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Chapter 19 – Viruses Definitions: Virus – particles of nucleic acid, protein (and in some cases lipids) that can reproduce only by infecting living cells Virulent – virus causes a disease immediately Temperate – virus does not cause a disease immediately Structure (of a typical virus): Core composed of genetic material o Either DNA or RNA Core surrounded by a protein coat o Called a capsid Characteristics: Not considered cells (or alive), must reproduce via host cell Are parasitic – cause harm to host cell for its own benefit Can be polyhedral, rod shaped, round 2 Major Classifications: DNA viruses – DNA codes directly for RNA which produces viral proteins (ex. Chicken pox) RNA viruses – “Retroviruses” – RNA makes DNA which then makes viral proteins (ex. HIV) Most commonly studied… Bacteriophage (aka phage) – virus that infects bacteria Reproduction – 2 methods o Lytic Cycle – virus enters cell, makes copies of itself, and causes cell to burst or lyse (“break open”) 1. Adsorption – virus attaches to a host cell 2. Entry – capsid contracts, injecting DNA into bacterial cell 3. Replication – viral DNA takes over, directing the cell to make more viral DNA and proteins 4. Assembly – new viruses are put together 5. Release – cell wall is digested and new virus particles are released which can then infect other cells o Lysogenic Cycle – virus embeds its DNA into the DNA of the host cell and is replicated along with the host cell’s DNA