= Solution
<Nucleic acid>[Nucleic acids] are directional sugar-phosphate polymers whose base sequences store information. Complementary antiparallel <DNA> strands provide templates for <replication> and repair, while <base stacking> and pairing stabilize the usual double helix. <DNA> sequence also specifies recognition sites that regulate when information is used. <RNA> generally has <ribose> and <uracil> and can fold through internal pairing into functional structures: <messenger RNA> carries coding information, <transfer RNA> links <Codon>[codons] with <Amino acid>[amino acids], and <ribosomal RNA> contributes catalytic machinery. Not all <RNA> is a passive messenger.
<Protein>[Proteins] have peptide-linked amino-acid sequences with chemically diverse <Side chain>[side chains]. Their primary sequence constrains secondary structures, tertiary folding and multi-subunit assembly, producing binding pockets, catalytic sites, structural fibers, channels and <Molecular motor>[molecular motors]. Function depends on conformational dynamics, chemical context and interactions; a linear sequence is not itself a complete account of an active structure. Folding errors and modifications illustrate the need to connect multiple organizational levels.
The central dogma concerns directional transfer of \i[sequence information]. <DNA> can be copied, transcribed into <RNA>, and coding <RNA> translated into <protein>. <Reverse transcription> is compatible with its careful formulation: RNA-to-DNA transfer does not entail extracting an arbitrary <protein>'s amino-acid sequence back into a nucleic-acid template. <RNA replication> also occurs. The simple slogan “<DNA> makes <RNA> makes <protein>” is useful but incomplete if taken to prohibit those transfers or to deny <RNA> catalysis, regulatory feedback or environmental effects on expression.
Amino-acid sequence is encoded through a triplet <genetic code>, with tRNAs and <Ribosome>[ribosomes] mediating <translation (biology)>[translation] rather than direct chemical affinity between each <codon> and its <amino acid>. Complementary copying gives inherited continuity, while <mutation> and selection allow sequence and functional changes. Thus molecular structure supports both stable heredity and adaptable biochemical machinery without making every cellular process a one-way causal chain.<Molecular Biology of the Cell, 4th edition by Bruce Alberts and others>[Chapters 3–6, protein structure, nucleic acids and sequence-information transfer]{ref}
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