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Virus and Bacteria Genetics
Chapter 18
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Viruses NOT living organisms (not made of cells) Only 2 components:
Nucleic acid (DNA or RNA) Protein coat (capsid) envelope with specific proteins to break into specific cell chapter 18.1
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Host range of viruses Cannot break into any cell, must enter by “tricking” a particular receptor protein Viruses often species-specific, even cell specific within larger hosts Ex: tobacco mosaic virus, HIV (helper T cells), cold virus (respiratory cells) Mutations might lead to “jumping” to a new species (bird flu human flu?)
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Viral life cycles We will discuss two strategies: 1) Lytic
2) Lysogenic “active” mode – reproducing new viruses, destroying host cell
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Viral life cycles Lysogenic – “stealth mode” – viral DNA joins host DNA code – will be copied in cell division trigger can “activate” virus and change into lytic mode
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Examples Lytic viruses – cold, flu
Lysogenic / lytic viruses – cold sores, HIV
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Retroviruses Infect with RNA code, not DNA
Carry enzyme called reverse transcriptase to use host nucleotides to make RNA DNA Ex: influenza, HIV These evolve very quickly (need for new flu vaccine every year)
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Retroviral life cycle
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Evolution of viruses Simple, but certainly could NOT have come first in history of life Evidence suggests they came about long after complex, multicellular life? Rogue transposon virus hypothesis?
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Bacteria Slide show by Kim Foglia (modified) Blue edged slides are Kim’s
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Bacteria Bacteria one-celled prokaryotes reproduce by mitosis
binary fission rapid growth generation every ~20 minutes 108 (100 million) colony overnight! dominant form of life on Earth incredibly diverse
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Bacterial genome Single circular chromosome haploid naked DNA
no histone proteins ~4 million base pairs ~4300 genes 1/1000 DNA in eukaryote Genome = all the DNA of an organism Eukaryotes • 1000 times more DNA • only 10 times more genes - introns, spacers, inefficiency How have these little guys gotten to be so diverse??
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Binary fission Replication of bacterial chromosome
Asexual reproduction offspring genetically identical to parent where does variation come from?
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Variation in bacteria Sources of variation spontaneous mutation
transformation plasmids DNA fragments transduction conjugation transposons bacteria shedding DNA
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Spontaneous mutation Spontaneous mutation is a significant source of variation in rapidly reproducing species Example: E. coli human colon (large intestines) 2 x 1010 (billion) new E. coli each day! spontaneous mutations for 1 gene, only ~1 mutation in 10 million replications each day, ~2,000 bacteria develop mutation in that gene but consider all 4300 genes, then: 4300 x 2000 = 9 million mutations per day per human host!
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Transformation Bacteria are opportunists
promiscuous!? Bacteria are opportunists pick up naked foreign DNA wherever it may be hanging out have surface transport proteins that are specialized for the uptake of naked DNA import bits of chromosomes from other bacteria incorporate the DNA bits into their own chromosome express new genes transformation form of recombination mix heat-killed pathogenic & non-pathogenic bacteria While E. coli lacks this specialized mechanism, it can be induced to take up small pieces of DNA if cultured in a medium with a relatively high concentration of calcium ions. In biotechnology, this technique has been used to introduce foreign DNA into E. coli. mice die
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Plasmids Small supplemental circles of DNA carry extra genes
,000 base pairs self-replicating carry extra genes 2-30 genes genes for antibiotic resistance can be exchanged between bacteria bacterial sex!! rapid evolution can be imported from environment Mini-chromosomes
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slide shows by Kim Foglia modified
GENE REGULATION slide shows by Kim Foglia modified Slides with blue edges are Kim’s
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Genes for antibiotic resistance = R Plasmids
Role in rapid evolution Method for spreading “antibiotic resistance”
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Plasmids & antibiotic resistance
Resistance is futile? 1st recognized in 1950s in Japan bacterial dysentery not responding to antibiotics worldwide problem now resistant genes are on plasmids that are swapped between bacteria
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TRANSDUCTION with viruses
Phage viruses carry bacterial genes from one host to another
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Conjugation - Bacteria “sex”
Animation Direct transfer of DNA between 2 bacterial cells that are temporarily joined results from presence of F (fertility) plasmid “male” extends sex pilli and attaches to “female” bacterium cytoplasmic bridge allows transfer of DNA
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TRANSPOSONS (Transposable elements)
“Jumping” genes Can move from one place to another 1st described by Barbara McClintock in corn Can move genes to new site
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Bacterial metabolism Bacteria need to respond quickly to changes in their environment if they have enough of a product, need to stop production why? waste of energy to produce more how? stop production of enzymes for synthesis if they find new food/energy source, need to utilize it quickly why? metabolism, growth, reproduction how? start production of enzymes for digestion STOP GO
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Remember Regulating Metabolism?
Feedback inhibition product acts as an allosteric inhibitor of 1st enzyme in tryptophan pathway but this is wasteful production of enzymes = inhibition - - Oh, I remember this from our Metabolism Unit!
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Different way to Regulate Metabolism
Gene regulation instead of blocking enzyme function, block transcription of genes for all enzymes in tryptophan pathway saves energy by not wasting it on unnecessary protein synthesis = inhibition - - - Now, that’s a good idea from a lowly bacterium!
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Gene regulation in bacteria
Cells vary amount of specific enzymes by regulating gene transcription turn genes on or turn genes off turn genes OFF example if bacterium has enough tryptophan then it doesn’t need to make enzymes used to build tryptophan turn genes ON example if bacterium encounters new sugar (energy source), like lactose, then it needs to start making enzymes used to digest lactose STOP Remember: rapid growth generation every ~20 minutes 108 (100 million) colony overnight! Anybody that can put more energy to growth & reproduction takes over the toilet. An individual bacterium, locked into the genome that it has inherited, can cope with environmental fluctuations by exerting metabolic control. First, cells vary the number of specific enzyme molecules by regulating gene expression. Second, cells adjust the activity of enzymes already present (for example, by feedback inhibition). GO
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Bacteria group genes together
Operon genes grouped together with related functions example: all enzymes in a metabolic pathway promoter = RNA polymerase binding site single promoter controls transcription of all genes in operon transcribed as one unit & a single mRNA is made operator = DNA binding site of repressor protein
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Operon model When gene is turned ON: Polymerase binds promoter Gene is transcribed RNA polymerase TATA gene1 gene2 gene3 gene4 DNA promoter operator 1 2 3 4 mRNA enzyme1 enzyme2 enzyme3 enzyme4 Slide by Kim Foglia modified
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So how can these genes be turned off?
Repressor protein binds to DNA at operator site blocking RNA polymerase blocks transcription
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Operon model Operon: operator, promoter & genes they control serve as a model for gene regulation RNA polymerase TATA gene1 gene2 gene3 gene4 DNA promoter operator 1 2 3 4 mRNA enzyme1 enzyme2 enzyme3 enzyme4
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Operon model GENE is TURNED OFF: Repressor binds to operator site Blocks RNA Polymerase RNA polymerase repressor TATA gene1 gene2 gene3 gene4 DNA promoter operator 1 2 3 4 mRNA enzyme1 enzyme2 enzyme3 enzyme4 repressor = repressor protein
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REPRESSIBLE OPERONS are ON Can be turned off
EX: trp operon makes enzymes used in tryptophan synthesis INDUCIBLE OPERONS are OFF Can be turned on EX: lac operon makes enzymes used in lactose digestion
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Repressible operon: tryptophan
Gene is on when tryptophan is needed Repressor protein exists as an inactive form Cell makes enzymes for tryptophan synthesis RNA polymerase TATA gene1 gene2 gene3 gene4 DNA promoter operator 1 2 3 4 mRNA enzyme1 enzyme2 enzyme3 enzyme4 repressor Inactive repressor protein
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Repressible operon: tryptophan
Synthesis pathway model When excess tryptophan is present, it binds to trp repressor protein & triggers repressor to bind to DNA blocks (represses) transcription RNA polymerase repressor trp TATA gene1 gene2 gene3 gene4 DNA promoter operator trp trp trp trp trp trp repressor repressor protein trp trp conformational change in repressor protein makes it ACTIVE! trp tryptophan trp repressor tryptophan – repressor protein complex trp
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Tryptophan operon When tryptophan is present
Don’t need to make tryptophan-building enzymes Tryptophan is allosteric regulator of repressor protein
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Inducible operon: lactose
Digestive pathway model GLUCOSE is food of choice Don’t need lactose digesting enzymes Gene is turned off RNA polymerase repressor TATA gene1 gene2 gene3 gene4 DNA promoter operator repressor ACTIVE repressor protein
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Inducible operon: lactose
Digestive pathway model When lactose is present, binds to lac repressor protein & triggers repressor to release DNA induces transcription RNA polymerase repressor TATA lac gene1 gene2 gene3 gene4 DNA promoter operator 1 2 3 4 mRNA enzyme1 enzyme2 enzyme3 enzyme4 repressor repressor protein lactose lac conformational change in repressor protein makes it INACTIVE! repressor lactose – repressor protein complex lac
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Lactose operon What happens when lactose is present?
Need to make lactose-digesting enzymes Lactose is allosteric regulator of repressor protein
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Jacob & Monod: lac Operon
1961 | 1965 Francois Jacob & Jacques Monod first to describe operon system coined the phrase “operon” Jacques Monod Francois Jacob
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Operon summary Repressible operon Inducible operon
usually functions in anabolic pathways synthesizing end products when end product is present in excess, cell allocates resources to other uses Inducible operon usually functions in catabolic pathways, digesting nutrients to simpler molecules produce enzymes only when nutrient is available cell avoids making proteins that have nothing to do, cell allocates resources to other uses
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