The Origin of DNA

THE ORIGIN OF DNA M. SLUYSER* Charles arranged his papers before him on the speaker's desk. "May I have the first slide, please?" he asked. The light ofthe projector winked on in the small lecture room, and the scientists craned their necks to peer at the screen. It showed a colored photograph ofthe Milky Way. "The previous two speakers have described the general properties of the stars and planets in the central region ofthis galaxy," Charles began, pointing at the screen. "I will tell you in some detail ofsome ofour findings on one ofthe planets, which I will call planet E, and which is part of a system of planets revolving around a medium-sized star. Next slide, please. This photograph gives us a closer look at planet E. As you see, it has only one moon. It has a radiation belt around it. Now, we became interested in planet E because it happens to possess an oxygen-rich atmosphere , a phenomenon which, as you know, is extremely rare. We therefore decided to pay it a visit." Charles paused and glanced down at his wife Emma, who was sitting in the front row. She gave him an encouraging nod. "One thing which struck us immediately," Charles continued, "was that there were lots of macromolecules on the surface of planet E. The funny thing was that most ofthese molecules formed aggregates and that these aggregates maintained a primitive form ofhomeostasis. There was a fierce competition between the aggregates in a struggle for survival, and in this struggle millions were butchered every second. The losses were replaced soon enough, however, because the remaining aggregates multiplied rapidly." Charles wiped his brow. It was hot in the lecture room. Perhaps the nitrogen conditioner wasn't working properly. * Antoni van Leeuwenhoek-Huis, Netherlands Cancer Institute, Amsterdam, The Netherlands. 325 "Naturally we wondered how these aggregates reproduced. We did some studies on this, and it appeared that the aggregates possessed a chemical substance which was able to direct the growth ofthese aggregates and of their offspring. I extracted some of this material and gave it to my technician, Miss Cherry Gaff, for analysis. She put it in the diffraction analyzer. 'Cricky!' she suddenly shouted. 'What's on?' I inquired. 'It's DNA!' she screamed excitedly, 'and the A's equal the T's, while the Cs equal the G's!' "I quickly looked at the photographs. One glance was enough. Miss Gaffwas right. The aggregates of the macromolecules on planet E contained complementary chains of deoxyribonucleotides. This DNA was used for their replication." There were gasps ofdisbelieffrom the audience. Several scientists held up their hands to ask a question, but Charles was already continuing his talk. "Miss Gaffand I analyzed DNA samples taken from thousands of different types of aggregates existent on planet E. We found that there were differences in nucleotide sequences between these DNA's, and then we made the starding discovery that we could arrange the DNA's in a graph. Last slide, please. Thank you. It shows that the DNA's of some aggregates on planet E were closely related, while others were vastly different. Our data can be explained only by assuming that one type of DNA was the ancestor of the other." Charles paused for effect. His next words seemed to drop like stones into a placid pool. "What I am about to say will perhaps seem blasphemous to some of the distinguished members of this audience. Our results suggest that chance mutations in the DNA of certain species on planet E may have led to the creation ofa variety ofnew species. The struggle for existence may have eliminated the molecular aggregates carrying the wrong type of DNA." There was a stunned silence as the implication of these words slowly dawned on the scientists. Then one ofthem, a highly esteemed biologist named Sam Wilberforce, slowly arose. He glared with hate-filled eyes at Charles. "Do you, sir," he thundered, "mean to imply that our DNA, the DNA ofthis distinguished gathering, which we all know to be immutable , is descended by chance from that ofa lowly molecular aggregate?" 326 M. Sluyser · Origin ofDNA Perspectives in Biology and Medicine · Winter 1969...