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In page RNA world:

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The hypothesized existence of an RNA world does not exclude a "Pre-RNA world", where a metabolic system based on a different nucleic acid is proposed to pre-date RNA. A candidate nucleic acid is peptide nucleic acid (PNA), which uses simple peptide bonds to link nucleobases.[1] PNA is more stable than RNA, but its ability to be generated under prebiological conditions has yet to be demonstrated experimentally.[citation needed]