Threose nucleic acid
Threose nucleic acid (TNA) is an artificial genetic polymer invented by Albert Eschenmoser. TNA has a backbone structure composed of repeating threose sugars linked together by phosphodiester bonds. Like DNA and RNA, TNA can store genetic information in strings of nucleotide sequences (G, A, C, and T). TNA is not known to occur naturally and is synthesized chemically in the laboratory under controlled conditions.
TNA can self-assemble by Watson-Crick pair bonding into duplex structures that closely approximate the helical geometry of A-form RNA.  TNA can also base pair opposite complementary strands of DNA and RNA, which makes it possible to share information with natural genetic polymers. This capability and chemical simplicity suggests that TNA could have preceded RNA as a genetic material.
Polymerases have been identified that can replicate TNA polymers in the laboratory. TNA replication occurs through a process that mimics RNA replication. In these systems, TNA is reverse transcribed into DNA, the DNA is amplified by PCR, and then forward transcribed back into TNA.
TNA replication coupled with in vitro selection has produced a TNA aptamer that binds to human thrombin. This example demonstrates that TNA is capable of heredity and evolution, which is a hallmark of life.
TNA has generated great interest in Synthetic Biology because TNA polymers are resistant to nuclease degradation. This property, coupled with its ability to undergo Darwinian evolution in a test-tube, provide a possible path to biologically stable molecules with relevance in material science and molecular medicine.
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