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Study suggests how DNA building block might have formed

Nov. 2, 2007
Courtesy University of Georgia
and World Science staff

Many ex­pe­ri­ments have shown it: sim­ple mo­le­cules can com­bine chem­ic­ally—out­side of liv­ing things—to form the build­ing blocks of DNA, the key com­po­nent of life. But just how this com­bina­t­ion oc­curs is un­known. Sci­en­tists want to find out, since that might ex­plain how DNA orig­i­nat­ed.

The basic structure of DNA. (Courtesy U.S. Nat'l Library of Medicine)
Now, chem­ists have pro­posed what they call the first de­tailed, fea­si­ble ac­count of how one of DNA’s ma­jor build­ing blocks could have aris­en on an ear­ly, life­less Earth. The nec­es­sary in­gre­di­ents: five cy­a­nide mo­le­cules, they said.

Where “bio­mole­cules,” such as DNA’s com­po­nents, ori­g­i­nat­ed is­n’t known, said Un­ivers­ity of Geor­gia chem­ist Paul von Ra­gué Schle­yer, one of the re­search­ers. 

“One can only spec­u­late. They could have formed from smal­ler mo­le­cules pre­s­ent on prim­i­tive Earth, ei­ther very slowly over mil­lions of years or rap­idly be­fore the Earth cooled down. As­ter­oids may have brought them from out­er space,” he added, thought this does­n’t ex­plain how they would have formed there.

DNA is life’s mo­lec­u­lar blue­print, passed from gen­er­a­t­ion to gen­er­a­t­ion. First iso­lat­ed in 1869 by a Swiss doc­tor from pus in dis­carded ban­dages, DNA’s struc­ture was dis­cov­ered in 1953. It’s shaped some­what like a twisted lad­der with rungs an­chored by in­ter­lock­ing pairs of two out of four mo­le­cules, known as nu­cleic ac­id bas­es. The four are ad­e­nine, gua­nine, cy­to­sine and thy­mine.

Schley­er’s team fo­cused on ad­e­nine be­cause of its prev­a­lence and abil­ity to form from sim­ple com­po­nents in the dark. Along with oth­er build­ing blocks of life, ad­e­nine has even been de­tected in out­er space, though there, the great dis­tances among its smal­l­er mo­lec­u­lar in­gre­di­ents make its emer­gence trick­i­er to ex­plain.

But many ex­pe­ri­ments have shown that sim­u­lated prim­i­tive Earth con­di­tions can lead to the forma­t­ion of es­sen­tial com­pounds of life in­clud­ing ami­no ac­ids, nu­cleotides and car­bo­hy­drates, the re­search­ers wrote in their stu­dy. The work was pub­lished Oct. 30 in the sci­ent­i­fic jour­nal Pro­ceed­ings of the Na­t­ional Academies of Sci­ence.

Re­mark­ably, they said, adenine has been found to arise from highly poi­son­ous cy­a­nide dis­solved in am­mo­nia and fro­zen in a re­frig­er­a­tor for 25 years. A high-temp­er­a­ture ex­pe­ri­ment de­signed to sim­u­late early volc­ano-like en­vi­ron­ments also pro­duced ade­nine. But the ques­tion is how.

Schley­er’s team de­vised an an­swer by solv­ing a se­ries of key rid­dles. They worked out pro­cesses in which five cy­a­nide mo­le­cules might com­bine to make ad­e­nine un­der ter­res­tri­al con­di­tions. The pro­pos­al was based on computer-assisted stud­ies that in­volved quan­tum me­chan­ics, the some­times illogical-seeming rules that go­vern atom­ic in­ter­ac­tions.

The re­search­ers said the re­port pro­vides a more de­tailed un­der­stand­ing of some of the pro­cesses of “chem­ical evo­lu­tion,” and a par­tial an­swer to the basic ques­tion of how life’s chem­istry emerged. The in­ves­ti­ga­t­ion should trig­ger si­m­i­lar probes into the ori­gins of the three re­main­ing bas­es and of oth­er bi­o­log­ic­ally rel­e­vant mol­e­cules, they added.

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Many experiments have shown it: simple molecules can combine chemically—outside of living things—to form the building blocks of DNA, the key component of all life forms. But just how this combination occurs is unknown. Scientists want to find out, since that might explain how DNA originated and made life possible. Now, chemists have proposed what they call the first detailed, feasible account of how one of DNA’s major building blocks could have arisen on an early, lifeless Earth. The necessary ingredients: five cyanide molecules, they said. Just where “biomolecules,” such as components of DNA, originated isn’t known, said University of Georgia chemist Paul von Ragué Schleyer, one of the researchers. “One can only speculate. They could have formed from smaller molecules present on primitive Earth, either very slowly over millions of years or rapidly before the Earth cooled down. Asteroids may have brought them from outer space,” he added, thought this doesn’t explain how they formed in space. DNA is life’s molecular blueprint, passed on from generation to generation. First isolated in 1869 by a Swiss doctor from pus in discarded bandages, DNA’s structure was solved in 1953. It’s shaped somewhat like a twisted ladder with rungs anchored by matching pairs of two of four molecules, known as nucleic acid bases: adenine, guanine, cytosine and thymine. Schleyer’s team focused on adenine because of its prevalence and ability to form from simple components in the dark. Along with other building blocks of life, adenine has even been detected in outer space, though there, the great distances among its smaller molecular ingredients make its emergence trickier to explain. But many experiments have shown that similated primitive Earth conditions can lead to the formation of essential compounds of life including amino acids, nucleotides and carbohydrates, the researchers wrote in their study, published Oct. 30 in the research journal Proceedings of the National Academies of Science. Remarkably, they said, highly poisonous cyanide dissolved in ammonia, and frozen in a refrigerator for 25 years, has produced adenine. So has a high-temperature experiment designed to simulate early volcano-like environments. But the question is how. Schleyer’s team devised an answer by solving a series of key riddles. They worked out processes in which five cyanide molecules might combine to form adenine under terrestrial conditions. The proposal was based on computer-assisted studies that involved quantum mechanics, the sometimes illogical-seeming rules that govern atomic interactions. The researchers said the report provides a more detailed understanding of some of the processes involved in “chemical evolution,” and a partial answer to the fundamental question of how the chemistry of life emerged. The investigation should trigger similar investigations of the primordial formation of the three remaining bases, as well as other biologically relevant molecules, they added.