ch8 microbial genetics
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CH8: Microbial Genetics
E. coli is found naturally in the human large intestine, and there it is beneficial. However, the strain designated E. coli O157:H7 produces Shiga toxin. How did E. coli acquire this gene from Shigella?
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Terminology Genetics: The study of what genes are, how they
carry information, how information is expressed, and how genes are replicated
Gene: A segment of DNA that encodes a functional product, usually a protein
Chromosome: Structure containing DNA that physically carries hereditary information; the chromosomes contain the genes
Genome: All the genetic information in a cell
Structure and Function of Genetic Material
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Terminology Genomics: The molecular study of genomes
(sequencing and molecular characterization of genomes).
Genotype: The genes of an organism Phenotype: Expression of the genes
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The Flow of Genetic Information
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DNA
Polymer of nucleotides: Adenine, thymine, cytosine, and guanine
Double helix associated with proteins
"Backbone" is deoxyribose-phosphate
Strands are held together by hydrogen bonds between AT and CG
Strands are antiparallel
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Semiconservative Replication
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DNA Synthesis
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DNA Synthesis
DNA is copied by DNA polymerase In the 5' 3' direction Initiated by an RNA primer Leading strand is synthesized continuously Lagging strand is synthesized discontinuously Okazaki fragments RNA primers are removed and Okazaki fragments joined
by a DNA polymerase and DNA ligase
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DNA Synthesis
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Replication of Bacterial DNA
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Important Enzymes DNA GyraseDNA Helicase DNA polymerase
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RNA and Protein SynthesisTranscription DNA is transcribed to make RNA (mRNA, tRNA, and
rRNA) Transcription begins when RNA polymerase binds to
the promoter sequence Transcription proceeds in the 5' 3' direction Transcription stops when it reaches the
terminator sequence
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Transcription
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The Process of Transcription
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ANIMATION Transcription: Overview
ANIMATION Transcription: Process
The Process of Transcription
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RNA Processing in Eukaryotes
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Translation
mRNA is translated in codons (three nucleotides)
Translation of mRNA begins at the start codon: AUG
Translation ends at nonsense codons: UAA, UAG, UGA
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The Genetic Code
64 total codons of which 61 sense codons encode the 20 amino acids
3 are non-sense codons or stop codons.
The genetic code is degenerate
tRNA carries the complementary anticodon
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The Genetic Code
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Simultaneous Transcription & Translation
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Figure 8.9-1
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Figure 8.9-2
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Regulation Constitutive genes are expressed at a fixed rate
(60-80%genes) Other genes are expressed only as needed
Repressible genes: Always active , can be turned off (repressed).
Inducible genes: Inactive , needs to be turned on or induced.
Catabolite repression: Inhibition of the metabolism of alternative carbon sources by Glucose.
Regulation of protein synthesis saves (conserve) energy for the organism.
The Regulation of Gene Expression
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Operon
Control of TranscriptionConsists of a promotor, an operator and series of genes
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• Inducible operons – must be activated by inducers
• Regulate catabolic pathway
• Lactose Operon
• Repressible operons – transcribed continually until deactivated by repressors
• Regulate anabolic pathway.
• Tryptophan Operon
Operons
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Induction
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Induction
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Repression
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Repression
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Lactose present, no Glucose
Lactose + glucose present
Figure 8.15
Catabolite Repression
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Figure 8.14b
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Mutation
A change in the genetic material Mutation is a change in the base sequence of DNA. Mutations may be neutral, beneficial, or harmful Mutagen: Agent that causes mutations Spontaneous mutations: Occur in the absence of a
mutagen
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Base substitution (point mutation)
Missense mutation
Mutation
Change in one base Result in change in
amino acid
Figure 8.17a, b
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Nonsense mutation
Mutation
Results in a nonsense codon
Figure 8.17a, c
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Mutation
Frameshift mutation Insertion or deletion of one or more nucleotide pairs
Figure 8.17a, d
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Spontaneous mutation rate = 1 in 109 replicated base pairs or 1 in 106 replicated genes
Mutagens increase to one in 105(10-5) or one in 103(10–3) per replicated gene
Point mutations concept map
The Frequency of Mutation
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Chemical Mutagens
Nucleoside analogs
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Chemical mutagens
Frameshift mutagens Cause small deletions or insertions Ex:Benzopyrene in smoke Aflatoxins Acridine orange
Frameshift mutagens are often carcinogens.
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Radiation
Ionizing radiation (X rays and gamma rays) causes the formation of ions that can react with nucleotides and the deoxyribose-phosphate backbone
Causes physical breakdown of DNA
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Radiation
UV radiation causes thymine dimers
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Repair Photolyases separate thymine dimers Nucleotide excision repair (can repair other
mutations also)
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Selection
Positive (direct) selection detects mutant cells because they grow or appear different
Negative (indirect) selection detects mutant cells because they do not grow Replica plating
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Replica Plating
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Ames Test for Chemical Carcinogens
90% of the chemicals found to be mutagenic also have been shown to be carcinogenic.
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• Exchange of DNA segments composed of homologous sequences
• Recombinants – cells with DNA molecules that contain new nucleotide sequences
• Vertical gene transfer – organisms replicate their genomes and provide copies to descendants
• Horizontal gene transfer – donor contributes part of genome to recipient; three types
• Transformation
• Transduction
• Bacterial Conjugation
Genetic Recombination and Transfer
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Vertical gene transfer: Occurs during reproduction between generations of cells.
Horizontal gene transfer: The transfer of genes between cells of the same generation.
Genetic Recombination
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Genetic Recombination
Exchange of genes between two DNA molecules Crossing over
occurs when two chromosomes break and rejoin
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Genetic TransformationGriffith’s Experiment
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• Griffiths experiment concluded transforming agent was DNA; one of conclusive pieces of proof that DNA is genetic material
• Cells that take up DNA are competent; results from alterations in cell wall and cytoplasmic membrane that allow DNA to enter cell
Transformation
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Bacterial ConjugationTransfer of DNA by cell to cell contact
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Transduction by a Bacteriophage• Generalized
transduction – transducing phage carries random DNA segment from donor to recipient
• Specialized transduction – only certain donor DNA sequences are transferred
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Plasmids
Conjugative plasmid: Carries genes for sex pili and transfer of the plasmid
Dissimilation plasmids: Encode enzymes for catabolism of unusual compounds
Bacteriocins plasmids R factors: Encode antibiotic resistance
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R Factor, a Type of Plasmid
Carry genes that confer resistance to antibiotics.
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Transposons
Segments of DNA that can move from one region of DNA to another
Contain insertion sequences for cutting and resealing DNA (transposase)
Complex transposons carry other genes
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Genes and Evolution
Mutations and recombination provide diversity Fittest organisms for an environment are selected by
natural selection