Bacterial Genetics

Bacterial Genetics

Bacterial Resistance

  • Bacterial cells acquire resistance through horizontal gene transfer.
  • Horizontal gene transfer: transfer of genes between cells of the same generation
Bacterial Conjugation
  • A donor cell transfers a plasmid to a recipient cell via an F pilus.
  • If the plasmid contains resistance genes, the recipient cell becomes resistant.
Bacterial Transformation
  • Uptake of a naked piece of DNA (e.g., a plasmid) by a cell.
  • Example: Uptake of the pGLO plasmid.

Plasmids and Antibiotic Resistance

  • Plasmids can carry multiple resistance genes.
  • Example: pLW1043 plasmid
    • Resistance to penicillin (beta-lactam genes).
    • Resistance to trimethoprim.
    • Genes for plasmid spread (F pilus formation).
    • Resistance to disinfectants.
    • Streptomycin family resistance.
    • Vancomycin resistance (a last line of defense antibiotic).

Vertical vs. Horizontal Gene Transfer

Vertical Gene Transfer
  • Transfer of genes from one generation to the next (e.g., parent to offspring).
Horizontal Gene Transfer
  • Transfer of genes between existing cells (e.g., via conjugation).
  • A donor cell transfers a plasmid to an acceptor cell.
  • Vertical transfer occurs when a cell divides into two, replicating the DNA and plasmid.

Mutations

Definition
  • Any change to the nucleotide sequence of DNA.
  • Can occur inside or outside of a gene.
  • May or may not result in a phenotypic change, unless it occurs in a gene.
Nucleotide Substitutions
  • Spontaneous mutations that change the identity of one nucleotide into another.
  • Transitions: Interchange of pyrimidines (T ↔ C) or purines (A ↔ G).
  • Transversions: Change of pyrimidines to purines or vice versa.
Perpetuation of Mutations
  • Wild type gene: Unmutated, normal function.
  • If DNA replication is normal, all progeny inherit the wild type sequence.
  • If there's an error during DNA replication, a mutation occurs.
  • This mutation can then be passed on to subsequent generations (vertical DNA transfer).
  • Example:
    • Original DNA sequence: CGTTAG
    • Replication error: CGTTCG (mutation)
    • Progeny inherit either the wild type or the mutant sequence.
Heritability of Mutations
  • Example: BRCA1 gene and breast cancer.
  • Heterozygous individuals (one normal, one mutant copy) have a 50% chance of passing on either the normal or mutant gene.

Types of Mutations

Codon Chart
  • Used to determine the amino acid sequence encoded by an RNA sequence.
  • Example: RNA sequence AUG GCA UAA
    • DNA sequence: TAC CGT ATT (minus strand).
    • Amino acid sequence: Methionine - Alanine - Stop
Missense Mutation
  • Changes the identity of the encoded amino acid.
  • Example: Change in DNA sequence leads to a change in the RNA sequence and thus in the amino acid sequence.
Nonsense Mutation
  • Changes the identity of an encoded amino acid into a stop codon.
  • Prematurely terminates translation.
  • Example: AGA (Arginine) → UGA (Stop).
Silent Mutation
  • Mutation occurs, but does not change the identity of the encoded amino acid.
  • Example: Mutation of GCA (Alanine) to GCU (also Alanine).
Point Mutations
  • Nucleotide substitutions that change just one nucleotide.
Insertions and Deletions (Indels)
  • Change the reading frame of the mRNA.
  • Affect the amino acid sequence.
  • Example: Addition of a nucleotide shifts all subsequent codons.

Spontaneous Mutations

Base Substitution
  • Nucleotide substitutions.
  • DNA strand separates, and the wrong nucleotide is incorporated during replication.
  • Leads to silent, missense, or nonsense mutations.
Deletion and Addition of Nucleotides
  • Can lead to a frame shift.
Jumping Genes
  • Mobile DNAs that can be excised from one chromosome and insert themselves into another.
  • Discovered by Barbara McClintock.
  • A large insertion mutation that can change the amino acid sequence of a gene.

Induced Mutations

  • Caused by mutagens (e.g., carcinogenic chemicals in cigarettes).
  • Chemical agents modify nucleotide bases.
  • Alkylating bases.
  • Base analogs.
  • Intercalating agents.
  • Radiation (e.g., UV radiation).
Examples of Induced Mutations
  • Deamination: Cytosine loses an amino group and becomes uracil.
  • Alkylation: Addition of an alkyl group, changing guanine into O6-methylguanine, which pairs with thymine instead of cytosine.
  • Oxidation: Changing guanine into 8-oxo-guanine, which pairs differently.
  • Base Analogs: Molecules that look like bases but pair differently (e.g., 5-bromouracil pairs with guanine instead of adenine).
  • Intercalating Mutagens: Molecules that insert between base pairs and act like a frameshift (e.g., ethidium bromide).
Radiation
  • UV radiation causes thymines to stick together, forming thymine dimers.
  • This creates a bulge in the DNA backbone, preventing DNA polymerase from copying the DNA.
  • Bacteria possess photolyase, an enzyme that uses light to break thymine dimers.
  • Humans have XPD, a gene that helps fix these mutations. Mutations in XPD cause xeroderma pigmentosum.
Xeroderma Pigmentosum
  • A rare hereditary defect of the enzyme system that repairs DNA after UV damage.
  • Individuals with this condition are very sensitive to light and develop lesions on their skin.

Repair of Errors in Nucleotide Incorporation

  • Enzymes cut into the sugar-phosphate backbone of the new daughter strand near a mismatched base.
  • Another enzyme removes the stretch of DNA surrounding the misincorporated nucleotide.
  • DNA polymerase inserts the correct nucleotides.
  • Ligase seals the gap in the phosphodiester backbone.
Repair of Oxidized Guanines
  • Glycosylase removes the base.
  • The backbone is cut, and some surrounding bases are removed.
  • DNA polymerase lays down the new sequence.
  • Ligase seals the gap.
Repair of Thymine Dimers
  • Photolyase breaks the thymine dimer.
Excision Repair
  • Enzymes cut on either side of the damage and remove the stretch.
  • DNA polymerase lays down the correct nucleotides.
  • Ligase seals the gap.