an exact copy but uses basic chemistry to create a complementary copy of the antisense strand, ergo an exact copy of the sense strand.

Tjian concentrates not on the chemical events themselves, but rather on how the RNA polymerase somehow knows where to go to begin and then where to end the transcript. The basic terrain he is exploring—the proteins called transcription factors—has signposts like promoter and enhancer regions, and introns and extrons, to guide this search for the "where" of transcription. The roles these regions play are largely unknown, and they offer a rich terra incognita for the molecular biologist and biochemist with a pioneering curiosity. As Tjian put it, "RNA polymerase is rather promiscuous" and not capable of discriminating discrete parts of the genome. It is the transcription factors that "seem to be designed to recognize very subtle differences in the DNA sequence of the template and can easily discriminate a real piece of information from junk. Their resolving power is very high," he said. Used as probes, they allow geneticists to home in on a piece of DNA as small as 6 to 8 nucleotides in length.

Drawing Lessons from Simpler Organisms

As Arnold Berk put it: "What are the punctuation marks?" Berk has done some important work on this question by probing a much simpler genome in a particular species of virus called adenovirus type 2, one of the 100 or so viruses that give humans colds. This diversity of known cold viruses allows Berk to deduce that "this is why you get a cold every year." Since a single exposure is sufficient to create a permanent immunity to its effect, it is comparatively safe to work with it in the laboratory. As compared to the 3 billion base pairs in the human genome, the adenovirus has only about 36,000. The logical inference is that—even though the virus does not have to perform differential calculus or ponder the philosophical and scientific implications of chaos theory—it has a more efficient genome. That is, there is a smaller proportion of junk or extra (that is, unidentified as to its clear function) DNA in the viral genome. "It grows well in the laboratory, it is convenient to work with," and its transcription behavior should be comparatively lucid.

The strategy of the virus, when it manages to get inside a host cell, is to exploit the cell's capacity to transcribe and translate DNA. The DNA of the virus says something like, "Make more of these." Instructions that say "make proteins" are obviously not hard to find, since they are the heart of any DNA code. But since it has been revealed that, in the human and most other genomes, so much other

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement