Chapter 19

• Proteins will bind to each other if they have complementary shapes and chemical properties
• RNA is synthesized during transcription
• Viruses are nonliving things because they cannot reproduce or carry out their metabolic activities outside of the host cell
• A virus consists of a nucleic acid surrounded by a protein coat
• The protein shell enclosing the viral genome is called a capsid
• Caspid

  The protein shell that encloses the viral genome
• The subunits of capsids are called capsomeres
• Some viruses also have viral envelopes that surround capsid.
• Viral envelopes are membranes derived from membranes of the host cell that cloaks the capsid, which in turn encloses a viral genome
• Viral envelopes contain host cell phospholipids and membrane proteins
• Viral envelopes help animal viruses infect their hosts
• Viruses are very small, smaller than ribosomes
• The host range of a virus refers to the limited number of species whose cells can be infected by a particular virus.
• A virus with a broad host range can affect many species while a virus with an extremely limited host range can sometimes only infect a single species.
• Bacteriophages are viruses that infect bacteria and contain complex capsids
• A virulent phage is a phage that replicates only by a lytic cycle
• A temperate phage is a phage that is capable of replicating by either a lytic cycle or lysogenic cycle
• A prophage is a phage genome that has been inserted into a specific site on a bacterial chromosome
• The portion of a phage that enters the host cell is its nucleic acid
• Natural selection favors bacterial mutants with surface proteins that are no longer recognized as receptors by a particular type of phage
• When phage DNA enters the bacterium, the DNA often is identified as foreign and cut up by restriction enzymes
• Restriction Enzymes

  • An endonuclase that recognizes and cuts DNA molecules foreign to a bacterium
• A provirus is a viral genome that is permanently inserted into a host genome.