' Microscopic (Life?) Forms That Reproduce with in a Living Cell

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

Classifying Viruses  capsid shape and size – pg. 385  polyhedral viruses – look like small  spherical viruses (ex. HIV) crystals (ex. polio)

 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)

Lytic Cycle during reproduction, the virus kills the host cell which results in the victim showing symptoms of disease

1. Adsorption  phage attaches to host cell wall at specific receptor sites

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

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

4. Assembly  viral nucleic acids and proteins are assembled to complete the virus particle, called a viron

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

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

1. Attachment  nucleic acid of virus attaches to DNA of host

2. Replication  host cell replicates and replicates viral DNA with it  this can go on for many years with no effect on the host

3. Activation  virus DNA begins lytic cycle (trigger unknown)

Source: http://science.howstuffworks.com/virus-human4.htm

______ 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

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