Article
A Little Truth about Genes
I am amazed at the inventiveness of early researchers in pursuit of the secrets of heredity. With no idea of the true chemistry of genes, investigators designed experiments to reveal genetic facts.
Geneticists of the 1930’s and ‘40’s believed, incorrectly, that genes must be made of protein. Yet during this time, George Beadle made his “one gene—one enzyme” discovery. The discovery came about because Beadle wondered what genes actually do in order to cause traits.
First Beadle investigated fruit fly eye colors. Normal eye color in these flies is a deep-red mixture of red and brown pigments. Two bright-red mutant colors are vermilion and cinnabar; these contain no brown. From mutant larvae, Beadle transplanted vermilion and cinnabar eye discs into normal larvae. The eyes developed normal color in the adult flies. Cross-mutant transplantation showed Beadle that the brown pigment resulted from a series of chemical reactions: a starting substance got changed to vermilion, which got changed to cinnabar, which got changed to brown. Each mutant lacked one of these reactions, but it was time-consuming to figure out which one.
To speed up his investigation, Beadle switched from fruit flies and eye colors, to red bread mold and the nutrients it manufactured. Beadle X-rayed the bread mold to cause mutations, then tested spores from the mold to see if they could grow on a minimal food containing only sugars and salts.
If a mold couldn’t develop on the minimal food, this meant it was missing a nutrient because of a mutation. Beadle tested to see if the mutation was in a single gene. If so, Beadle then added a supplement, such as a vitamin or an amino acid. If that didn’t make the mold grow, he tried a different supplement, until he found the missing nutrient.
During growth, each mutant mold accumulated a chemical. This chemical came from the reaction step where the mutant got stuck. The mutants could be arranged in order, according to where they got stuck, and this order showed the reaction steps in the manufacture of the supplement nutrient.
Beadle knew that each chemical reaction is controlled by an enzyme. So each mutant mold must be missing the enzyme that could change its accumulated chemical to the next one in the series. Since each mutant was missing a single gene, each of those genes must give rise to a single enzyme. “One gene—one enzyme!”
Geneticists of the 1930’s and ‘40’s believed, incorrectly, that genes must be made of protein. Yet during this time, George Beadle made his “one gene—one enzyme” discovery. The discovery came about because Beadle wondered what genes actually do in order to cause traits.
First Beadle investigated fruit fly eye colors. Normal eye color in these flies is a deep-red mixture of red and brown pigments. Two bright-red mutant colors are vermilion and cinnabar; these contain no brown. From mutant larvae, Beadle transplanted vermilion and cinnabar eye discs into normal larvae. The eyes developed normal color in the adult flies. Cross-mutant transplantation showed Beadle that the brown pigment resulted from a series of chemical reactions: a starting substance got changed to vermilion, which got changed to cinnabar, which got changed to brown. Each mutant lacked one of these reactions, but it was time-consuming to figure out which one.
To speed up his investigation, Beadle switched from fruit flies and eye colors, to red bread mold and the nutrients it manufactured. Beadle X-rayed the bread mold to cause mutations, then tested spores from the mold to see if they could grow on a minimal food containing only sugars and salts.
If a mold couldn’t develop on the minimal food, this meant it was missing a nutrient because of a mutation. Beadle tested to see if the mutation was in a single gene. If so, Beadle then added a supplement, such as a vitamin or an amino acid. If that didn’t make the mold grow, he tried a different supplement, until he found the missing nutrient.
During growth, each mutant mold accumulated a chemical. This chemical came from the reaction step where the mutant got stuck. The mutants could be arranged in order, according to where they got stuck, and this order showed the reaction steps in the manufacture of the supplement nutrient.
Beadle knew that each chemical reaction is controlled by an enzyme. So each mutant mold must be missing the enzyme that could change its accumulated chemical to the next one in the series. Since each mutant was missing a single gene, each of those genes must give rise to a single enzyme. “One gene—one enzyme!”
Article author
About the Author
Julie Simon Lakehomer is writing a book about thirteen geneticists eager to ferret out the secrets of inheritance. These researchers committed themselves to conversation with the DNA universe until they transformed what was known about heredity.
Julie has degrees from the U. of Chicago and the U. of Ill. at Chicago. She taught sciences at a Chicago area high school. During her fiction period she published and won awards for her short stories. Recently her first science article, “A New Look at Mendel,” appeared in the Summer 2007 edition of The journal of the Washington Academy of Science. More of her articles appear on her website, “The Pursuit of Wonder” at www.juiesimonlakehomer.com.
Julie is active in the American Association for the Advancement of Science, the National Science Teachers Association, the Association of Women in Science, Graduate Women in Science, the National Education Association, and the International Women’s Writing Guild.
Julie has degrees from the U. of Chicago and the U. of Ill. at Chicago. She taught sciences at a Chicago area high school. During her fiction period she published and won awards for her short stories. Recently her first science article, “A New Look at Mendel,” appeared in the Summer 2007 edition of The journal of the Washington Academy of Science. More of her articles appear on her website, “The Pursuit of Wonder” at www.juiesimonlakehomer.com.
Julie is active in the American Association for the Advancement of Science, the National Science Teachers Association, the Association of Women in Science, Graduate Women in Science, the National Education Association, and the International Women’s Writing Guild.
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