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