intracellular organelle, site of protein biosynthesis resulting from translation of messenger RNA (mRNA), consisting of two subunits
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Ribosomes are important cell organelles. It does RNA translation, building proteins from amino acids using messenger RNA as a template. Ribosomes are found in all living cells, prokaryotes as well as eukaryotes.

A ribosome is a mixture of protein and RNA that starts being made in the nucleolus of a cell. The nucleolus is found in the center of the nucleus. The nucleus is protected by the nuclear envelope, and lets things out through the nuclear pores.

The job of the ribosome is to make new proteins. It does this by moving along a strand of RNA and building a protein based on the code it reads. Making a protein this way is called translation. Ribosomes are usually found in the rough endoplasmic reticulum, but can also be found throughout the cytoplasm.

Ribosomes read the sequence of messenger RNAs and assemble proteins out of amino acids bound to transfer RNAs.
Translation of mRNA (1) by a ribosome (2)(shown as small and large subunits) into a polypeptide chain (3). The ribosome begins at the start codon of mRNA (AUG) and ends at the stop codon (UAG).

Ribosome structureEdit

Figure 2 : Large (red) and small (blue) subunit fit together

Ribosomes are made out of two things: a small ribosomal subunit that reads the mRNA, while the large subunit joins amino acids to form a polypeptide chain. Each subunit is composed of one or more ribosomal RNA (rRNA) molecules and a variety of proteins.

Eukaryotes have 80S ribosomes, each consisting of a small (40S) and large (60S) subunit.[1][2][3] Their small subunit has a 16S RNA sub-unit (consisting of 1540 nucleotides) bound to 21 proteins. The large subunit has a 5S RNA (120 nucleotides), a 28S RNA (4700 nucleotides), a 5.8S RNA (160 nucleotides) subunits and 46 proteins.[2][4][5]


  1. The unit of measurement is the Svedberg unit, a measure of the rate of sedimentation in centrifugation rather than size. This accounts for why fragment numbers do not add up (80S is made of 40S and 60S).
  2. 2.0 2.1 Ben-Shem A. et al 2011. "The structure of the eukaryotic ribosome at 3.0 Å resolution". Science. 334 (6062): 1524–1529. doi:10.1126/science.1212642. PMID 22096102.
  3. Rabl et al 2010. "Crystal structure of the eukaryotic 40S ribosomal subunit in complex with initiation factor 1". Science. 331 (6018): 730–736. doi:10.1126/science.1198308. PMID 21205638.
  4. Alberts, Bruce et al 2002. The molecular biology of the cell. 4th ed, Garland Science, 342. ISBN 0-8153-3218-1
  5. Klinge et al 2011. "Crystal structure of the eukaryotic 60S ribosomal subunit in complex with initiation factor 6". Science. 334 (6058): 941–948. doi:10.1126/science.1211204. PMID 22052974.