Genetic Variation in Bacteria

Genetic Variation

Variation

  • Prokaryotes can enhance their survival chances by producing diverse proteins.
  • Asexual reproduction and binary fission usually create copies of the original DNA.
  • Genetic variation in bacteria arises through:
    • Mutation
    • Genetic Recombination
  • Rapid reproduction rates in bacteria allow new mutations to quickly increase genetic diversity.
  • Genetic diversity is further enhanced by the recombination of DNA from different bacterial cells.

Mutation

  • Mutation is a significant source of bacterial variation.
  • Mutation: A change in the DNA (genetic code) of an organism.
  • A change in DNA can lead to altered characteristics in an organism.
  • Cells make mistakes even though they are good at copying during division.
  • Mutation rate: Approximately 1% of binary fissions result in mutations.
  • Example: In a population of 1,000,000 E. coli cells, about 10,000 will have mutations.
  • Every generation produces some mutants.

Effects of Mutation

  • Most mutations are harmful and cause cell death.
  • Rare mutations can increase a cell's fitness.
  • Example: A mutation could lead to antibiotic resistance.
  • Frequency: Maybe 1 in 1,000,000 cells will continue to grow due to a beneficial mutation.

Fast Reproduction with Mutations

  • If bacteria have mutations for antibiotic resistance, they survive and reproduce quickly.
  • This leads to a new population of antibiotic-resistant bacteria.
  • Model:
    • A random mutation creates a resistant cell (red).
    • Within 3 generations, the resistant cell expands in the population.
    • Other variants are either stable (yellow) or disappear (pink).

Mechanisms of Genetic Recombination

  • Prokaryotic DNA can be altered through:
    • Transformation
    • Conjugation
    • Transduction
    • Transposition (also in eukaryotes)
  • These mechanisms of gene transfer and genetic recombination in prokaryotes result in great diversity.

Transformation

  • Transformation: The alteration of a prokaryotic cell's genes by taking up foreign DNA from the surrounding environment.
  • Harmless Streptococcus pneumoniae bacteria can transform into pneumonia-causing cells through transformation.

Evidence That DNA Can Transform Bacteria

  • Frederick Griffith's research in 1928 revealed the genetic role of DNA.
  • Griffith worked with two strains of bacteria:
    • Pathogenic “S” (smooth) strain
    • Harmless “R” (rough) strain
  • When heat-killed remains of the pathogenic strain were mixed with living cells of the harmless strain, some living cells became pathogenic.
  • Transformation: A change in genotype and phenotype due to the assimilation of foreign DNA.

The Griffith Experiment

  • Experiment:
    • Rough strain (nonvirulent) → mouse lives
    • Smooth strain (virulent) → mouse dies
    • Heat-killed smooth strain → mouse lives
    • Rough strain & heat-killed smooth strain → mouse dies

Conclusion of Griffith Experiment

  • The dangerous, mouse-killing gene (DNA) from the S strain was absorbed by the R strain.
  • The S strain was then TRANSFORMED into a killer bacteria
  • Prokaryotes can incorporate pieces of DNA into their genes.

Conjugation

  • Conjugation: The direct transfer of genetic material (plasmids) between prokaryotic cells that are temporarily joined.
  • The transfer is one-way: One cell donates DNA via a sex pilus, and the other receives the genes.

Plasmids

  • Plasmids are smaller, circular DNA molecules independent of the bacterial chromosome.
  • Plasmids contain genes for adaptations like:
    • Resistance against antibiotics
    • Making a sex-pilus (F-pilus)
    • Making toxins
    • Guarding against heavy metal toxicity.

F Plasmids

  • The presence of an F plasmid gives the prokaryotic cell the ability to have fertility by forming a sex pilus.
  • This allows the prokaryote to donate DNA to other prokaryotes in its colony, increasing their genetic variability.
  • Note: Fertility can also be present if the "F" factor is located in the bacterial chromosome.

R Plasmids

  • R plasmids give a bacteria cell antibiotic resistance.
  • Antibiotic resistance gives the bacterial cell immunity to certain types of antibiotics.
  • When a bacterial population is exposed to an antibiotic, individuals with the R plasmid will survive and increase in the overall population.

Transduction

  • Transduction: The process by which DNA is transferred from one prokaryote to another by a virus.

Transposition

  • Transposition: Segments of DNA (transposons) change their position in the genome, either within or between chromosomes.
  • Transposons are also known as "jumping genes."
  • They are found in both prokaryotes and eukaryotes.
  • A significant portion of the human genome (50%) consists of transposons.
  • Genetic variability is increased through this process.

Transposition Operation

  • The transposon is cut from its original position and pasted to its new location, which can alter the function of the disrupted gene.