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Chapter 19 I. Viruses 1. bacteriophages (phages) a) capsid (capsomeres) b) genome (usually DNA) c) cycles (lytic / lysogenic) Lytic Cycle Virulent phages use just this cycle ( ) 1) viral DNA enters bacterium 2) host’s DNA hydrolyzed 3) phage DNA directs formation of viral DNA and proteins 4) self assembly 5) new viruses “burst” out Lysogenic Cycle 1. Viral DNA enters bacterium 2. Viral DNA integrates into host chromosome 3. viral DNA now called a prophage 4. bacteria reproduces, daughter cells all receive prophage 5. prophage randomly exits to initiate lytic stage Final notes A temperate phage can use both lytic and lyosogenic cycles • Bacteria contain “restriction enzymes” to cleave foreign DNA Animal viruses a)Usually contain an “envelope” surrounding the capsid -from? b) Can be RNA or DNA viruses c) Involves formation of “proviruses” which are permanent in host genome d) Have cycles “sort of” like bacteriophages HIV retrovirus ( ) • Carries RNA and reverse trancriptase in capsid Viruses and cancer Tumor viruses disrupt normal cell cycle: a) some may carry oncogenes- genes to send cell cycle out of control b) some viruses may turn on protooncogenes at inappropriate times Plant viruses I. Horizontal transmission – plant is infected from external source of virus( insect vector) II. Vertical transmission – plant is infected from a parent (cuttings, or infected seeds) Bacteria chapter 27 • Bacterial genomes are larger than viral genomes, but much smaller than a typical eukaryotic genome • Most DNA in a bacterium is found in a single circular chromosome that is composed of double-stranded DNA found in the nucleiod region. • Many bacteria also contain extra DNA in plasmids- beneficial but not normally essential • Bacterial reproduction reproduce by binary fission, which is preceeded by DNA replication. • rapid reproductive rate,useful for genetic studies. • Binary fission is asexual reproduction that produces daughter cells that are genetically identical to the parent Genetic recombinations 1. Transformation – the process during which a bacterial cell assimilates foreign DNA from the surroundings. • Specialized proteins on the cell membrane of some bacteria facilitate this kind of DNA uptake. 2. Transduction – the process of gene transfer from one bacterium to another by a bacteriophage (a virus) • General transduction occurs when random pieces of host cell DNA are packaged within a phage capsid during the lytic cycle. • specialized transduction involves a temperate phage • Conjugation – the direct transfer of genes between two cells that are temporarily joined • A donor bacterium (F+ or Hfr) produces a tube, or pilus, that connects to the recipient bacterium. • Transposons – DNA sequences that can move from one chromosomal site to another • When the transposon’s genes are not replicated before the move it is called conservative transposition • When the transposon’s genes are replicated before the move it is called replicative transposition Gene regulation and Operons I. Operon a) promoter b) operator c) genes to be transcribed(transcription unit) Repressible operons ex) trp operon (tryptophan) 1. operon is usually turned “on” 2. repressor protein (made at a distant regulatory gene) is manufactured in an “off” formation 3. tryptophan acts as a corepressor to activate the repressor to shut off own production Inducible operons ex) lac operon – for lactose hydrolysis 1. operon is usually off because repressor protein (from distant regulatory gene) codes for repressor in “on” formation 2. allactose (lactose isomer) acts as inducer to turn “off” repressor – thus induces the turning on of a gene for its own breakdown. In the presence of lactose Cyclic AMP (cAMP) accumulates in a cell without glucose cAMP activates CRP (cAMP receptor protein) which binds to promoter area to increase rate of transcription of lac gene. When glucose is present, cAMP levels fall Transcription of lac gene slows due to inactive CRP