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Lecture 10

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Biology 1001A
Tom Haffie

October 18/2011 Chapter 3.3 and HIV (Lecture 10) What is a Virus? • A non-living infectious agent that contains either DNA or RNA surrounded by a protein coat • Recognition proteins enabling the virus to attach to host cells extend from the surface of infectious viruses • Viruses reproduce by entering a host cell and directing the cellular machinery to make new particles of the same kind • Unaffected by antibiotics and most other treatment methods. Many viruses have great genetic variability Evolution in Action: HIV • Human Immunodeficiency Virus • A retrovirus that contains two copies of single-stranded RNA o The virus’ genome enters the host cell along with reverse transcriptase(viral enzyme), which copies the viral RNA into a complementary strand of DNA o A second strand of DNA is then synthesized, using the first strand as a template. o The resulting DNA integrates into the host cell’s DNA as a provirus and is transcribed by the host cell into mRNA • HIV infects and hijacks immune cells • Immune system eventually collapses • AZT was developed to block reverse transcription o Replaces –OH on pentose sugar with N 3 • Drug became less and less effective o Some mutations led to AZT resistance (Reverse transcriptase can’t pick up the AZT) • Viral population becomes resistant • Evolution of AZT resistance o Process:  Random mutations to reverse transcriptase produce virions that vary in AZT resistance  AZT resistance is heritable  During AZT treatment, many virions fail to reproduce  variants that persist are those that can reproduce in this environment (presence of AZT) o Outcome:  Increasing proportion of AZT-resistant virions (viral population changes over time) • Host-parasite evolution works both ways o Deletion in an immune receptor confers HIV resistance (HIV cannot attach to cell) Summary: Evolution in Action • Evolution is often very fast • Even non-living things can evolve • HIV evolution involves topics such as o Mutation and variation o Natural selection o Evolutionary history 3.3- Evolution • A gradual change in the characteristics of a population of organisms over time • The theory of evolution explains both the unity and the diversity of all life; it tells us that all organisms alive today descended from a common ancestor o All organisms share features such as the use of ATP as a cellular energy source and DNA as genetic material (unity) o Species change over time as a result of natural selection (diversity)
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