![]() Scientists studying the origins of DNA are confronted with a paradox. “The important issue is whether or not it is possible to make TNA using potentially prebiotic chemistry. Leslie Orgel of the Salk Institute for Biological Studies. “Since the direct evidence has disappeared, it will require an inventive chemist to construct a persuasive scenario,” says Dr. Because the conditions of a primitive Earth were so different – little atmospheric oxygen, high ultraviolet radiation, possibly higher temperatures and volcanic activity – chemicals may have combined in very different ways than they do in today’s environment. The fact that TNA is currently synthetic doesn’t exclude the possibility that it could have formed on early Earth. “The reason for synthesizing and studying it,” Eschenmoser explains, is “to screen the structural neighborhood of RNA for potential nucleic-acid alternatives that could also have fulfilled the function of a genetic system.” Indeed, says Eschenmoser, “talking about TNA as a a possible ancestor of RNA is actually premature.”īut scientists can examine the basic properties of TNA and determine whether they could have formed in a prebiotic-Earth environment. Since we can’t go back in time to witness the evolution of nucleic acids, we will never be able to prove whether natural TNA made an appearance on Earth. ![]() Scientists have to create it in the lab in order to study it. It is also the fact that, unlike ribose, “the simplest formation of threose requires only a single type of starting material.” “But it is not only the number of the carbon atoms that makes threose an intrinsically simpler molecule than ribose,” Eschenmoser says. Under nonbiological conditions, threose forms easily than ribose. In TNA that backbone is composed of sugar molecules – threose – that contain only four carbon atoms (Figure 3). The backbone of DNA and RNA is composed of sugar molecules – ribose for RNA and deoxyribose for DNA – that contain five carbon atoms. Keeping the image of the double helix in mind, the bases form the steps of a spiral staircase while the sugar-phosphate backbones form the railing (Figure 1). In RNA, the T is replaced by a U – uracil. The sugar-phosphate backbone of DNA provides the structural support for a coded sequence of information-bearing molecules – the bases adenine, thymine, cytosine and guanine (A, T, C and G, for short). ![]() Part of this simplicity stems from the structure of its sugar-phosphate backbone. TNA seems to be very similar to RNA in some regards, but overall it is a simpler molecule. They study the properties of the alternatives, such as TNA, and compare them with corresponding properties of RNA. To investigate potential RNA precursors, the scientists have been creating nucleic acids that are structurally similar to RNA. ![]() A second requirement is that it should be a simpler molecule than RNA.Īccording to Eschenmoser, the synthesis of TNA is part of a comprehensive decade-long project aimed at understanding the origin of RNA. This ability is thought to be one of the requirements of any system that would be considered a possible ancestor of RNA. The TNA strands can also pair up with complementary strands of RNA and DNA. They found that complementary TNA strands can form stable double helices. Albert Eschenmoser and his colleagues at the Scripps Research Institute in La Jolla, California, and the Federal Institute of Technology in Zurich, Switzerland, chemically synthesized TNA in a number of steps. ![]() So what is the ancestor of RNA? One recent report suggests that it may have been yet another nucleic acid called (L)-a-threofuranosyl oligonucleotides, also known as TNA.ĭr. But while RNA is slightly simpler than DNA, it too is very complex. Scientists have put forth the theory that RNA – ribonucleic acid (Figure 2) – was the predecessor to DNA and evolved into that more-complex molecule. It could not have appeared spontaneously it must have evolved from a simpler form. But DNA – deoxyribonucleic acid – is highly complex. We all know that DNA (Figure 1) makes up the building blocks for life on Earth. What did it develop from? Astrobiologists examine possible ancestors of DNA: nucleic acids called PNA, p- RNA, and TNA. But it is a highly complex molecule, and could not have arranged itself spontaneously. DNA is the building block for life on Earth. ![]()
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