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Biology, 6th Edition

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Greg Steinberg AP Biology Chapter 18 Microbial Models: The Genetics of Viruses and Bacteria A. The Genetics of Viruses a. Size i. The smallest viruses can be smaller than a ribosome. ii. Even the largest viruses are very small. b. Viral Genomes i. Viruses can consist of a variety of genetic material: Double stranded DNA, single strand DNA, single strand RNA, and double strand RNA. ii. The type of genetic material determines the name of the virus; either a DNA Virus or RNA Virus. c. Capsids and Envelopes i. The protein shell that covers the viruses genetic material is known as a capsid. ii. There are multiple shapes for a capsid. iii. Some viruses have envelopes that protect their capsid (viruses such as influenza). iv. Some envelopes also have proteins and glycoproteins. v. Bacteriophages are viruses that infect bacteria; these are the most complex capsid. d. Host Cells i. Viruses can only reproduce by infected a host cell. ii. A virus can only infect a limited variety of cells: this is known as their host range. iii. When the genome of the virus enters the cell, the cell is known as infected. iv. The genome then takes over the cell, and forces the cell to make replications of the virus. v. Most viruses use the cells on DNA polymerase to splice their genome into the cells. vi. RNA Viruses must use their own polymerases to splice. vii. The mRNA coded by the virus’ genome is used to make the capsid, as well as other molecules needed by the virus. e. The Lytic Cycle i. This is the cycle from when the phage infects the cell until the cell dies and releases the newly made phages. ii. A phage that uses the lytic cycle for reproduction is known as virulent phages. f. The Lysogenic Cycle i. The lysogenic cycle replicates the phage without actually destroying the host cell.
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ii. Phages that can do both cycles are known as temperate phages.
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