CHAPTER 36 NOTES VIRUSES Virus an obligate dependent intracellular must enter host cell parasite reproduce at expense of host cell their own o Are not cells or made up of cells so are not organisms o Cannot manufacture their own ATP amino acids or nucleotides cannot produce proteins on o Are not alive but have a genome are superbly adapted and evolve Viruses enter host cells take over biosynthetic machinery and use it to manufacture more viruses o Each type of virus infects a specific unicellular species or cell type in a multicellular species 36 1 Why do biologists study viruses Viruses participate in organismal diversity by introducing foreign genes into cellular genomes o Promote lateral gene transfer o Also contribute their own genetic material to organisms some of which has evolved to be part of human genome e g a viral gene for a protein is used for placenta development Epidemic a disease that rapidly affects a large number of individuals over a widening area o Pandemic an epidemic that is worldwide e g the Spanish flu in 1918 1919 o Virulent tending to cause severe disease Acquired immune deficiency syndrome AIDS an affliction caused by the human immunodeficiency virus HIV response to invading pathogens o HIV parasitizes helper T cells of the immune system which are crucial to the immune system s o cells destroyed by HIV cells produced to replace those o Too few T cells left to fight off pathogens opportunistic infections prove fatal o AIDS has already killed 30 million people currently 34 million infected with HIV o HIV infection rates highest in east and central Africa o People infected with HIV in late teens or twenties die of AIDS in their twenties thirties 36 2 How do biologists study viruses Researchers have two goals 1 developing vaccines that help hosts fight off disease if they become infected and 2 developing antiviral drugs that prevent a virus from replicating efficiently inside the host Viruses are 20 300nm in diameter so researchers can isolate a virus by passing infected culture through a filter that is large enough for viruses but too small for cells and then exposing susceptible host cells to the filtrate to confirm the presence of a virus Viruses are studied and characterized by 1 the structure of the extracellular infectious particle the virion 2 the nature of the genetic material that is transmitted by virions from one host to another and 3 variations in how viruses replicate Analyzing morphological traits o Viruses are either enclosed by just a shell of protein called a capsid or both a capsid and a membrane like envelope infecting a new cell material The capsid protects the genome while outside the host and releases the genome while Non enveloped viruses naked viruses use only the capsid to protect their genetic Enveloped viruses have an additional layer consisting of viral proteins embedded in a phospholipid bilayer derived from a membrane found in a host cell o Most virions are helical or icosahedral in shape bacteriophage T4 and smallpox are more complex Analyzing genetic material viral proteins mRNAs o Positive sense RNA virus genome contains same sequences as the mRNA required to produce o Negative sense RNA virus base sequences in the genome are complementary to those in viral o Ambisense RNA virus genome has at least one other strand that contains two regions one is positive sense the other negative sense Viruses infect host cells via o Replicative growth produces the next generation of virions and often kills the host cell 6 phases 1 Attachment to a host cell and entry into the cytosol 2 Transcription of the viral genome and production of viral proteins 3 Replication of the viral genome 4 Assembly of a new generation of virions 5 Exit from the infected cell 6 Transmission to a new host o A dormant manner that suspends production of virions and allows the virus to coexist w the host for a period of time Bacteriophage a virus that infects bacterial cells o Replicates using the lytic cycle which ends with lysis destruction of the cell How do viruses enter a cell o Plant viruses are inserted directly into the host cell cytosol via abrasions or the mouthparts of o Bacteria animal cell viruses must first attach to a specific molecule on the cell wall plasma sucking or biting insects membrane Bacteriophages attach to the host and then use lysozyme to degrade part of the cell wall and expose the membrane then insert their genome by the process of uncoating which can vary Animal cell viruses first attach to virus receptors membrane proteins or carbohydrates attached to glycoproteins glycolipids in the host cell s plasma membrane o HIV binds to CD4 membrane proteins on helper T cells CD4 and a co receptor CXCR4 and CCR5 to fuse the virion envelope and plasma membrane Tested by researchers by using antibodies to bind to CD4 and block HIV o The virion can be internalized by endocytosis where the virus is pulled into the cell in an endosome vesicle which then acidifies and fuses the viral envelope and endosomal membrane Used by naked viruses to enter animal cells and HIV o Drugs that interfere with viral infection or replication of viruses antivirals How do viruses produce proteins o Viruses lack ribosomes amino acids ATP and most other machinery required for translation o Viral mRNAs that code for envelope proteins are translated as if they were mRNAs for the cell s own membrane proteins translated by ribosomes on rough ER and carbohydrates added o mRNAs that code for proteins that make up the capsid are translated by free ribosomes in the cytosol o Long polypeptide sequences may be cut into individual proteins by viral enzymes called proteases critical step in HIV researchers looking for ways to stop it from functioning How do viruses copy their genomes o RNA replicases function as RNA dependent RNA polymerases which synthesize RNA from an RNA template using ribonucleotides from the host cell First convert genome into double stranded RNA then produce multiple copies of original strand o Reverse transcriptase makes complementary DNA cDNA from single stranded RNA template then synthesizes the second complementary DNA strand double stranded DNA DNA copy inserted into host genome and used to produce viral and genomic mRNAs Used by retroviruses backward viruses How are new virions assembled o Viral genomes packaged into capsids sometimes with enzymes like polymerases o Enveloped viruses use the host endomembrane system to transport envelope proteins to the
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