Lecture Notes: Beadle and Tatum's Genetic Experimentation

Key Figures

• George Beadle and Edward Tatum
  • Conducted experiments on the red bread mold Neurospora crassa in 1941.
  • Demonstrated that each gene encodes one enzyme that regulates specific chemical events.
  • Built upon Sir Archibald Garrod's 1902 theory regarding hereditary diseases as inborn errors of metabolism.

Background

• Neurospora crassa
  • Primarily a haploid organism (one copy of each gene).
  • Grows on minimal medium containing simple sugars, inorganic salts, and vitamin biotin.
  • Converts simple substances to amino acids and vitamins necessary for growth.

Hypothesis and Methodology

• Objective:

  • Mutate individual genes and identify nutritional mutants unable to grow on minimal medium.
  • Mutants should grow if enzyme product is supplemented.

• Process:

  • Utilized X-rays to induce mutations in Neurospora based on Herman Muller's 1927 discovery.
  • Grew irradiated offspring on complete medium with all vitamins and amino acids.
  • Tested growth on minimal medium to identify genetic mutations.

Experimental Findings

• Culture 299

  • Did not grow on minimal medium.
  • Grew on minimal medium with vitamin supplements, indicating inability to synthesize a vitamin.
  • Identified missing vitamin as B6 via single vitamin supplementation tests.

• Significance:

  • Proved mutation in the gene responsible for the enzyme in Vitamin B6 pathway.
  • Established that genetic mutations affect metabolic pathways.

Further Investigations

• Arginine Synthesis Pathway

  • Involves stepwise conversion: precursor → ornithine → citrulline → arginine.
  • Mutations identified through inability to perform specific steps.

• Strain Specific Findings:

  • Mutant 1:
    • Could not synthesize ornithine.
    • Able to grow when ornithine was added.
  • Mutant 2:
    • Affected in the synthesis of arginine precursor citrulline.
    • Citrulline supplementation restored growth.
  • Mutant 3:
    • Affected in final conversion to arginine.
    • Required arginine supplementation for growth.

Conclusion

• One Gene-One Enzyme Hypothesis
  • Each gene directs the formation of one specific enzyme.
  • Validated Garrod's hypothesis that gene mutations lead to specific metabolic defects.
  • Established that mutations inactivate enzyme proteins necessary for nutrient synthesis.