Chapter 27 Viruses

27.1 The Nature of Viruses Viruses possess only a portion of the properties of organisms. Parasitic chemicals (segments of DNA of RNA wrapped in a protein coat). Each type of virus can replicate in only a very limited number of cell types. These cells are referred to as its host range. Ex: rabies infects neurons; hepatitis replicates in liver cells. Vary greatly in appearance and size (smaller than bacteria). Tobacco Mosaic Virus- first virus discovered in Structure A core of nucleic acid surrounded by a protein coat/capsid. DNA or RNA can be linear or circular; single stranded or double stranded. Viruses that contain RNA that are reverse transcribed to DNA are called retroviruses. Shape Most are helical (rod-like) or isometric (roughly spherical)

27.2 Bacteriophage: Bacterial Viruses Bacteriophages Viruses that infect bacteria; many are large complexes with large amounts of DNA. Replication Viruses use the host’s cellular machinery by getting the host to translate their genes into proteins. These proteins lead to the production of more viruses. Life Cycles 1. Lytic Cycle Virus attaches to host (attachment), injects in its DNA (penetration), replication of virus using host machinery(synthesis), and virus kills host cell. (Causes lysing or rupturing) Considered virulent; example: cold or flu Life Cycle of T2 Phage

27.2 cont’d 2. Lysogenic Life Cycle Do not immediately kill the cells they infect, instead they integrate their own nucleic material into the host’s genome. (While residing there it is called a prophage) Example: λ phage = E. coli Prophage may, at a later time, exit the genome and initiate virus replication. Considered temperate; example: herpes Molecular Biology |

27.3 Human Immunodeficiency Virus- HIV AIDS (acquired immune deficiency syndrome) is caused by HIV. First reported case was in Infected humans have varying degrees of resistance to HIV. HIV compromises the host immune system by attacking the CD4 T helper cells These cells are responsible for rousing the defending cells into action. Without them the body cannot ward off bacteria and viruses of any kind. Clinical symptoms do not develop until after long latency period (8 to 10 years). Individuals infected with HIV are monitored for CD4 T helper cell counts. When count drops, individual is considered to have AIDS.

27.3 Infection Cycle 1.Attachment- can only attach to a CD4 T cells and macrophages; does so by recognizing the cell’s receptors. 2.Entry into macrophage- contents of virus enter the cell. 3.Replication- viral RNA makes a double stranded DNA which is then replicated by host cell. Viruses are released. Treatment 1.The FDA currently lists 35 antiretroviral drugs that are used in AIDS therapy. These target four aspects of the HIV life cycle: viral entry, genome replication, integration of viral DNA and maturation of HIV proteins. 2.Vaccines- has been highly unsuccessful; high rate of HIV mutation has been a problem for development. HIV subunit in trials was not producing a strong cellular immune response.

27.4 Other Viral Diseases The most lethal virus in human history would have to be the flu of It is thought to have infect 1/3 of the world’s population. The three general “types” of flu viruses are distinguished by their capsid protein. (Look like a rods studded with spikes) Type A viruses cause most of the serious flu epidemics in humans. These can also occur in mammals and birds. Types B and C viruses are restricted to humans and rarely cause serious health problems. Subtypes of the flu virus differ in their protein spikes. Mutation and recombination of viruses is what makes it difficult to create new vaccines for new strains of the flu.

27.4 cont’d

27.4 Other Viral Diseases Emerging Viruses- viruses that originate in one organism and then pass to another and cause disease. Ex: hantavirus- associated with rodents; transmitted by fecal contamination in areas of human habitation. Ex: hemorrhagic fever: Ebola- natural host is unknown- has lethality rates in excess of 90% in isolated outbreaks. Ex: Severe Acute Respiratory Syndrome (SARS)- associated with civets (a weasel-like mammal). Recent data has shown bats to be the natural reservoir for the SARS virus.

27.4 cont’d Viruses & Cancer 1.Chronic hepatitus B & liver cancer 2.Human papillomavirus & cervical cancer 3.Viruses contribute to 15% of human cancers Prions- proteinaceous infectious particles; responsible for transmissible spongiform encephalopathies. Ex: mad cow disease Viroids- tiny, naked molecules of RNA that are important infectious disease agents in plants. Not clear how they cause disease, although there are some intriguing hints that they might use the plant siRNA machinery to affect gene expression.