Molecular Biology: Nucleic Acid. The Genetic Code

But how did the nucleic acid molecule manage to pass on information concerning physical characteristics? The answer to that was made known through the work of the American geneticists, George Wells Beadle (1903- ) and Edward Lawrie Tatum (1909- ). In 1941, they began experiments with a mold called Neurospora crassa, one that was capable of living on a nutrient medium containing no amino acids. The mold could manufacture all its own amino acids out of simpler nitrogen-containing compounds.

If the molds were subjected to X rays, however, mutations were formed and some of these mutations lacked the ability to form all their own amino acids. One mutated strain might, for instance, be unable to form the amino acid, lysine, but would have to have it present in the nutrient mixture in order to grow. Beadle and Tatum were able to show that this inability was caused by the lack of a specific enzyme that the ordinary unmutated strain possessed.

They concluded that it was the characteristic function of a particular gene to supervise the formation of a particular enzyme. The nucleic acid molecules passed on in sperm and egg possessed within themselves the capacity of producing a particular set of enzymes. The nature of this set governed the cell chemistry; and the nature of the cell chemistry produced all the characteristics concerning whose heredity scientists inquired. Thus, one passed from DNA to physical characteristics.

The production of enzymes by the genes must, however, clearly be performed through intermediaries, since the DNA of the genes remained within the nucleus while protein synthesis went on outside the nucleus. With the advent of the electron microscope, the cell was studied in new and much subtler detail and the exact site of protein synthesis was found.

Organized granules, much smaller than the mitochondria and therefore called "microsomes" (from Greek words meaning "small bodies"), had been noted in great numbers within the cell. By 1956, one of the most assiduous of the electron microscopists, the Rumanian-American, George Emil Palade (1912- ), had succeeded in showing that the microsomes were rich in RNA. They were therefore renamed "ribosomes," and it was these ribosomes that proved to be the site of protein manufacture.

The genetic information from the chromosomes must reach the ribosomes and this was done through a particular variety of RNA called "messenger-RNA." This borrowed the structure of a particular DNA molecule within the chromosomes, and traveled out with that structure to a ribosome on which it layered itself. Small molecules of "transfer-RNA," first studied by the American biochemist, Mahlon Bush Hoagland (1921- ), attached themselves to specific amino acids; then, carrying the amino acids, attached themselves to matching spots on the messenger-RNA.

The chief remaining problem was to decide how a particular molecule of transfer-RNA came to attach itself to a particular amino acid. The simplest solution would be to imagine an amino acid attaching itself to a purine or pyrimidine of the nucleic acid; a different amino acid to each purine or pyrimidine. However, there are about twenty different ammo acids and only four purines and pyrimidines to a nucleic acid molecule. For that reason, it seems clear that a combination of at least three nucleotides must be matched to each amino acid. (There are 64 different possible combinations of three nucleotides.)

Matching the trinucleotide combination to the amino acid has been the great biological problem of the early 1960s and this is usually referred to as "breaking the genetic code." Men such as the Spanish-American biochemist, Severo Ochoa (1905- ), have been active in this respect.