Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 27.2 Rapid Reproduction, Mutation, and Genetic Recombination Promote Genetic Diversity in Prokaryotes
Prokaryotes don't reproduce sexually, yet they show extraordinary genetic variation. This variation—key to their evolutionary success—arises from a combination of rapid reproduction, spontaneous mutations, and genetic recombination.
Rapid Reproduction and Mutation
- Prokaryotes reproduce by binary fission, often within hours.
- Though mutation rates per gene are low, the sheer number of replications results in many mutations:
- Example: E. coli in the human gut produces ~2,000 mutations per gene per day across 20 billion cells.
- These mutations generate genetic diversity, enabling rapid adaptation to new environments, as demonstrated by long-term E. coli experiments over 20,000 generations.
Genetic Recombination in Prokaryotes
Unlike eukaryotes, prokaryotes do not undergo meiosis, but they can exchange genes via:
1. Transformation
- Uptake of foreign DNA from surroundings.
- Example: Nonpathogenic Streptococcus can become pathogenic by acquiring virulence genes.
- Many bacteria actively absorb DNA from closely related species.
2. Transduction
- Phages (viruses) accidentally transfer bacterial DNA between cells.
- If the transferred DNA integrates into the host genome, a recombinant cell results.
3. Conjugation
- Direct DNA transfer between temporarily joined cells via a pilus.
- Requires the F factor (F plasmid or integrated into the chromosome) for pilus formation and DNA donation.
- F+ cells donate DNA.
- F– cells receive DNA and can become F+.
- Hfr cells (F factor integrated into chromosome) can transfer chromosomal genes, increasing recombination.
R Plasmids and Antibiotic Resistance
- R plasmids carry resistance genes to antibiotics (e.g., tetracycline, ampicillin).
- R plasmids often also carry F-like genes for conjugation, enabling them to spread resistance rapidly.
- Some R plasmids carry resistance to up to 10 antibiotics, raising serious public health concerns.
In a Nutshell
Despite lacking sexual reproduction, prokaryotes achieve high genetic diversity through rapid binary fission, mutations, and recombination mechanisms like transformation, transduction, and conjugation. These processes drive adaptation and the evolution of traits like antibiotic resistance.