Genetic Recombination in Microbiology

Genetic Recombination

• Prokaryotic cells can take up and incorporate foreign DNA from the surrounding environment through transformation.
• Transduction is the movement of genes between bacteria by bacteriophages (viruses that infect bacteria).
• Horizontal gene transfer refers to the movement of genes among individuals from different species.
• Conjugation is the process where genetic material is transferred between prokaryotic cells.

Conjugation

• In bacteria, DNA transfer is one-way, from a donor cell to a recipient cell.
• A donor cell attaches to a recipient by a pilus, pulls it closer, and transfers DNA.
• The F factor is required for the production of pili.

Metabolism and Nutrition

• Prokaryotes can be categorized by how they obtain energy and carbon:
  • Phototrophs obtain energy from light.
  • Chemotrophs obtain energy from chemicals.
  • Autotrophs require CO2 as a carbon source.
  • Heterotrophs require an organic nutrient to make organic compounds.

Nutrition Modes

• Energy and carbon sources are combined to give four major modes of nutrition.

Nitrogen Metabolism

• Nitrogen is essential for the production of amino acids and nucleic acids.
• In nitrogen fixation, some prokaryotes convert atmospheric nitrogen (N2) to ammonia (NH3).
• In the cyanobacterium Anabaena, photosynthetic cells and nitrogen-fixing cells called heterocysts exchange metabolic products.

Prokaryotic Metabolism

• Obligate aerobes require O2 for cellular respiration.
• Obligate anaerobes are poisoned by O2 and use fermentation or anaerobic respiration.
• Facultative anaerobes can survive with or without O2.

Prokaryotic Diversity

Domain Archaea

• Archaea share certain traits with bacteria and other traits with eukaryotes.
• Some archaea live in extreme environments and are called extremophiles.
• Extreme halophiles live in highly saline environments.
• Extreme thermophiles thrive in very hot environments.
• Methanogens produce methane as a waste product.

Domain Bacteria

• Proteobacteria include photoautotrophs, chemoautotrophs, and heterotrophs.
• Some proteobacteria are anaerobic, and others are aerobic.
• Chlamydia are parasites that live only within animal cells.
• Spirochetes are helical heterotrophs.
• Cyanobacteria are photoautotrophs that generate O2.
• Gram-positive bacteria include actinomycetes, Bacillus anthracis, Clostridium botulinum, and some Staphylococcus and Streptococcus.

Ecological Importance

• Prokaryotes play a major role in the recycling of chemical elements between the living and nonliving components of ecosystems.
• Chemoheterotrophic prokaryotes function as decomposers, breaking down dead organisms and waste products.
• Prokaryotes can increase the availability of nitrogen, phosphorus, and potassium for plant growth.

Symbiosis

• Symbiosis is an ecological relationship in which two species live in close contact: a larger host and smaller symbiont.
• Some prokaryotes have fimbriae, which allow them to stick to their substrate or other individuals in a colony.
• Pili (or sex pili) are longer than fimbriae and allow prokaryotes to exchange DNA.

Locomotion

• Many bacteria exhibit taxis, the ability to move toward or away from a stimulus.
• Chemotaxis is the movement toward or away from a chemical stimulus.
• Most motile bacteria propel themselves by flagella scattered about the surface or concentrated at one or both ends.
• Bacterial flagella are composed of a motor, hook, and filament.

Internal Organization

• Prokaryotic cells usually lack complex compartmentalization.
• Some prokaryotes have in-foldings of the plasma membrane that perform metabolic functions.
• Most of the genome consists of a circular chromosome located in the nucleoid region.
• Some species of bacteria also have smaller rings of DNA called plasmids.

Prokaryotic Reproduction

• Bacteria are small and reproduce by binary fission (1-3 hours).
• They have short generation times.
• Many prokaryotes form metabolically inactive endospores, which can remain viable in harsh conditions for centuries.
• Three factors contribute to genetic diversity in bacteria: rapid reproduction, mutation, and genetic recombination.
• Rapid reproduction and mutation allow for rapid evolution.
• Genetic recombination through conjugation, transduction, and transformation increases genetic diversity in bacteria.