The beauty of the DNA/RNA system is that it solves many functions by the one phenomenon of base pairing. The double-stranded nature of DNA and its packaging make it a stable molecule resistant to damage. When a cell divides, the entire complement of DNA must be replicated so that each daughter cell receives a complete copy. To achieve this, the two strands, call them A and B, separate and are matched nucleotide-by-nucleotide by a new concatenation of nucleotides. This generates two new double helices,AB and BA (and these are chemically identical—the order we write them in is strictly arbitrary). Uncorrected copying errors (mutations) can occur in the replication process, which is vital to evolution, but basically the DNA is preserved for posterity.
Meanwhile, RNA is the local, contextual, temporary tool for making polypep-tides. A given gene can be transcribed many times to make for abundant protein, but the mRNA itself is not replicated and is eventually degraded. RNA transcription is based on base complementarity and so is the tRNA anticodon system. Thus all these functions are based on the same base-complementarity system. While this is general and applies between strands, the linear sequence along strands is used to carry the specific coding information.
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