Genes are strung along chromosomes and contain directions for the operation and growth of individual cells. But how can a molecule of nucleic acid (a gene) direct an entire complex cell to perform in a certain way? George Beadle answered this critical question and vastly improved our understanding of evolutionary genetics.
Beadle discovered that each gene directs the formation of a particular enzyme. Enzymes then swing the cell into action.
His discovery filled a huge gap in scientists’ understanding of how DNA blueprints are translated into physical cell-building action. Beadle’s groundbreaking work shifted the focus of the entire field of genetics research from the qualitative study of outward characteristics (what physical deformities are created by mutated genes) to the quantitative chemical study of genes and their mode of producing enzymes.
George Beadle was supposed to be a farmer. He was born on a farm outside Wahoo, Nebraska, in 1903. But a college study of the genetics of hybrid wheat hooked Beadle on the wonder of genetics. Genetics instantly became his lifelong passion.
In 1937, at the age of 34, Beadle landed an appointment with the genetics faculty at Stanford University. Stanford wanted to develop their study of biochemical genetics. The study of genetics was 80 years old. But biochemical genetics, or the molecular study of how genetics signals were created and sent to cells, was still in its infancy. Beadle teamed with microbiologist Edward Tatum to try to determine how genes exercise their controlling influence.
In concept their work was simple. In practice it was painstakingly tedious and demanding.
They searched for the simplest life form they could find, choosing the bread mold Neurospora because its simple gene structure had been well documented. They grew trays upon trays of colonies of Neurospora in a common growth medium. Then Beadle and Tatum bombarded every colony with X-rays, which were known to accelerate genetic mutations.
Within 12 hours most colonies continued to grow normally (they were unmutated), a few died (X-rays had destroyed them), and a precious few lived but failed to thrive (gene mutations now made them unable to grow).
The interesting group was this third one because it had undergone some genetic mutation that made it impossible for the mold to grow on its own. If Beadle and Tatum could discover exactly what this mutated mold now needed in order to grow, they would learn what its mutated gene had done on its own before it was damaged.
Beadle and Tatum placed individual spores from one of these colonies into a thousand different test tubes, each containing the same standard growth medium. To each tube they added one possible substance the original mold had been able to synthesize for itself but that the mutated mold might not be producing. Then they waited to see which, if any, would begin to thrive.
Only one tube began to grow normally, tube 299, the one to which they had added vitamin B6. The mutation to the mold’s gene must have left the mold unable to synthesize vitamin B6 and thus unable to grow. That meant that the original gene had produced something that made the cells able to synthesize the vitamin on their own. The second step of Beadle and Tatum’s experiment was to search for that something.
Beadle found that when he removed, or blocked, certain enzymes the mold stopped growing. He was able to trace these enzymes back to genes and to show that the mutated gene from tube 299 no longer produced that specific enzyme.
Through this experiment Beadle discovered how genes do their job. He proved that genes produce enzymes and that enzymes chemically direct cells to act. It was a discovery worthy of a Nobel Prize.
Humans have between 25,000 and 28,000 genes. Different genes direct every aspect of your growth and looks. Some do nothing at all. Called recessive genes, they patiently wait to be passed on to the next generation, when they might have the chance to become dominant and control something.