<p> Viruses  microscopic (life?) forms that reproduce with in a living cell  size range from 0.01 – 0.03 um  Viruses have no cellular structures, no cytoplasm, no organelles, or cell membrane  consists of two parts: 1. outer protein coat = capsid 2. inner nucleic acid (DNA or RNA); double or single stranded; linear or circular  exhibit some characteristics of living things and some of non-living things:  do not respire  do not respond to stimulus  do not grow  DO reproduce  evolve?  little more than mobile genes that parasitize cells</p><p>Classifying Viruses  capsid shape and size – pg. 385  polyhedral viruses – look like small  spherical viruses (ex. HIV) crystals (ex. polio)</p><p> cylindrical viruses (tobacco mosaic)  head and tail viruses – polyhedral head, a protein tail and tail fibers (ex. T4) Viral Reproduction  must reproduce inside a living cells  virus is specific to type of cell  bacteriophage = virus that invades bacterial cells (used during gene transfer technology)</p><p>Lytic Cycle during reproduction, the virus kills the host cell which results in the victim showing symptoms of disease</p><p>1. Adsorption  phage attaches to host cell wall at specific receptor sites</p><p>2. Entry  phage releases enzyme to break down cell wall  nucleic acid passes into host; leaves empty capsid outside  some animal viruses enter the cell by phagocytosis  some plant viruses are injected into cells by insects</p><p>3. Replication  nucleic acid is replicated either in nucleus or cytoplasm of host, using DNA and/or RNA of host  RND or DNA is copied and protein for capsid is also produced</p><p>4. Assembly  viral nucleic acids and proteins are assembled to complete the virus particle, called a viron</p><p>5. Release  new viruses release an enzyme to break cell wall of host and are released  could be up to 300 viruses produced in one cell</p><p>Source: http://science.howstuffworks.com/virus-human.htm Provirus (Lysogenic Cycle)  some viruses may remain inactive for many generations  new viral DNA is created and releases by host cells but cell does not die  HIV virus, herpes virus</p><p>1. Attachment  nucleic acid of virus attaches to DNA of host</p><p>2. Replication  host cell replicates and replicates viral DNA with it  this can go on for many years with no effect on the host</p><p>3. Activation  virus DNA begins lytic cycle (trigger unknown)</p><p>Source: http://science.howstuffworks.com/virus-human4.htm</p><p>______ many (70%) viruses are retroviruses  these have RNA which when injected into the host is incorporated into the DNA  the RNA viruses have higher mutation rates than DNA viruses  the HIV virus is a retrovirus that is also a provirus  this is why people can test positive for HIV but be healthy for many years Viruses and Biotechnology  for fast cloning, a gene can be spliced into viral DNA so it is replicated quickly</p><p>Origin of Viruses  must have evolved after the first cells came into existence  fragments of nucleic acid that escaped cells that then became parasites on similar cells  each type of virus is probably more related to its host cell than to other viruses Source: http://science.howstuffworks.com/virus-human.htm Source: http://science.howstuffworks.com/virus-human4.htm</p>