 Viruses- particles of nucleic acid, protein and sometimes lipids  Most viruses are so small, they can only be seen through a powerful electron microscope.  A typical virus is composed of a core of DNA or RNA surrounded by a protein coat.  The more complex the virus is the more genes it contains  They enter a living cell and once inside, use machinery of infected cell to produce more viruses

 They reproduce at a good rate, but only in living host cells.  They can mutate.

Nonliving characteristics of viruses  They are acellular, that is, they contain no cytoplasm or cellular organelles.  viruses don't grow and divide. Instead, new viral components are synthesized and assembled within the infected host cell.  They possess DNA or RNA but never both.

 Capsid- a protein coat  The capsid enable’s host cells to enter the virus  The capsid proteins “trick” the virus by binding the surface of the cell allowing the capsid to come inside the cell  The cell transcribes and translates the viral genetic information into viral capsid proteins.  Sometimes it causes the host cell to make copies of the virus, and it causes the host cell to be destroyed

 Bacteriophages- viruses that infect bacteria  Once the virus is inside the host cell two different process may occur  In a lytic infection, a virus enters a cell and replicates itself and causes the cell to burst.  In the lysogenic infection the virus replicates itself in a way that doesn’t kill the host cell immediately.

Lytic and Lysogenic Infections

 Prophage - viral DNA that is embedded in the host cell’s DNA  Prophage may remain part of the DNA for many generations before being active  A virus may not always stay in the prophage  Eventually one factor will activate the DNA of a prophage causing it to remove itself form the host cell DNA and direct the synthesis of a new virus particles

 Retroviruses- viruses that contain RNA as their genetic information  When retroviruses infect a cell they produce a DNA copy of their RNA  The DNA is then inserted into the DNA of the host cell

 In order for a virus to grow it must infect a living cell  Viruses are parasites  After infecting living cells viruses can reproduce  Viruses are borderline of living and none living things  Viruses are smaller and simpler than cells

1. What are the two ways the two ways viruses cause infections?

1.Lysogenic and lytic

2. What are typical viruses composed of?

 Core of DNA or RNA surrounded by protein coat

 What is the viruses protein coat called?

 capsid

 How do viruses “reproduce”?

 They enter a living cell and once inside, use machinery of infected cell to produce more viruses