AskDefine | Define ribosome

Dictionary Definition

ribosome n : an organelle in the cytoplasm of a living cell; ribosomes attach to mRNA and move down it one codon at a time and stop until tRNA brings the required amino acid; when a ribosome reaches a stop codon it falls apart and releases the completed protein molecule for use by the cell; "the ribosome is the site of protein synthesis"

User Contributed Dictionary



ribosome (plural ribosomes )
  1. small particles found in a cell involved in the production of proteins by translating messenger RNA


Extensive Definition

Ribosomes (from ribonucleic acid and "Greek: soma (meaning body)") are complexes of RNA and protein that are found in all cells. Prokaryotic ribosomes from archaea and bacteria are smaller than most of the ribosomes from eukaryotes such as plants and animals. However, the ribosomes in the mitochondrion of eukaryotic cells resemble those in bacteria, reflecting the evolutionary origin of this organelle.
The function of ribosomes is the assembly of proteins, in a process called translation. Ribosomes do this by catalysing the assembly of individual amino acids into polypeptide chains; this involves binding a messenger RNA and then using this as a template to join together the correct sequence of amino acids. This reaction uses adapters called transfer RNA molecules, which read the sequence of the messenger RNA and are attached to the amino acids.


Ribosomes are about 20nm (200 Ångström) in diameter and are composed of 65% ribosomal RNA and 35% ribosomal proteins (known as a Ribonucleoprotein or RNP). They translate messenger RNA (mRNA) to build polypeptide chains (e.g., proteins) using amino acids delivered by transfer RNA (tRNA). Their active sites are made of RNA, so ribosomes are now classified as "ribozymes."
Ribosomes build proteins from the genetic instructions held within messenger RNA. Free ribosomes are suspended in the cytosol (the semi-fluid portion of the cytoplasm); others are bound to the rough endoplasmic reticulum, giving it the appearance of roughness and thus its name, or to the nuclear envelope. As ribozymes are partly constituted from RNA, it is thought that they might be remnants of the RNA world. Catalysis of the peptide bond involves the C2 hydroxyl of RNA's P-site adenosine in a protein shuttle mechanism. The full function (i.e. translocation) of the ribosome is reliant on changes in protein conformations. Ribosomes are sometimes referred to as organelles, but the use of the term organelle is often used only in reference to sub-cellular components that include a phosholipid membrane, which ribosomes, being entirely particulate, do not. For this reason, ribosomes may sometimes be described as "non-membranous organelles".
Ribosomes are an extremely important structure in the cell. Ribosomes were first observed in the mid-1950s by Romanian cell biologist George Palade using an electron microscope as dense particles or granules for which he would win the Nobel Prize. The term "ribosome" was proposed by scientist Richard B. Roberts in 1958: The structure and function of the ribosomes and associated molecules, known as the translational apparatus, has been of research interest since the mid-twentieth century and is a very active field of study today.
Ribosomes consist of two subunits (Figure 1) that fit together (Figure 2) and work as one to translate the mRNA into a polypeptide chain during protein synthesis (Figure 3). Prokaryotic subunits consist of one or two and eukaryotic of one or three very large RNA molecules (known as ribosomal RNA or rRNA) and multiple smaller protein molecules. Crystallographic work has shown that there are no ribosomal proteins close to the reaction site for polypeptide synthesis. This suggests that the protein components of ribosomes act as a scaffold that may enhance the ability of rRNA to synthesize protein rather than directly participating in catalysis (See: Ribozyme).


In prokaryotic cells, ribosomes synthesize with cytoplasm to enable the transcription of multiple ribosome gene operons. In eukaryotes and some prokaryotic cells, the process takes place both in the cell cytoplasm and in the nucleolus of eukaryotic cells. It involves the coordinated function of over 200 proteins in the synthesis and processing of the four rRNAs, as well as assembly of those rRNAs with the ribosomal proteins.

Ribosome locations

Ribosomes are classified as being either "free" or "membrane-bound."

Free ribosomes

Free ribosomes are "free" to move about anywhere in the cytoplasm (within the cell membrane). Proteins that are formed from free ribosomes are used within the cell. Proteins containing disulfide bonds using cysteine amino acids cannot be produced outside of the lumen of the endoplasmic reticulum.

Membrane-bound ribosomes

When certain proteins are synthesized by a ribosome they can become "membrane-bound". The newly produced polypeptide chains are inserted directly into the endoplasmic reticulum by the ribosome and are then transported to their destinations. Bound ribosomes usually produce proteins that are used within the cell membrane or are expelled from the cell via exocytosis.
Free and membrane-bound ribosomes differ only in their spatial distribution; they are identical in structure and function. Whether the ribosome exists in a free or membrane-bound state depends on the presence of a ER-targeting signal sequence on the protein being synthesized.


The ribosomal subunits of prokaryotes and eukaryotes are quite similar.
Prokaryotes have 70S ribosomes, each consisting of a small (30S) and a large (50S) subunit. Their large subunit is composed of a 5S RNA subunit (consisting of 120 nucleotides), a 23S RNA subunit (2900 nucleotides) and 34 proteins. The 30S subunit has a 1540 nucleotide RNA subunit (16S) bound to 21 proteins.
The various ribosomes share a core structure which is quite similar despite the large differences in size. The extra RNA in the larger ribosomes is in several long continuous insertions, such that they form loops out of the core structure without disrupting or changing it. Even though mitochondria possess ribosomes similar to the bacterial ones, mitochondria are not affected by these antibiotics because they are surrounded by a double membrane that does not easily admit these antibiotics into the organelle.

Atomic structure

In Figure 3, both ribosomal subunits (small and large) assemble at the start codon (towards the 5' end of the mRNA). The ribosome uses tRNA which matches the current codon (triplet) on the mRNA to append an amino acid to the polypeptide chain. This is done for each triplet on the mRNA, while the ribosome moves towards the 3' end of the mRNA. Usually in bacterial cells, several ribosomes are working parallel on a single mRNA, forming what we call a polyribosome or polysome.


See also

ribosome in Arabic: ريبوسوم
ribosome in Bosnian: Ribozomi
ribosome in Bulgarian: Рибозома
ribosome in Catalan: Ribosoma
ribosome in Czech: Ribozom
ribosome in Welsh: Ribosom
ribosome in Danish: Ribosom
ribosome in German: Ribosom
ribosome in Modern Greek (1453-): Ριβόσωμα
ribosome in Spanish: Ribosoma
ribosome in Esperanto: Ribosomo
ribosome in Persian: ریبوزوم
ribosome in Faroese: Ribosom
ribosome in French: Ribosome
ribosome in Galician: Ribosoma
ribosome in Korean: 리보솜
ribosome in Croatian: Ribosomi
ribosome in Indonesian: Ribosom
ribosome in Interlingua (International Auxiliary Language Association): Ribosoma
ribosome in Italian: Ribosoma
ribosome in Hebrew: ריבוזום
ribosome in Latin: Ribosoma
ribosome in Luxembourgish: Ribosom
ribosome in Lithuanian: Ribosoma
ribosome in Macedonian: Рибозом
ribosome in Malay (macrolanguage): Ribosom
ribosome in Dutch: Ribosoom
ribosome in Japanese: リボソーム
ribosome in Norwegian: Ribosom
ribosome in Occitan (post 1500): Ribosòma
ribosome in Polish: Rybosom
ribosome in Portuguese: Ribossomo
ribosome in Romanian: Ribozom
ribosome in Russian: Рибосома
ribosome in Simple English: Ribosome
ribosome in Slovak: Ribozóm
ribosome in Slovenian: Ribosom
ribosome in Serbian: Рибозом
ribosome in Serbo-Croatian: Ribozom
ribosome in Finnish: Ribosomi
ribosome in Swedish: Ribosom
ribosome in Thai: ไรโบโซม
ribosome in Vietnamese: Ribosome
ribosome in Turkish: Ribozom
ribosome in Ukrainian: Рибосома
ribosome in Chinese: 核糖体
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