= Cell nucleus
{wiki}
The <cell nucleus> is a membrane-bounded compartment characteristic of <eukaryotic> <biological cells>, containing most of the cellular <DNA> in a typical nucleated <biological cell>. Its double <nuclear envelope> separates this <DNA> from the surrounding <cytoplasm>. Most does not mean all: <mitochondria> and <chloroplasts> retain their own <DNA>. Nor does every specialized <eukaryotic> <biological cell> retain a <cell nucleus>: mature mammalian <red blood cells> have lost theirs. <Prokaryotes> organize their <DNA> in a <nucleoid> without a <nuclear envelope>, and still have <ribosomes>. See https://www.genome.gov/genetics-glossary/Nucleus[NHGRI: nucleus], https://openstax.org/books/biology-2e/pages/23-1-eukaryotic-origins[Clark, Douglas and Choi, Biology 2e, 23.1: organelle DNA]{ref} and https://openstax.org/books/biology-2e/pages/40-2-components-of-the-blood[Clark, Douglas and Choi, Biology 2e, 40.2: mature mammalian red blood cells].
Follow one nuclear <protein>-coding <gene>: its <DNA> is used in <transcription> to make <RNA>; after the appropriate processing, <messenger RNA> is exported through a <nuclear pore complex>; a <ribosome> in the <cytoplasm> then uses it in <biological translation> to build an <amino acid> chain. The <DNA> remains in the <cell nucleus>. The <RNA> is a template, not the material converted into a <protein>. The <ribosome> diagram below follows this route; <gene expression> gives a complementary <nucleotide sequence> example. See https://www.ncbi.nlm.nih.gov/books/NBK26932/[Alberts et al.: selective RNA export and protein import].
This architecture makes <gene expression> interesting in a spatial way: nuclear <transcription> and cytoplasmic <biological translation> are separated by a selective boundary. A <messenger RNA> can be processed before export and use, while <proteins> needed for nuclear functions travel inward. Inside the <cell nucleus>, the <cell nucleolus> helps assemble <ribosome> subunits for export. Thus the compartment housing most <DNA> also helps build the machinery that interprets its messages elsewhere.
Robert Brown described and named the <cell nucleus> in his account of <plant> <biological cells>, read to the Linnean Society in 1831 and published in its Transactions in 1833. This historical observation identified a recurring structure, without establishing its later-understood role in <DNA> storage or <gene expression>. See https://doi.org/10.1111/j.1095-8339.1829.tb00158.x[Brown, On the Organs and Mode of Fecundation in Orchideæ and Asclepiadeæ, Transactions 16, 685–738 (1833)]. This is a documented contribution rather than a claim that no one had observed the structure earlier.
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