Biology II Virsus
Feb 23, 2016
Biology II
Virsus
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
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.
• 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
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.
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
Two Structural Types
• Helical- continuous helix of capsomers forming a cylindrical nucleocapsid
• Icosahedral- 20 sided with 12 corners
• 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
Viral Infection
• Once the virus is inside the host cell, two different process may occur– Lytic infection– Lysogenic Infection
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
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
Retroviruses
• Viruses that contain RNA as their genetic information
• When retroviruses infect a cell, they produce a DNA copy of their RNA
• Ex: HIV
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