BIOLOGY Viruses

Figure 27.2 The three domains of life

Table 27.2 A Comparison of the Three Domains of Life

Viruses (Latin, poison) Are noncellular Size 20-200 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

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

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.

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

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

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

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.

Reproduction of HIV-1

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

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