Mary Lou was especially excited this evening at 6:30 as she drove with her parents to the college science-fair presentations. Not only was she told that her group had made a rather momentous discovery regarding nucleic acid synthesis, but - and this was even better - her parents were to meet Jimmy's parents for the first time. But this meeting was almost called off because Mary Lou's mother was to meet her long-lost sister at 8:30 that evening. This science fair thing had made this evening's scheduling very tight.

    Her brother Jason and his raggamuffin freshman friend Wilbie were very pleased to be helping Cheryl put up their poster, but it was dangerous work for the guys as they stuck themselves with tacks several times as they were entranced by being so close to their smart and beautiful classmate. This is what their poster was all about:

    Wilbie had gotten Dr. Sørensen over at Syntek to help them radioactively label the E. coli cells in their cultures with various radioisotopes of elements common to DNA - namely, tritium (₃H) and C-14 (₁₄C) both separately incorporated in thymidine, as well as radio-phosphorous (₃₂P) fed as phosphate - all in something like nutrient broth. They wanted to do an initial "Goldie Locks" experiment because these three radio-isotopes emit radiation at greatly different energies: some might be too weak, others just right, and some too strong. They wanted to find something not too hot or too cold - but just right for their needs. But first they had a brief lecture on beta-radiation and how the nucleus of the element was transmutated into that of one higher on the periodic table without losing either mass or momentum. Finally, these three cultures were grown in these radioactive media overnight and then drops were diluted and fixed on slides in preparation to do "autoradiography," where the radiation exposes photographic material. When the smudges were dry, the slides were taken into the dark room and a photographic emulsion (like that on unexposed film) was spread as thin layers over the smudges, and these were allowed to dry in the dark. The slides were then put into a light-tight box and stored over a weekend so as to allow some of the radio-isotopes to "decay" and emit their high-energy β-particles to "expose" some of the silver bromide molecules in the photographic emulsions. Upon returning on Monday, the slides were photographically developed, and inspected under the microscope with the results as shown here.

    Upon very high magnification they saw that the weak betas from tritium barely left detectible trails, while those of ₃₂P inundated the view with streaks rendering the view a meaningless hodge-podge web of streaks. Only with C-14 were nice points of nuclear decay seen to be countable. Thus C-14 became their radioisotope of choice.

    Starting again using only the C-14-thymidine, they grew a culture many generations so as to obtain uniform incorporation of the radioisotope. Then they "washed" the cells free of excess ₁₄C-thymidine by centrifuging them out of their old medium and resuspending the cells in new broth (with no radioisotopes but with a lot of non-radioactive C-12-thymidine). Thus any newly synthesized cell material would not be radioactive. They then followed the optical density of the culture as it continued to grow. Each time the culture had doubled (about each 23 minutes), they took a drop of the culture and put it on a microscope slide, and subjected it in the darkroom to overlaying with emulsion and storing it away for a few days.

    Once again, upon returning afer a few days, the slides were photographically developed, and inspected under the microscope. What they saw in the "zero-time" slide was that each cell had about 80 very small black dots of silver grains bunched together. After one generation, each cell had about 40 bunched dots. After two generations, the "AUTORADIOGRAPH" showed about half of the cells with 40 bunched dots, and the remaining cells with no dots. Thereafter, as the generations progressed, there were a few cells with 40 dots, and an ever-growing number of undotted cells. All this is shown in the figure below.

    While Cheryl, Wilbie and Jason were explaining all this to the crowd, Jimmy brought his parents over to meet his fiancee's parents, before taking them to the poster. As Jason stood gazing at talkative Cheryl, Wilbie nudged him to remind him that she was so much older than he was. "Maybe she is now - proportionately," Jason whispered back. "But I'm catching up with her," he said with a satisfied smile. "Now I am only 80% as old as she is, but in four years I'll be nearly 84% as old." Here's what the crowd was excited about:

When Jimmy's mother saw Mary Lou's mother, she stopped, tilted her head, and was about to say something when Mary Lou's mother screeched: "Sis, it's been so long!" Hugs and tears were aplenty.