A View of Life

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 1. Helicalspirals of many protein units called capsomeres 2. Icosahedronhas 20 triangular subunits

3. Bacteriophagewith polyhedral head, tail sheath, and tail fibers May also have membranous envelope and glycoprotein spikes. Viruses shapes:

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 1. 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

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