1. Biology II Virsus

2. Brief History Many years the cause viral infections such as smallpox and polio were unknown even though we knew they were transferred from person to person – Louis Pasteur was certainly on the right track when he postulated that rabies was caused by a living thing smaller than bacteria – In 1884 he was able to develop the first vaccine for rabies

3. Viruses Virus- from the Latin virus means toxin or poison Particles of nucleic acid, protein, and sometimes lipids Viruses can reproduce only by infecting living cells A typical virus is composed of a core of DNA or RNA surrounds by a protein coat Viruses are very small.

4. The general plan of virus organization is the utmost in simplicity and compactness Viruses contain only those parts needed to invade and control a host cell – An external coating and a core containing one or more nucleic acid stands of either DNA or RNA

5. Capsid A viruses protein coat The capsid proteins of a typical virus bind to receptors on the surface of a cell and trick the cell into allowing it inside Once inside, the viral genes are expressed cause the host cell to make copies of the virus and in the process the host cell is destroyed.

6. Fucntion of Capsid The outermost covering of a virus is indispensable to viral fucntion – It protects the nucleic acid from the effects of various enzyme and chemical when the virus is outside the host cell Capsids are also responsible for helping to introduce the viral DNA or RNA into a suitable host cell

7. Two Structural Types Helical- continuous helix of capsomers forming a cylindrical nucleocapsid Icosahedral- 20 sided with 12 corners

8. Because viruses must bind precisely to proteins on the cell surface and then use a hosts genetic system, most viruses are highly specific to the cells they infect

9. Viral Infection Once the virus is inside the host cell, two different process may occur – Lytic infection – Lysogenic Infection

10. Lytic Infection Virus attaches to host cell Injects its DNA Host makes RNA from viral DNA Cell begins to make copies of Virus New viruses form Host cell bursts

12. Lysogenic Infection Virus attaches to host cell Injects DNA Viral DNA incorporates itself into the host DNA Viral DNA can be dormant Once it becomes active it follows the 4 processes in the lytic cycle

14. Retroviruses Viruses that contain RNA as their genetic information When retroviruses infect a cell, they produce a DNA copy of their RNA Ex: HIV

15. Viruses and Living Cells Viruses must infect a living cell in order to grow and reproduce They, also take advantage of the host’s respiration, nutrition, and all the other functions that occur in living things. Therefore, viruses are considered to be parasites