= Ribosome
{wiki}
A <ribosome> is the molecular machinery for <biological translation>: it reads <messenger RNA> and joins <amino acids> into a <polypeptide> that can become a <protein>. Both <prokaryotes> and <eukaryotes> have <ribosomes>. A <ribosome> consists of <ribosomal RNA> and <proteins> organized into small and large subunits, with no enclosing membrane. <Ribosomal RNA> contributes to both structure and the <active site> that forms <peptide bonds>; a <ribosome> is therefore not merely a collection of <proteins>. See https://openstax.org/books/biology-2e/pages/4-3-eukaryotic-cells[Clark, Douglas and Choi, Biology 2e, 4.3: ribosome composition] and https://pubmed.ncbi.nlm.nih.gov/10937990/[Nissen et al., The structural basis of ribosome activity in peptide bond synthesis, Science 289, 920–930 (2000)].
\Image[/ribosome-translation-schematic.png]
{title=Nuclear <transcription>, <RNA> export and <ribosome>-mediated <biological translation>}
{description=Original schematic of a nuclear <protein>-coding <gene> in a nucleated <eukaryotic> <biological cell>. The upper route tracks <DNA> to <RNA> by <transcription>, <RNA> export, then <biological translation>. The lower example reads three <messenger RNA> <codons> from $5'$ to $3'$ and shows a growing <polypeptide> segment from its <N-terminus> toward its <C-terminus>. Shapes and distances are illustrative; processing, flanking signals, additional machinery and termination are omitted.}
For a computed decoding example, choose the <reading frame> starting at the first displayed <nucleotide> of $5'\text{-}\mathrm{AUG\ GCU\ GAA}\text{-}3'$. Under the standard <genetic code>, AUG specifies <methionine> (Met), GCU specifies <alanine> (Ala), and GAA specifies a <glutamic acid residue> (Glu) in the growing <polypeptide>. Nine <nucleotides> divide into three <codons>, giving the ordered three-<amino acid residue> segment Met–Ala–Glu. The <ribosome> reads the <messenger RNA> $5'\to3'$, and the chain grows from its <N-terminus> toward its <C-terminus>. If synthesis begins at this AUG in an appropriate <eukaryotic> initiation context, these are the first three <amino acid residues> before any subsequent processing. See https://openstax.org/books/biology-2e/pages/15-1-the-genetic-code[Clark, Douglas and Choi, Biology 2e, 15.1: standard codon assignments].
For this three-<amino acid residue> segment, joining the three delivered <amino acids> requires two <peptide bonds>, one between Met and Ala and another between Ala and Glu. There is no <stop codon> in the displayed <nucleotide sequence>, so this is a growing segment rather than a complete released <protein>. An isolated nine-<nucleotide> <RNA> is not claimed to supply all signals needed for initiation. This small calculation separates three jobs: the <messenger RNA> specifies order, <transfer RNA> delivers the <amino acids>, and the <ribosome> catalyzes their joining. Each new <amino acid residue> is added at the <C-terminus>. See https://openstax.org/books/biology-2e/pages/15-5-ribosomes-and-protein-synthesis[Clark, Douglas and Choi, Biology 2e, 15.5: initiation and chain elongation].
The beauty of a <ribosome> is its reusable chemistry: the same machinery can assemble many different <proteins> by following different <messenger RNA> templates. The <genetic code> connects an alphabet of <nucleotides> to an alphabet of <amino acids>, while <transfer RNA> links recognition of the message to delivery of building material. Changing the message can therefore change the product without building a different <ribosome> for every <protein>.
In 1955 George E. Palade described small particles in the <cytoplasm>, both free and associated with membranes, in https://pmc.ncbi.nlm.nih.gov/articles/PMC2223592/[A small particulate component of the cytoplasm, Journal of Biophysical and Biochemical Cytology 1, 59–68]. The paper documents the structural observations, rather than the full modern decoding mechanism. Complementary biochemical work by Hoagland, Stephenson, Scott, Hecht and Zamecnik described a soluble <RNA> intermediate in <protein> synthesis in 1958, helping establish the role now assigned to <transfer RNA>; see https://pubmed.ncbi.nlm.nih.gov/13538965/[A soluble ribonucleic acid intermediate in protein synthesis, Journal of Biological Chemistry 231, 241–257]. Together, structural observations and biochemical experiments connected visible cellular machinery with its working parts.
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