Probing the Mystery of Life

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Robert W. Holley

The scientific quest to decipher biology's ultimate mystery—How do cells make protein, the building blocks of life?— gained fresh impetus in 1945. It followed announcement of the discovery that a nucleic acid carries and transmits the genetic blueprint for the manufacture of protein. The question matters because how and where cells make protein determine not only why a human differs from a mouse, but also why an individual person is unlike any other on earth.

A big step toward understanding this elemental mystery came in 1965 when a team of five ARS and three Cornell University scientists determined the molecular structure of one of the RNA's, ribonucleic acid. This marked the first time that the structure of an RNA—or any ribonucleic acid—was determined. The achievement won the research team's leader, former ARS biochemist Robert W. Holley, a share of the 1968 Nobel Prize for medicine or physiology. Holley was one of three Americans, working independently, honored for "interpreting the genetic code and its function in protein synthesis."

The research team's achievement was to depict the structure of a "transfer" RNA molecule—designated tRNA— whose function is to carry activated amino acids to protein-building sites within the cell.

For 7 years Holley's team engaged in a kind of molecular cartography. Their working terrain, bundled up inside a tiny yeast cell, was the tRNA molecule. The molecule itself is a chain of smaller organic molecules that can be compared to a string of pearls. Each pearl is strung along a strand composed of a sugary substance called ribosephosphate. Several organic bases combine with the molecules of ribosephosphate to form the pearls, which are called nucleotides.

How the nucleotides are arranged along the strand is important because RNAs faithfully follow the DNA molecules' genetic blueprint in assembling the order of amino acids.

The resulting chain of hundreds or even thousands of aminoacids is a protein. Arrangement of the nucleotides along the strand also determines the structure of the tRNA molecule.

Holley's team determined the tRNA's structure by using two enzymes to split the molecule into pieces. Each enzyme split the molecule at location points for specific nucleotides. By a process of "puzzling out" the structure of the pieces split by the two different enzymes, then comparing the pieces from both enzyme splits, the team eventually determined the entire structure of the molecule. Since the molecule transports the amino acid alinine to its appropriate protein-building site, it was designated "alinine-tRNA."

By harnessing the Holley team's method, other scientists determined the structures of the remaining tRNA's. A few years later the method was modified to help track the sequence of nucleotides in various bacterial, plant, and human viruses. Modified further, the Holley team's approach is playing a role in determining the sequence of DNAs in today's chromosomal research.