WONDER OF THE MOMENT

         Since the 1960’s, we have discovered a lot about the evolution of cells.

         Fossil evidence indicated that bacteria had not only been the first living creatures, but they had had the earth to themselves for two billion years. Bacteria are single-celled organisms. Each one carries its genes, made of DNA, in a ring-shaped chromosome folded up in a special region of the cell. Smaller rings of genes, called plasmids, sometimes accompany this chromosome.

         Over two billion years, plenty of mutations took place in bacterial genes, resulting in vast numbers of different bacterial species. Also, being single-celled, bacteria were, and are, capable of picking up chromosome fragments from one another, introducing even more new species.

         About a billion and a half years ago, a new type of organism appeared in the fossil record. Like bacteria, they consisted of single cells. But unlike bacteria, these cells carried their chromosomes enclosed within a special membrane. These membrane-enclosed chromosomes formed a “nucleus” in the new cell type. To distinguish bacteria from the new cells, biologists call bacteria “prokaryotic,” meaning “before the nucleus;” and they called nucleated cells “eukaryotic,” meaning “true nucleus.” Besides the nucleus, the new eukaryotic cells contained a number of infinitesimal organs, called “organelles.” Some of these organelles were photosynthetic and made sugar from light energy. Some did the opposite, extracting energy from sugar to run cell processes.

         Over the next billion and a half years, mutations and gene trading resulted in vast numbers of new eukaryotic species. In some cases, eukaryotic cells joined into multicellular species, such as plants, animals and fungi.

         As François Jacob famously wrote, evolution acts like a tinkerer. Old devices and mechanisms get put to new uses. So it was unlikely that eukaryotic cells had sprung up on their own. It was much more likely that they had somehow evolved out of prokaryotic cells.

         In 1967, Lynn Margulis at Boston University suggested that the first eukaryotic cell could actually have been a group of prokaryotic cells that began living together. In fact, she found that the photosynthetic organelles, called “chloroplasts,” are quite similar to certain photosynthetic bacteria. She also found that the energy-harvesting organelles, called “mitochondria,” are quite similar to certain oxygen-using bacteria. And it turned out that chloroplasts and mitochondria have their own genes, exactly as we might expect, if they were actually bacteria that just happened to be living inside another cell. Margulis’ idea is called the “endosymbiont hypothesis” or the “endosymbiont theory.” It is the beginning of some interesting stories about cell evolution. Stay tuned!

I am amazed at the inventiveness of early researchers in pursuit of the secrets of heredity.  With no idea of the true chemistry of genes, investigators designed experiments to reveal genetic facts.

          Geneticists of the 1930’s and ‘40’s believed, incorrectly, that genes must be made of protein.  Yet during this time, George Beadle made his “one gene—one enzyme” discovery.  The discovery came about because Beadle wondered what genes actually do in order to cause traits. 

          First Beadle investigated fruit fly eye colors.  Normal eye color in these flies is a deep-red mixture of red and brown pigments.  Two bright-red mutant colors are vermilion and cinnabar; these contain no brown.  From mutant larvae, Beadle transplanted vermilion and cinnabar eye discs into normal larvae.  The eyes developed normal color in the adult flies.  Cross-mutant transplantation showed Beadle that the brown pigment resulted from a series of chemical reactions: a starting substance got changed to vermilion, which got changed to cinnabar, which got changed to brown.  Each mutant lacked one of these reactions, but it was time-consuming to figure out which one. 

          To speed up his investigation, Beadle switched from fruit flies and eye colors, to red bread mold  Beadle X-rayed the bread mold to cause mutations, then tested spores from the mold to see if they could grow on a minimal food containing only sugars and salts, and the nutrients it manufactured.

          If a mold couldn’t develop on the minimal food, this meant it was missing a nutrient because of a mutation.  Beadle tested to see if the mutation was in a single gene.  If so, Beadle then added a supplement, such as a vitamin or an amino acid.  If that didn’t make the mold grow, he tried a different supplement, until he found the missing nutrient.

          During growth, each mutant mold accumulated a chemical.  This chemical came from the reaction step where the mutant got stuck.  The mutants could be arranged in order, according to where they got stuck, and this order showed the reaction steps in the manufacture of the supplement nutrient.

          Beadle knew that each chemical reaction is controlled by an enzyme.  So each mutant mold must be missing the enzyme that could change its accumulated chemical to the next one in the series.  Since each mutant was missing a single gene, each of those genes must give rise to a single enzyme.  “One gene—one enzyme!”