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In a second letter, they described that mechanism: how the DNA molecule unwinds and unzips itself right down the middle during cell division, its base pairs breaking apart at their hydrogen bonds. Then by drawing on the free-floating material surrounding them in the nucleus of the cell, the two separated strands link up with complementary base-and-strand units along their entire length, forming two exact copies of the original double helix. Thus DNA faithfully passes its genetic information on to new cells and to future generations.
Ingenious as the theory was, scientists still demanded proof that the molecule actually replicated itself. That proof was quick to come. By 1956, Arthur Kornberg, then at Washington University in St. Louis, discovered an enzyme, or natural chemical catalyst (which he named "DNA polymerase") that was apparently critical to some of the activities of the double helix. Once he obtained enough of the enzyme, he placed it in a test-tube brew with a bit of natural DNA, one of whose strands was incomplete, the four bases (A, T, C, G) and a few other off-the-shelf chemicals. True to his expectations—and the Watson-Crick theory—the incomplete segment picked up its complementary nucleotides from the brew to form a complete double helix.
Implicit in the Watson-Crick model were the workings of DNA's other essential function: how it orders the production of proteins. These are also long and twisted helical molecules, but they are the actual building blocks rather than the genetic blueprints for living things. As such, proteins are immensely varied; there are many thousands of different kinds in the human body alone. The distinctive proteins that make up the cells of the eye, for example, differ from those of the kidneys or muscles. Despite their variety, however, all proteins are built from some of only 20 smaller and simpler molecules, called amino acids. How then, scientists asked themselves, did the isolated double helix, locked in the nucleus of the cell, direct the assembly of amino acids into protein in other parts of the cell?
Scientists suspected that DNA had a helper, a single-stranded chemical first cousin called ribonucleic acid (RNA). Most of the cell's RNA is found in ribosomes. These are globular bodies in the material outside the cell's nucleus that seem to be highly active centers of protein synthesis. But if this ribosomal RNA played a role in protein making, how did it obtain and execute the instructions from the master molecule DNA inside the nucleus?
In 1955, after wrestling with the question, Francis Crick postulated (and Harvard Biochemists Paul Zamecnik and Mahlon Hoagland confirmed) a second form of RNA, which was later found to carry specific amino acids floating in the cytoplasm to the ribosomes; this substance became known as transfer RNA. Then in the early 1960s, biologists discovered a third kind of RNA—shortly after its existence had been theorized by Jacques Monod and François Jacob of France's Pasteur Institute. Called messenger RNA, it provided the missing piece in the molecular puzzle. It was formed on an uncoiled strip of DNA in the nucleus, imprinted with the particular "message" encoded in that portion—or gene—of the staircase, and then sent off with these instructions to the protein-making ribosomes.