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Before DNA, before RNA: Life in the hodge-podge world
Gerhard Schulz/osf/getty
THIS WEEK
Life’s origins in the hodge-podge world backbone has a key advantage over those of RNA and DNA, says John Chaput of Arizona State University in Tempe. Where RNA uses ribose and DNA deoxyribose, TNA uses threose. That makes it the smallest molecule of the three, and could mean it forms more easily. To see if TNA could have done the work of RNA, Chaput and his colleagues took a variety of TNAs and evolved them in the presence of a protein. After three generations, a TNA turned up that could bind to the protein and had a complex, folded threedimensional shape like an enzyme (Nature Chemistry, DOI: 10.1038/nchem.1241). These are features needed to make a TNA
Michael Marshall
Take note, DNA and RNA: it’s not all about you. Life on Earth may have begun with a splash of TNA, a different kind of genetic molecule altogether. That is not to say that TNA will dethrone RNA entirely as the original molecule of life. Instead, it looks like the very first life forms may have used a mix of these and other nucleic acids for genetic material. Bar some viruses, most life forms around today use DNA to store genetic information and RNA to execute the instructions encoded by that DNA. However, researchers studying the origins of life have long thought that RNA was the first genetic material. “It looks like the very first According to this “RNA world” life forms may have used hypothesis, the earliest forms of life used RNA for everything, with a mix of TNA, RNA and other nucleic acids” little or no help from DNA. A key piece of evidence for the RNA world (New Scientist, 13 August enzyme that, like RNA, could 2011, p 32) is that the molecule is a control a chemical reaction. jack of all trades. It can both store Chaput thinks TNA probably genetic information and act as an was not the original genetic enzyme. Now it seems that TNA – material, though – if only because a chemical relative of RNA – can the chemistry of early Earth was perform one of RNA’s critical so messy that TNA would not have actions, suggesting it might have arisen on its own. Rather, many been just as capable. different molecules probably Whatever happened at the formed in a genetic hodge-podge. dawn of life, TNA is not found in “The most likely scenario is that nature today. However, its sugar nature sampled lots of different
–Anything goes in TNA world–
things,” says Chaput. That scenario is in line with a recent study by Nobel prizewinner Jack Szostak of Harvard University and colleagues. The team created mosaic nucleic acids that were half DNA, half RNA. Like Chaput’s TNA, some of these could bind to target molecules (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1107113108). However, there are problems
Candidates for life’s first molecules DNA and RNA are not the only genetic molecules out there. Others have advantages that suggest they too might have helped get life started billions of years ago BACKBONE
SEEN IN NATURE
POINTS IN FAVOUR
RNA
Ribose (sugar)
✓
Can store genetic information and execute the orders in that information
DNA
Deoxyribose (sugar)
✓
Found in all life forms on Earth; embodies the genetic code of life
TNA
Threose (sugar)
✘
May act as an enzyme to execute the genetic code; simple, stable structure
PNA
Peptide
✘
Forms a double helix like DNA (stable); long PNA molecules can be made under conditions similar to prebiotic Earth, even at temperatures of 100 °C
GNA
Glycerol
✘
Forms a double helix like DNA (stable); has the simplest structure of all
10 | NewScientist | 14 January 2012
with the hodge-podge world hypothesis. For one thing, there is no trace of TNA or its synthetic cousins (see table) in modern organisms. By contrast, RNA, like DNA, is essential to life today and there is evidence to suggest that it came about before proteins. For another, although TNA looks simpler than RNA, we can’t be sure it was easier to make some 4 billion years ago. As John Sutherland of the MRC Laboratory of Molecular Biology in Cambridge, UK, points out, no one has actually made it under the conditions that existed on Earth before life began. “Looks might be deceptive,” he says. We still know very little about what TNA can do, says Chaput, because the technology to evolve the molecules in the lab is so new. The research, he says, is just getting going. n
THIS WEEK
Life’s origins in the hodge-podge world backbone has a key advantage over those of RNA and DNA, says John Chaput of Arizona State University in Tempe. Where RNA uses ribose and DNA deoxyribose, TNA uses threose. That makes it the smallest molecule of the three, and could mean it forms more easily. To see if TNA could have done the work of RNA, Chaput and his colleagues took a variety of TNAs and evolved them in the presence of a protein. After three generations, a TNA turned up that could bind to the protein and had a complex, folded threedimensional shape like an enzyme (Nature Chemistry, DOI: 10.1038/nchem.1241). These are features needed to make a TNA
Michael Marshall
Take note, DNA and RNA: it’s not all about you. Life on Earth may have begun with a splash of TNA, a different kind of genetic molecule altogether. That is not to say that TNA will dethrone RNA entirely as the original molecule of life. Instead, it looks like the very first life forms may have used a mix of these and other nucleic acids for genetic material. Bar some viruses, most life forms around today use DNA to store genetic information and RNA to execute the instructions encoded by that DNA. However, researchers studying the origins of life have long thought that RNA was the first genetic material. “It looks like the very first According to this “RNA world” life forms may have used hypothesis, the earliest forms of life used RNA for everything, with a mix of TNA, RNA and other nucleic acids” little or no help from DNA. A key piece of evidence for the RNA world (New Scientist, 13 August enzyme that, like RNA, could 2011, p 32) is that the molecule is a control a chemical reaction. jack of all trades. It can both store Chaput thinks TNA probably genetic information and act as an was not the original genetic enzyme. Now it seems that TNA – material, though – if only because a chemical relative of RNA – can the chemistry of early Earth was perform one of RNA’s critical so messy that TNA would not have actions, suggesting it might have arisen on its own. Rather, many been just as capable. different molecules probably Whatever happened at the formed in a genetic hodge-podge. dawn of life, TNA is not found in “The most likely scenario is that nature today. However, its sugar nature sampled lots of different
–Anything goes in TNA world–
things,” says Chaput. That scenario is in line with a recent study by Nobel prizewinner Jack Szostak of Harvard University and colleagues. The team created mosaic nucleic acids that were half DNA, half RNA. Like Chaput’s TNA, some of these could bind to target molecules (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1107113108). However, there are problems
Candidates for life’s first molecules DNA and RNA are not the only genetic molecules out there. Others have advantages that suggest they too might have helped get life started billions of years ago BACKBONE
SEEN IN NATURE
POINTS IN FAVOUR
RNA
Ribose (sugar)
✓
Can store genetic information and execute the orders in that information
DNA
Deoxyribose (sugar)
✓
Found in all life forms on Earth; embodies the genetic code of life
TNA
Threose (sugar)
✘
May act as an enzyme to execute the genetic code; simple, stable structure
PNA
Peptide
✘
Forms a double helix like DNA (stable); long PNA molecules can be made under conditions similar to prebiotic Earth, even at temperatures of 100 °C
GNA
Glycerol
✘
Forms a double helix like DNA (stable); has the simplest structure of all
10 | NewScientist | 14 January 2012
with the hodge-podge world hypothesis. For one thing, there is no trace of TNA or its synthetic cousins (see table) in modern organisms. By contrast, RNA, like DNA, is essential to life today and there is evidence to suggest that it came about before proteins. For another, although TNA looks simpler than RNA, we can’t be sure it was easier to make some 4 billion years ago. As John Sutherland of the MRC Laboratory of Molecular Biology in Cambridge, UK, points out, no one has actually made it under the conditions that existed on Earth before life began. “Looks might be deceptive,” he says. We still know very little about what TNA can do, says Chaput, because the technology to evolve the molecules in the lab is so new. The research, he says, is just getting going. n