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Unformatted text preview: Chapter 18 The Genetics of Viruses and Bacteria • A virus has a genome (genetic material) but can reproduce only within a host cell. • Structure of Viruses- Viruses are infectious p articles consisting of nucleic acid and enclosed in a protein co a nd in so m e cases a m e m branous e nvelope.- A virus is called a DNA virus or RNA virus according to the kind of nucleic acid that m akes up its g eno m e. It m ay be double-strand ed or single strand ed.- The viral geno m e is e nclosed by a protein coat called a capsid. - Som e viruses (influenza virus) also have a m e m branous envelope containing viral proteins that h elp viruses e nt er cells. • General Features of Viral Reproductive Cycles- Each type of virus can infect only a limited range of host cells, called its host rang e.- Viruses identify their host cells by a “lock-a nd-key” fit betwe en proteins on the outsid the virus and sp ecific receptor molecules on the surface of cells.- A virus enters the host cell and is uncoated, releasing viral DNA and capsid proteins- Host enzym es replicate the viral g eno m e that entered the cell. Meanwhile other host e nzym es transcribe the viral geno m e into viral mRNA, which are us ed by other e nzym to m ake more viral proteins.- Viral geno m es and capsid proteins self-ass e m ble into new viruses which exit the cell. • Reproductive Cycles of Phages- Bacteriophag es or phag es are viruses that infect bacteria.- Phages reproduce by two alternative m ech anisms: the lyctic cycle a nd the lysogenic cycle.- A phage that reproduces by a lyctic cycle is a virulent phage. The lyctic cycle consists 5 steps: 1. Attachment: The T4 phage uses its tail fibers to bind to the outer surface of the E. col cell....
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