3 Polio Vaccines / Drugs What are vaccines and how do they work? Introduce body to harmless or weakened strain of the virus, so that your immune system learns to recognize the virus prior to invasion Few drugs around to fight viruses, most interfere with DNA, RNA, or protein synthesis Smallpox Emerging Viruses HIV Ebola Influenza From where do these viruses emerge? From mutated versions of current viruses Jump from current host to new host Move from a previously isolated region of the world Deer Mouse – Vector of Hantavirus SARS (severe acute respiratory syndrome), a recently emerging viral disease (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. Viroids and Prions Viroids are naked circular RNA that infect plants Prions are proteins that infect cells Examples of prions seen in Scrapies in sheep, mad-cow disease, and Creutzfeldt-Jakob disease in humans How can a prion spread infection? Altered versions of proteins that can alter other proteins Model for how prions propagate Prion Original prion Many prions Normal protein New prion Replication of the Bacterial Chromosome E. coli E. coli Dividing Transformation of Bacteria - Griffith Bacterium releasing DNA with plasmids Plasmids Transduction Bacterial Mating “Male” with the F factor Conjugation and recombination in E. coli Viruses and Cancer Hepatitus B virus can cause liver cancer Some viral genes can trigger cancerous genetic conditions Oncogenes = viral genes that trigger cancerous characteristics proto-oncogenes = genes already found in normal cells, usually regulate growth factors Insertion sequences, the simplest transposons Insertion of a transposon and creation of direct repeats Anatomy of a Composite Transposon