= Gene expression
{wiki}
<Gene expression> uses a <gene>'s information to produce functional <RNA> or a <protein>. <Transcription> makes <RNA> from a <DNA> template; for a <protein>-coding <gene>, <biological translation> then uses <messenger RNA> to specify an <amino acid> chain. Many <genes> instead produce functional <RNA>, including <transfer RNA> and <ribosomal RNA>, without a translated <protein> product. See https://www.ncbi.nlm.nih.gov/books/NBK26887/[Alberts et al., Molecular Biology of the Cell, 4th edition (2002): from DNA to RNA].
Follow the chosen <coding strand> $5'\text{-}\mathrm{ATGGCTTAA}\text{-}3'$: <transcription> of its complementary template gives $5'\text{-}\mathrm{AUGGCUUAA}\text{-}3'$ in <messenger RNA>. In the stated <reading frame>, its three <codons> are AUG, GCU and UAA: <methionine>, <alanine> and a <stop codon>. Thus nine <nucleotides> specify two <amino acid> residues followed by termination, not three residues. This is an illustrative coding segment embedded in suitable expression and initiation context, not a claim that an isolated nine-<nucleotide> <RNA> is sufficient to initiate <biological translation>.
\Image[/gene-expression-transcription-translation.png]
{title=<Gene expression>: oriented <DNA> template, <messenger RNA> <codons> and the resulting <amino acid> order}
{description=The <coding strand> matches the displayed <messenger RNA> after T is replaced by U; <RNA polymerase> uses the opposite, complementary <DNA> strand. The <ribosome> reads AUG, GCU and UAA from $5'$ to $3'$. AUG and GCU specify <methionine> and <alanine>; UAA signals termination without supplying an <amino acid>. The chain is shown from its <N-terminus> to its <C-terminus>. The labels summarize successive operations, not simultaneous binding of all depicted components; flanking signals and machinery are omitted.}
The distinction between copying and use explains how <biological cells> can preserve the same <DNA> yet produce different products. <DNA replication> duplicates the record; <gene expression> selects and uses parts of it. <Transcription> does not use all <DNA> at once, and <gene expression> can be controlled at several steps, including <transcription>, <RNA> processing and <biological translation>. See https://openstax.org/books/biology-2e/pages/16-1-regulation-of-gene-expression[Clark, Douglas and Choi, Biology 2e: regulation of gene expression]. The arrows describe template-directed synthesis: the <DNA> does not physically turn into a <protein>, and the <messenger RNA> does not supply the <amino acids> from which that <protein> is assembled.
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