Larralde calculates that its half-life in a room-temperature primordial soup would have been less than a year-which wouldn’t have allowed it much time to bind with bases and phosphates and form RNA.Įven with plenty of time, ribose would have trouble hooking up with phosphate. At the boiling point, ribose had a half-life of only 73 minutes. She put ribose in a variety of solutions designed to span the gamut of possible primordial soups, heated them up, and measured how much ribose remained as time passed. That’s not an encouraging percentage, but Larralde has discovered something far more disheartening. A fraction of 1 percent of those rings are ribose. Chains of formaldehyde molecules spontaneously curl into rings, forming various sugars. On the early Earth, ribose probably formed out of formaldehyde, which is a single carbon atom bonded to a water molecule it has been found not only in Miller’s spark chambers but also in comets, which in Earth’s youth were constantly pelting it. Rosa Larralde, another student of Miller’s (now at Harvard), took a close look at ribose. The backbone, as recent experiments have helped show, is the Achilles’ heel of RNA. The sugars alternate with phosphate molecules, each of which consists of a phosphorus atom surrounded by four oxygen atoms two of those oxygens bind to sugar carbons on either side, thus linking two sugars together. Every base is attached to a sugar molecule called ribose (or deoxyribose, in the case of DNA). The backbone is made of two repeating building blocks. DNA and RNA require not only bases but a backbone on which to hang them. That lagoon, then, might have given rise to all four RNA bases. If you had urea in the ocean and seawater evaporated in a lagoon, you’d get a very high concentration. The secret, they found, was to load up their flasks with urea, a carbon-nitrogen compound likely to have formed in the early ocean. This spring Miller and one of his graduate students, Michael Robertson, finally finished the job: they discovered reactions that could efficiently produce copious amounts of the other two bases, cytosine and uracil. In the 1960s researchers were able to synthesize two of RNA’s bases-adenine and guanine-from precursor molecules that are likely to have been present on the early Earth. Less fragile than RNA and thus more secure as a storehouse of genetic information, DNA ultimately grabbed control of life, demoting RNA to the rank of errand boy.īut could RNA itself have formed spontaneously and abundantly in the primordial soup? From a genetic point of view the key components of both RNA and DNA are the four bases that make up the genetic alphabet. The picture of the primordial soup as an RNA world has now become conventional wisdom only later, in this view, did organisms evolve DNA. That immediately made it a strong candidate for the first biomolecule. But in the early 1980s, researchers discovered that RNA could catalyze some chemical reactions-acting like a protein as well as an information carrier. Each needed the other to exist, and so, like chicken and egg, neither could come first. DNA and RNA were only information carriers, and proteins could only do chemical chores. Researchers believed that these building blocks might have formed into the first genes.ĭespite Miller’s work and other work that flowed from it, a messy paradox remained: the division of labor in the cell was too clean. The energy caused the molecules to combine into many simple organic compounds. Miller is famous for a 1953 experiment in which he mixed up atmospheric gases and water in a flask and put a spark to it, thus simulating lightning ripping through primeval skies. Some fascinating hints are now emerging, many of them from the lab of chemist Stanley Miller at the University of California at San Diego. Might the first organisms instead have used some kind of pre-NA? This system, so universal and uniform, poses a puzzle: where did it all come from? Complex as it is, it is hard to see how it could have sprung full-blown from the primordial soup. Every cell of every organism, with the exception of a few viruses, encodes genetic information in DNA DNA dispatches a single strand of RNA to make proteins and the proteins do all the cellular grunt work. Underneath life’s variety, from mites to mastodons, there’s a profound sameness.
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