Microbial Genetics

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Microbial Genetics
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Terminology

• Genetics
• Study of what genes are
• how they carry information
• how information is expressed
• how genes are replicated
• Gene
• Segment of DNA that encodes a functional
  product, usually a protein

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Terminology

• Genome
• All of the genetic material in a cell
• Genomics
• Molecular study of genomes
• Genotype
• Genes of an organism
• Phenotype
• Expression of the genes

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E. coli
Figure 8.1a
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DNA

• Polymer of nucleotides adenine, thymine,
  cytosine, guanine
• Double helix associated with proteins
• "Backbone" is deoxyribose-phosphate
• Strands held together by hydrogen bonds between
  AT and CG
• Strands are antiparallel

Figure 8.4
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DNA
Figure 8.3
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DNA
Figure 8.5
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DNA

• DNA is copied by DNA polymerase
• In the 5? ? 3? direction
• Initiated by an RNA primer
• Leading strand synthesized continuously
• Lagging strand synthesized discontinuously
• Okazaki fragments
• RNA primers are removed and Okazaki fragments
  joined by a DNA polymerase and DNA ligase

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DNA
Figure 8.6
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DNA

• DNA replication is semiconservative

Figure 8.7
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Figure 8.8
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RNA processing in Eukaryotes
Figure 8.12
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Translation

• mRNA is translated in codons (3 nucleotides)
• Translation of mRNA begins at the start codon
  AUG
• Translation ends at a STOP codon UAA, UAG, UGA

Figure 8.2
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The Genetic Code
Figure 8.9
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Translation
Figure 8.10.1
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Translation
Figure 8.10.2
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Translation
Figure 8.10.3
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Translation
Figure 8.10.4
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Translation
Figure 8.10.5
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Translation
Figure 8.10.6
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Translation
Figure 8.10.7
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Translation
Figure 8.10.8
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Translation
Figure 8.11
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Mutation

• Change in the genetic material
• 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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Mutation

• Base substitution (point mutation)
• Missense mutation

• Change in one baseResult in change in amino acid

Figure 8.17a, b
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Mutation

• Nonsense mutation

• Results in a nonsense codon

Figure 8.17a, c
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Mutation

• Insertion or deletion of one or more nucleotide
  pairs

• Frameshift mutation

Figure 8.17a, d
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Mutagen

• Ionizing radiation
• formation of ions that can react with nucleotides
  and the deoxyribose-phosphate backbone.
• Nucleotide excision repairs mutations

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Mutagen

• UV radiation causes thymine dimers
• Light-repair separates thymine dimers

Figure 8.20
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The Frequency of Mutation

• Spontaneous mutation rate 1 in 109 replicated
  base pairs or 1 in 106 replicated genes
• Mutagens increase to 105 or 103 per replicated
  gene

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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.

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Replica Plating
Figure 8.21
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The Ames Test for Chemical Carcinogens
Figure 8.22
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Genetic Transfer and Recombination

• Vertical gene transfer
• Horizontal gene transfer

• Occurs during reproduction, between generations
  of cells
• Transfer of genes between cells of the same
  generation

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Transformation
Figure 8.24
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Recombination
Figure 8.25
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Conjugation
Figure 8.27a
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Conjugation
Figure 8.27b
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Conjugation
Figure 8.27c
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Plasmids
Figure 8.29
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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

Figure 8.30a, b
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Regulation of Bacterial Gene Expression

• Constitutive enzymes are expressed at a fixed
  rate
• Other enzymes are expressed only as needed
• Repressible enzymes
• Inducible enzymes

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Repression
Figure 8.13
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Operon
Figure 8.14.1
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Regulation of Gene Expression
Figure 8.14.2
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Regulation of Gene Expression
Figure 8.14.3
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Regulation of Gene Expression
Figure 8.14.4
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Regulation of Gene Expression
Figure 8.14.5
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Regulation of Gene Expression
Figure 8.15