= Solution
<DNA> is a directional polymer with a sugar-phosphate backbone linked by <Phosphodiester bond>[phosphodiester bonds]. Its <deoxyribose> sugars carry <adenine>, <thymine>, <guanine> or <cytosine>. In the usual cellular duplex, antiparallel strands form a double helix with complementary A–T and G–C pairing, stabilized by <base stacking> as well as <hydrogen bonding>. The sequence can vary without changing the basic backbone, allowing a large information capacity. Complementarity provides a template for copying and repair; the two strands are not merely two redundant lists stored without a copying mechanism. Major and <Minor groove>[minor grooves] permit <Protein>[proteins] to recognize sequence-dependent chemical patterns. Eukaryotic <Chromosome>[chromosomes] package <DNA> around <Histone>[histones], while bacterial <Chromosome>[chromosomes] are usually supercoiled and associated with other <Protein>[proteins]; not all cellular <DNA> is nuclear or linear.
<RNA> uses <ribose> with a reactive 2-prime hydroxyl and usually <uracil> in place of <thymine>. It is often a single chain, but intramolecular complementary segments form stems, loops and elaborate three-dimensional folds. <Messenger RNA> conveys coding information to <Ribosome>[ribosomes]; <transfer RNA> combines an <anticodon> with an amino-acid attachment site; <ribosomal RNA> has structural and catalytic roles in <translation (geometry)>[translation]. Regulatory and catalytic <RNA>[RNAs] show why it is inadequate to describe <RNA> solely as a transient <DNA> copy. <RNA>'s local pairing and folding let sequence encode both information and molecular machinery. <DNA>'s chemical stability and duplex repair make it especially suitable for long-term inheritance.
<Replication> is highly faithful but imperfect. Heritable sequence changes, <recombination (cosmology)>[recombination] and <gene duplication> create variation, and selection, <Genetic drift> and population history determine its spread. Thus the same chemistry explains both continuity of lineages and their divergence; <mutation> is not directed toward what an organism needs. Conserved genes, homologous sequences and the widely shared <genetic code> support common ancestry. <RNA> catalysis makes an early RNA-based stage of <evolution> a plausible hypothesis, not a direct historical observation or a deduction that all modern <RNA> preceded <DNA> in a demonstrated sequence. Structure constrains function and evolutionary possibilities without uniquely determining a species' history.<Molecular Biology of the Cell, 4th edition by Bruce Alberts and others>[Chapter 4, The Structure and Function of DNA; Chapter 6, RNA structure and translation]{ref}
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