Red Bread Mold Provided Insight into the Role of Genes

The common red bread mold, Neurospora crassa, proved to be an ideal organism for studying the relationship between genes and enzymes. Although we commonly see this mold on stale bread, Neurospora is an extremely independent organism that can synthesize almost all the organic compounds it needs. It can grow on a simple nutrient solution (minimal medium) that contains a few minerals, a single vitamin, and an energy source such as sucrose.

Besides being easy to grow, Neurospora is genetically ideal as an experimental subject. For most of its life, Neurospora has just one copy of each gene. Most plants and animals, in contrast, have two copies of each chromosome and thus two copies of each gene. Consequently, the effects of a defective gene may be masked by a normal gene on the second chromosome. In Neurospora the effects of a defective gene cannot be masked, because it does not supply another copy of that gene.

In the early 1940s, geneticists George Beadle and Edward Tatum bombarded Neurospora with X-rays. The high energy of X-rays causes mutations, which are changes in the base sequence of DNA. Eventually the X-rays produced hundreds of different mutations that affected the nutritional requirements of the mold. Each mutant mold was no longer able to grow on minimal medium unless a specific nutrient – for example, one of the B vitamins or a certain amino acid – was added. Beadle and Tatum concluded that each of these mutations inactivated a specific enzyme that normally allowed the mold to synthesize a nutrient. Their experiments supported the hypothesis that each gene codes for a single enzyme. A few years later, to determine the series of chemical reactions by which normal molds synthesize the amino acid arginine, biochemists used mutant Neurospora that couldn’t grow on minimal medium unless arginine was added. They found that the mold uses the following biochemical pathway:

enzyme A enzyme B

ornithine -> citrulline -> arginine

By starting with minimal medium and then adding one precursor molecule in the pathway of arginine synthesis at a time, the researchers found that the mutant lacked a single enzyme that catalysed one specific step in arginine synthesis. This finding further supported the hypothesis that each gene encoded the information needed for the synthesis of a specific enzyme.

1 Neurospora is very independent because it can synthesize the organic compounds it needs. T

2 Neurospora cannot grow on a minimal medium. F: “It can grow on a simple nutrient solution (minimal medium) that contains a few minerals, a single vitamin, and an energy source such as sucrose.”

3 Neurospora has just one copy of each gene that is why it is very useful in experiments. T

4 George Beadle and Edward Tatum wanted to see the DNA of Neurospora with the help of X-ray diffraction. T

5 The mutations caused by X-rays made Neurospora need no nutrients. F: “Eventually the X-rays produced hundreds of different mutations that affected the nutritional requirements of the mold. Each mutant mold was no longer able to grow on minimal medium unless a specific nutrient – for example, one of the B vitamins or a certain amino acid – was added.”

6 The experiment of Beadle and Tatum proved that each enzyme was encoded by one gene. T

7 Some years later mutant Neurospora was used that could not grow unless citrulline was added to the medium. F: “A few years later, to determine the series of chemical reactions by which normal molds synthesize the amino acid arginine, biochemists used mutant Neurospora that couldn’t grow on minimal medium unless arginine was added.”

8 The experimental mutant mold lacked many enzymes. F: “the researchers found that the mutant lacked a single enzyme that catalysed one specific step in arginine synthesis.”

Unit 8