RNA likely came first. Evidence suggests RNA predates DNA in evolutionary history.
Evidence Supporting RNA as the Precursor to DNA
Several lines of evidence point to RNA's primacy over DNA:
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Simpler Structure: RNA is structurally simpler than DNA. It has a single strand and uses ribose sugar, whereas DNA has a double helix structure and deoxyribose sugar. This simplicity suggests it could have formed more easily in early Earth conditions.
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Catalytic Activity: RNA can act as both a carrier of genetic information and a catalyst (ribozymes). This dual functionality is crucial in a world where proteins (enzymes) might not have been as readily available. DNA, on the other hand, primarily serves as a storage molecule and lacks catalytic activity.
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Role in Fundamental Processes: RNA plays a central role in essential cellular processes, such as protein synthesis (mRNA, tRNA, rRNA). These processes are fundamental to life, suggesting that RNA was involved early on.
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Chemical Differences: Chemical differences between RNA and DNA, like the presence of uracil in RNA instead of thymine in DNA, and the 2' hydroxyl group on ribose, provide clues that DNA evolved from RNA. The enzymatic conversion of ribonucleotides to deoxyribonucleotides likely occurred later in evolution.
The "RNA World" Hypothesis
The idea that RNA came first is the basis of the "RNA world" hypothesis. This hypothesis proposes that early life was based on RNA, which handled both information storage and catalysis. Over time, DNA evolved to become the primary storage molecule due to its greater stability, and proteins became the dominant catalytic molecules because of their wider range of chemical functionalities.
Summary
While it's impossible to definitively prove which molecule came first, the current evidence strongly supports the RNA world hypothesis. The simpler structure, catalytic ability, and critical roles in cellular processes suggest that RNA predates DNA in the history of life.
