Bacterial Plasmid and Chromosome Transfer Mechanisms

Genetic Change in Bacteria

• Two mechanisms of genetic change in bacteria: mutation and horizontal gene transfer
• Changes are passed to progeny by vertical gene transfer

Mutation

• A change in an organism's DNA that alters its genotype
• Can occur spontaneously or be induced by mutagens
• Can result in a change in the phenotype of the organism

Types of Mutations

• Point mutations: a change in a single nucleotide
• Frameshift mutations: the insertion or deletion of one or more nucleotides, leading to a change in the reading frame of the genetic code
• Deletions: the removal of one or more nucleotides
• Insertions: the addition of one or more nucleotides

Effects of Mutations

• May result in a change in the function of a protein
• Can affect the growth and survival of an organism
• Can influence an organism's response to environmental factors

Induced Mutations

• Caused by external factors such as radiation and chemicals
• Can be used intentionally to generate mutations in bacteria

Repair of Damaged DNA

• Enzymes can repair damaged DNA by fixing errors in nucleotide incorporation
• Two mechanisms of repair: proofreading by DNA polymerase and mismatch repair
• SOS repair is a last-ditch repair mechanism used when other systems fail

Screening for Possible Carcinogens

• The Ames test measures the effect of a chemical on the reversion rate of a histidine-requiring Salmonella auxotroph
• Mutagens increase the frequency of spontaneous reversions

Plasmid Transfer

• Plasmids are small, self-replicating circular DNA molecules
• Can be transferred from one bacterium to another through conjugation
• F plasmid is involved in conjugation and is integrated into the chromosome of the donor cell

Chromosome Transfer

• Involves the transfer of part of the chromosome from one bacterium to another
• F′ plasmid is formed when a small piece of the chromosome is removed with the F plasmid

Genome Variability

• Much variation in the genes of different strains of a single species
• Less than 50% of the genes are shared between different strains of E. coli

Transposons

• Pieces of DNA that can move from one location to another in a cell's genome
• Can move into other replicons in the same cell
• Can integrate into the chromosome via non-homologous recombination

Genomic Islands

• Large DNA segments in a genome that originated in other species
• Nucleotide composition is very different from the rest of the genome
• Characteristics encoded by genomic islands include the use of specific energy sources and acid tolerance

Phage DNA

• Certain types of bacteriophages can insert their DNA into the host cell chromosome
• Phage DNA becomes part of the host cell's genome and is replicated and passed on to progeny cells

Bacterial Defenses Against Invading DNA

• Restriction-modification systems recognize and destroy foreign DNA that enters the cell
• CRISPR systems include small segments of phage DNA that recognize specific DNA and direct DNA-cutting enzymes to invading DNA