= Solution
The <central dogma of molecular biology> concerns sequence information: <DNA> can be copied by <DNA replication>; <transcription> makes <RNA> from <DNA>; and <translation> makes a <protein> from messenger <RNA>. The familiar $\mathrm{DNA}\to\mathrm{RNA}\to\mathrm{protein}$ diagram is not an assertion that all information transfer is one-way: <reverse transcription> and <RNA replication> are compatible with the dogma. Its key exclusion is transfer of a <protein>'s amino-acid sequence back into nucleic-acid sequence by a general templating mechanism.
<DNA> is a polymer of deoxyribonucleotides with a sugar–phosphate backbone joined by <phosphodiester bonds>. Its usual <DNA double helix> contains antiparallel strands with complementary bases, adenine paired with thymine and guanine with cytosine. The base sequence stores inherited information; complementary pairing supports accurate copying and repair. A strand has distinct $5'$ and $3'$ ends, and ordinary chain synthesis extends the $3'$ end. Packaging into chromosomes compacts and regulates access to the genetic material.
<RNA> uses ribose, with a $2'$ hydroxyl group, and uracil in place of thymine. It is usually single-stranded but forms extensive paired secondary and tertiary structures. Messenger <RNA> conveys coding information, transfer <RNA> couples codons to amino acids, and ribosomal <RNA> provides both structural organization and catalysis within the <ribosome>. Other <RNA> molecules regulate expression or catalyse reactions. Thus <RNA> is not merely a passive intermediate.
<Proteins> are polypeptides of amino acids linked by <peptide bonds>, read from an amino terminus to a carboxyl terminus. Their primary <sequence> guides <protein folding>; backbone <hydrogen bonds> organize helices and sheets, while hydrophobic, electrostatic and other interactions produce tertiary and sometimes quaternary <protein structure>. Their functions include <catalysis>, transport, signalling, mechanical support and molecular movement. Sequence alone does not guarantee function: folding, modifications, partners and cellular conditions matter. During <translation>, a <ribosome> reads successive triplet <codons> in messenger <RNA>, with charged transfer <RNA> molecules supplying the corresponding amino acids; initiation and termination define the reading frame.
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