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Unit 6c Viruses Structure of a virus • To be a virus, you need: – Genetic material (DNA or RNA) – Protein coat (to enclose genetic material) – Some are membrane-bound (helps them fuse with host cell) Viral Replication • Viruses replicate rapidly within host cell and use “component assembly model” to make many viruses within one host cell • Two main ways virus replicate within host cell – Lytic Cycle (main mechanism) – Lysogenic Cycle Lytic Cycle • Virus attaches to host cell → injects its genetic information (DNA or RNA) into cell • Genetic information goes into nucleus & is replicated by host cell • Some replicated DNA is transcribed into mRNA • mRNA exits nucleus & is translated into protein • Duplicated DNA & viral proteins form new viruses within the cell → cell lyses (breaks open) → virus is released & infects neighboring cells Lysogenic Cycle • Bacteriophage inject genetic material into host bacteria, genetic material integrates into host cell's DNA, bacteria continues to replicate DNA and divide • Essentially virus “hides out” in DNA of bacteria • At some point, virus transfers to lytic cycle • Ex: Cold sores (Herpes Simplex Virus or HSV-1) Genetic Exchange b/n Viruses • If two similar viruses happen to be in the same cell during lytic cycle, exchange of genetic material may occur – Increases variation among viruses which can lead to creation of new strains of virus Lysogenic Cycle & Virulence of Bacteria • When bacteriophage infect bacteria & enter lysogenic cycle, they can change the virulence of the bacteria • Ex: Cholera (vibrio cholerae) bacteria by itself is not toxic until it's infected by CTX phage virus RNA viruses (aka Retroviruses) • Use RNA as genetic information • Contain enzyme called reverse transcriptase which takes RNA and makes it DNA, DNA enters nucleus, etc... – BAD NEWS – no proofreading in reverse transcriptase and it is very error-prone – High error rate → increase pathogenicity of virus via genetic variation Ex: HIV Virus • • “Large” retrovirus (60x smaller than rbc) with high mutation rate – HIV generates 1010 virions per day with approximately 3x10-5 mutation rate per bp per replication cycle – BAD: Create vaccine → HIV mutates → vaccine no longer works Most treatment for HIV is based on targeting reverse transcriptase One good mutation... • People with CCR5-delta32 mutation (deletion of 36bp) are resistant to HIV – • Mutation causes helper T cells to have a nonfunctional receptor protein that makes it impossible for HIV to enter the cell 5-15% of Europeans vs. < 1% Africians and Asians