1. BIOLOGY
Viruses

2. Figure 27.2 The three domains of life

3. Table 27.2 A Comparison of the Three Domains of Life

4. Viruses (Latin, poison)
Are noncellular
Size nm
Always have at least two parts:
--an outer capsid composed of
protein subunits
--an inner core of nucleic acid
DNA or RNA
May be surrounded by membranous envelope
May also contain various proteins, especially enzymes such as polymerases

5. Viruses Are called obligate intracellular parasites, which means they
cannot live outside of a living cell
Each kind of virus usually infects one kind of organism and/or one kind of tissue,
e.g. hepatitis virus only infects liver cells
Believed that viruses are derived from the kind of cell they infect and must have evolved after cells
They can mutate and, therefore, they evolve; e.g. flu viruses are constantly mutating
Figure 18.x4 Hepatitis

6. Viruses shapes:
Helical—spirals of many protein units called capsomeres
Icosahedron—has 20 triangular subunits
Bacteriophage—with polyhedral head, tail sheath, and tail fibers
May also have membranous envelope and glycoprotein spikes.

7. Virus Classification Classification is based on: Type of nucleic acid.
Size and shape.
Presence / absence of outer envelope.

8. Viral Reproduction Lytic cycle may be divided into five stages:
Attachment
Penetration
Biosynthesis
Maturation
Release

9. Viral Reproduction Lysogenic Cycle
Phage becomes a prophage that is integrated into the host genome.
Becomes latent, and later may reenter the lytic cycle.

10. Reproduction of Animal Viruses
After animal viruses enter the host cell, uncoating releases viral DNA or RNA and reproduction occurs.
If viral release occurs by budding, the viral particle acquires a membranous envelope.
Retroviruses
Contain reverse transcriptase which carries out RNA  cDNA transcription.

11. Reproduction of HIV-1

12. Infectious Agents Smaller than a Virus
Viroids
naked circular strands of RNA several hundred nucleotides long that infect plants
replicate in host cells using cellular enzymes
symptoms include abnormal development and stunted growth in the plants
one viroid disease has killed over 10 million coconut palms in the Philippines

13. Infectious Agents Smaller than a Virus
Prions
protein particles that can cause other proteins in the cell to have a misshapen tertiary structure
cause a number of degenerative brain diseases, including scrapie in sheep, mad cow disease, and Creutzfeldt-Jakob disease in humans