Bacterial Genetics

• The F plasmid is an episome that can be transferred to other bacteria through conjugation.
• The F plasmid 


  is an episome that can be transferred to other bacteria through conjugation.
• Barbara McClintock was awarded the Nobel Prize

  1983
• Barbara McClintock concluded that segments of DNA moved in and out of genes involved in color
• Genotype


  Sequence of nucleotides in DNA
• The idea of DNA movement was established

  1970s
• Model system of genetic change

  • E. coli
• Phenotype


  Observable characteristics influenced by genotype and environmental conditions
• Barbara McClintock's idea of DNA movement was met with skepticism as DNA was believed to be stable
• Barbara McClintock published her results

  1950
• Bacterial replication is like "cloning" but introduces genetic changes via rare mutations
• Organisms adapt to ever-changing environments through natural selection
• Terms related to genetic change in bacteria

  • Auxotroph
  • Prototroph
  • Wild type
• General mechanisms for bacteria to adjust to new circumstances

  • Regulation of gene expression
  • Genetic change
• Base substitutions are more common in aerobic environments due to reactive oxygen species
• Base substitution outcomes

  • Synonymous mutation: codes for same amino acid
  • Missense mutation: creates codon for different amino acid
  • Nonsense mutation: creates a stop codon
• Base substitution is the most common type of mutation, involving the incorrect incorporation of a nucleotide during DNA synthesis
• Deletion or addition of nucleotides can cause frameshift mutations depending on the number involved
• Mutations occur at characteristic rates and can be passed to progeny
• Spontaneous mutations are random genetic changes resulting from normal cell processes
• Bacteria have two mechanisms of genetic change: Mutation and Horizontal gene transfer
• Barbara McClintock (1902 to 1992) observed that kernel colors in corn were not inherited in a predictable manner
• Base Analogs

  • Resemble nucleobases, but have different hydrogen-bonding properties
  • Can be incorporated into DNA by DNA polymerase
  • Wrong nucleotide is incorporated into complementary strand during DNA replication
  • 5-bromouracil resembles thymine, often base-pairs with guanine
  • 2-amino purine resembles adenine, often pairs with cytosine
• Can oxidize nucleobase guanine

  DNA polymerase often mispairs with adenine instead of cytosine
• Repair of Damaged DNA
• Transposons (Jumping Genes)
• Intercalating Agents

  • Flat molecules that intercalate between adjacent bases in DNA strand
  • Pushes nucleotides apart, produces space
  • Increases chances of insertions or deletions during replication
  • Often result in premature stop codon
  • Transposons can be introduced intentionally to generate mutations
  • Transposon inserts into cell’s genome
  • Generally, inactivates gene into which it inserts
• Chemical Mutagens


  • Chemicals that modify nucleobases
  • Change base-pairing properties; increase chance of incorrect nucleotide incorporation
• Induced Mutations
• Transposons (Jumping Genes)

  1. Pieces of DNA that can move from one location to another in a cell’s genome

  2. Process of transposition

  3. Insertional inactivation: gene into which transposon jumps is inactivated; function disrupted

  4. Most transposons have transcriptional terminators

  5. Block expression of downstream genes in operon
• Produced from O2
• Mismatch Repair

  1. Fixes errors missed by DNA polymerase

  2. Enzyme cuts sugar-phosphate backbone of new DNA strand

  3. Another enzyme degrades short region of DNA strand with error

  4. Methylation of DNA indicates template strand

  5. Newly synthesized strand is unmethylated

  6. DNA polymerase, DNA ligase fill in and seal the gap
• Deletion or Addition of Nucleotides

  1. Impact depends on number of nucleotides involved

  2. Addition or deletion of three pairs changes one codon

  3. Causes one amino acid more or less

  4. Impact depends on location within protein

  5. Addition or deletion of one or two pairs yields frameshift mutation

  6. Different set of codons translated

  7. Often results in premature stop codon

  8. Shortened, nonfunctional protein
• Repair of Damaged Nucleobases

  1. Base excision repair uses DNA glycosylase to remove damaged nucleobase

  2. Another enzyme cuts DNA at this site

  3. A DNA polymerase degrades a short section to remove damage; then synthesizes correct replacement

  4. DNA ligase seals gap
• Radiation can be used as a mutagen
• Repair of Errors in Nucleotide Incorporation
• Repair of Thymine Dimers

  1. Photoreactivation: light repair

  2. Enzyme uses energy from light

  3. Breaks covalent bonds of thymine dimer

  4. Nucleotide Excision repair: dark repair

  5. Enzyme removes damaged region

  6. DNA polymerase, DNA ligase fill in and seal the gap
• SOS Repair
• Mutagen


  Agent that induces genetic changes
• SOS Repair


  Last-ditch repair mechanism used when other systems fail, induced following extensive DNA damage that stalls DNA and RNA polymerases, includes a DNA polymerase that synthesizes even in extensively damaged regions, results in SOS mutagenesis