Ch 19 Viruses 1 Chapter 19 Viruses A Borrowed Life T4 bacteriophage infects e Coli o o o By injecting its DNA into the cell the virus sets in motion a genetic takeover of bacterium recruiting cellular machinery to mass produce many new viruses Recall that bacteria and other prokaryotes are cells much smaller and more simply organized than cells of eukaryotes Virus lacks the structure and metabolic machinery found in a cell is an infectious particle consisting of little more than such as plants and animals genes packaged in a protein coat Viruses cannot reproduce or carry out metabolic activities outside of a host cell Experiments with viruses provided important evidence that genes are made of nucleic acids and they were critical in working out the molecular mechanisms of the fundamental processes of DNA replication transcription and translation Concept 19 1 A virus consists of a nucleic acid surrounded by protein coat o The Discovery of Viruses scientific inquiry In 1883 Adolf Mayer a German scientist discovered that he could transmit the disease from plant to plant by rubbing sap extracted from diseased leaves onto healthy plants Mayer suggested that unusually small bacteria that were invisible caused the disease Dimitri Ivanowsky clung to the hypothesis that bacteria caused tobacco mosaic disease He reasoned that the bacteria were small enough to pass through the filter or made a toxin that could Matinus Beijernick carried out experiments that showed that the infectious agent in the filtered sap could do so replicate The pathogens replicated only within the host it infected The mysterious agent of mosaic disease could not be cultivated in petri dish He imagined a replicating particle much smaller and simpler than bacteria Wendell Stanley crystallized the infectious particle now known as tobacco mosaic virus TMV o Structure of viruses The tiniest viruses are only 20 nm in diameter smaller than a ribosome The structure of a virus reveals that it is an infectious particle consisting of nucleic acid enclosed in a protein coat and for some viruses surrounded by a membranous envelope The genome of a virus can consist of double stranded DNA single stranded DNA double stranded RNA or single stranded RNA depending on the type of virus A virus is called a DNA virus or an RNA virus based on the kind of nucleic acid that makes up its o In either case the genome is usually organized as a single linear or circular molecule of The smallest viruses known have only 4 genes in their genome while the largest have several hundred to a The protein shell enclosing the viral genome is called a capsid Depending on the type of virus the capsid may be rod shaped polyhedral or more complex in shape genome nucleic acid thousand like t4 Capsids are built from a large number of protein subunits called capsomeres Rod shaped viruses are commonly called helical viruses because they are made from over a thousand molecules of a single type of protein arranged in a helix An icosahedron is a polyhedral capsid with triangular facets thus these and other similarly shaped viruses are referred to as icosahedral viruses Adenoviruses infect the respiratory tracts of animals are icosahedron Viral envelopes which are derived from the membranes of the host cell contain host cell phospholipids and Ch 19 Viruses 2 The membranous envelope surrounds the capsids of influenza viruses and other viruses found in They also contain proteins and glycoproteins proteins with carbohydrates covalently attached Many of the most complex capsids are found among the viruses that infect bacteria called bacteriophages or membrane proteins animals simply phages Of the seven phages the even numbered phages T2 T4 and T6 found in E coli were similar in structure Their capsids have elongated icosahedral heads enclosing their DNA Attached to the head is a protein tailpiece with fibers by which the pages attach to a bacterium T even use their elaborate tail apparatus 19 1 Summary to inject DNA into a bacterium Researchers discovered viruses in the late 1800s by studying a plant disease tobacco mosaic disease A virus is a small nucleic acid genome enclosed in a protein capsid and sometimes a membranous viral envelope containing viral proteins that help viruses enter cells The genome may be single or double stranded DNA or RNA Concept 19 2 Viruses replicate only in host cells o General features of viral replicative cycles Viruses lack metabolic enzymes and equipment for making proteins such as ribosomes They can only replicate within a host cell Each particular virus can infect cells of only a limited number of host species called the host range of the virus Viruses usually identify host cells by a lock and key fit between viral surface proteins and specific receptor molecules on the outside of cells West Nile is a virus that infects mosquitoes birds horses and humans Measles virus can only infect humans o Viral infection of multicellular eukaryotes is usually limited to particular tissues A viral infection begins when a virus binds to a host cell and the viral genome makes it way inside Other viruses are taken up by endocytosis or in the case of enveloped viruses by fusion of the viral envelope with the plasma membrane manufacture viral proteins Once the viral genome is inside the protein it can reprogram the cell to copy the viral nucleic acid and o The host provides the nucleotides for making viral nucleic acids as well as enzymes ribosomes tRNAs amino acids ATP and other components needed form making the viral o Many DNA viruses use the DNA polymerase of the host cell to synthesize new genomes along the templates provided by the viral DNA After the viral nucleic acid molecules and capsomeres are produced they spontaneously self assemble proteins into new viruses The simplest type of viral replicative cycle ends with the exit of hundreds or thousands of viruses from the infected host cell a process that often damages or destroys the cell The viral progeny that exit a cell have the potential to infect additional cells spreading the viral Ch 19 Viruses 3 infection o Replicative Cycles of Phages cycle The lytic cycle kills the host cell Some double stranded DNA viruses can replicate two alternative mechanisms the lytic cycle and the lysogenic A phage replicative cycle that culminates in death of the host cell is known as the lytic cycle The last stage of infection the bacterium lyses breaks open and release the phages that were produced within the