Presentation is loading. Please wait.
Published byJulia Wheeler Modified over 7 years ago
WHAT DOES IT MEAN TO BE ALIVE?
Are the tiny viruses infecting this E. coli cell alive? 0.5 mm
“I’D LIKE TO INFECT YOU WITH MY DNA” (VIRAL REPRODUCTION) ;)
VIRAL VIDEO LINKS NPR podcastNPR podcast : how viruses invade your body SciShowSciShow: Top 5 Deadliest Disease!!
LECTURE PRESENTATIONS For CAMPBELL BIOLOGY, NINTH EDITION Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson © 2011 Pearson Education, Inc. Lectures by Erin Barley Kathleen Fitzpatrick Viruses Chapter 19
Concept 19.1: A virus consists of a nucleic acid surrounded by a protein coat Viruses were detected indirectly long before they were actually seen © 2011 Pearson Education, Inc.
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 In 1935, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV) © 2011 Pearson Education, Inc.
Figure 19.2 Extracted sap from tobacco plant with tobacco mosaic disease RESULTS Passed sap through a porcelain filter known to trap bacteria Healthy plants became infected Rubbed filtered sap on healthy tobacco plants 1234
Structure of Viruses Viruses are not cells, they are infectious particles © 2011 Pearson Education, Inc.
Capsids and Envelopes A capsid is the protein shell that encloses the viral genome Capsids are built from protein subunits called capsomeres A capsid can have various structures © 2011 Pearson Education, Inc.
Examples of viral structure Capsomere of capsid RNA Capsomere DNA Glycoprotein Glycoproteins Membranous envelope RNA Capsid Head DNA Tail sheath Tail fiber 18 250 nm 80 225 nm 70–90 nm (diameter) 80–200 nm (diameter) 20 nm 50 nm (a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses(d) Bacteriophage T4
Concept 19.2: Viruses replicate only in host cells Viruses are obligate intracellular parasites, which means they can replicate only within a host cell using the cell’s parts, much like… © 2011 Pearson Education, Inc.
General Features of Viral Replicative Cycles Once a viral genome has entered a cell, the cell begins to manufacture viral proteins The virus makes use of host enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules Viral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses © 2011 Pearson Education, Inc.
Animation: Simplified Viral Reproductive Cycle Right-click slide / select “Play”
VIRUS 2134 Entry and uncoating Replication Transcription and manufacture of capsid proteins Self-assembly of new virus particles and their exit from the cell DNA Capsid HOST CELL Viral DNA mRNA Capsid proteins A simplified viral replicative cycle.
Replicative Cycles of Phages Phages are the best understood of all viruses Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle (see animations that follow) © 2011 Pearson Education, Inc.
Animation: Phage T4 Lytic Cycle Right-click slide / select “Play”
Figure 19.5-1 Attachment 1
Figure 19.5-2 Attachment 21 Entry of phage DNA and degradation of host DNA
Figure 19.5-3 Attachment 213 Entry of phage DNA and degradation of host DNA Synthesis of viral genomes and proteins
Figure 19.5-4 Attachment 2143 Entry of phage DNA and degradation of host DNA Synthesis of viral genomes and proteins Assembly Phage assembly Head TailTail fibers
Figure 19.5-5 Attachment 21543 Entry of phage DNA and degradation of host DNA Release Synthesis of viral genomes and proteins Assembly Phage assembly Head TailTail fibers
The Lysogenic Cycle The lysogenic cycle replicates the phage genome without destroying the host The viral DNA molecule is incorporated 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 © 2011 Pearson Education, Inc.
Animation: Phage Lambda Lysogenic and Lytic Cycles Right-click slide / select “Play”
An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode Phages that use both the lytic and lysogenic cycles are called temperate phages © 2011 Pearson Education, Inc.
Figure 19.6 New phage DNA and proteins are synthesized and assembled into phages. The cell lyses, releasing phages. Phage Phage DNA The phage injects its DNA. Bacterial chromosome Lytic cycle lytic cycle is induced or Phage DNA circularizes. Certain factors determine whether lysogenic cycle is entered Lysogenic cycle Prophage Daughter cell with prophage Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Cell divisions produce a population of bacteria infected with the prophage. The bacterium reproduces, copying the prophage and transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a prophage.
Capsid RNA Envelope (with glycoproteins) Capsid and viral genome enter the cell HOST CELL Viral genome (RNA) Template mRNA ER Capsid proteins Copy of genome (RNA) New virus Glyco- proteins The replicative cycle of an enveloped RNA virus.
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 (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome) © 2011 Pearson Education, Inc.
Glycoprotein Reverse transcriptase HIV Viral envelope Capsid RNA (two identical strands) HOST CELL Viral RNA Reverse transcriptase RNA-DNA hybrid DNA NUCLEUS Provirus Chromosomal DNA RNA genome for the next viral generation mRNA New virus HIV Membrane of white blood cell 0.25 m HIV entering a cell New HIV leaving a cell HIV: a retrovirus
© 2011 Pearson Education, Inc. Animation: HIV Reproductive Cycle Right-click slide / select “Play”
Concept 19.3: Viruses, viroids, and prions are formidable pathogens in animals and plants © 2011 Pearson Education, Inc.
Why viruses cause disease in animals, other than by lysis 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 Others have molecular components such as envelope proteins that are toxic © 2011 Pearson Education, Inc.
Vaccines are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the harmful pathogen –Vaccines can prevent certain viral illnesses Viral infections cannot be treated by antibiotics Antiviral drugs can help to treat, though not cure, viral infections © 2011 Pearson Education, Inc.
Emerging Viruses Mostly spread from animals or massively mutated pre-existing viruses –The 2009 flu pandemic was likely passed to humans from pigs; for this reason it was originally called the “swine flu” © 2011 Pearson Education, Inc.
Figure 19.9 (c) 1918 flu pandemic 2009 pandemic screening (b) 2009 pandemic H1N1 influenza A virus (a) 1 m
Viroids and Prions: The Simplest Infectious Agents Viroids are small circular RNA molecules that infect plants and disrupt their growth Prions are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals –Prions propagate by converting normal proteins into the prion version –Scrapie in sheep, mad cow disease, and Creutzfeldt- Jakob disease in humans are all caused by prions © 2011 Pearson Education, Inc.
Figure 19.11 Prion Normal protein Original prion New prion Aggregates of prions
Test your understanding
Test Your Understanding, question 8 Time A B Number of bacteria Number of viruses
Test Your Understanding, question 6
© 2023 SlidePlayer.com Inc.
All rights reserved.