Table of Contents
The cell is comprised of many organelles. An organelle is a structure surrounded by lipid bilayers. In this regard, nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, and chloroplast (plastid) are regarded as organelles whereas ribosomes and nucleosomes are not. In the same way, lysosomes and vacuoles, would not qualify as an organelle because they are single-membrane bounded cytoplasmic structures. Other references, though, are less restrictive. An organelle is one that which acts as a specialized subunit inside the cell that performs a specific function. In this case, there are two types of organelles: (1) membrane-bound organelles (included are double-membraned and single-membraned cytoplasmic structures) and (2) non-membrane-bound organelles. Examples of membrane-bound organelles are nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, plastids, lysosomes and vacuoles. Examples of non-membrane-bound organelles are ribosomes, spliceosome, vault, proteasome, DNA polymerase III holoenzyme, RNA polymerase II holoenzyme, photosystem I, ATP synthase, nucleosome, centriole, microtubule-organizing center, cytoskeleton, flagellum, nucleolus, stress granule, etc.
The ribosome is a cytoplasmic structure that is minute and sphere-shaped. It is composed of protein and ribonucleic acid (RNA). It serves as the site of protein synthesis. In mid-1950s, ribosomes were first observed as dense particles or granules by George Palade with his electron microscope. In 1958, the term ribosome was proposed by the scientist, Richard B. Roberts.
A ribosome is a particle consisting of two subunits that fit together and work as one to build proteins according to the genetic sequence held within the messenger RNA (mRNA). Ribosomes are typically composed of two subunits: the large and small subunits. They join as one during translation; together, they catalyze the translation of mRNA into a polypeptide chain during protein synthesis, and since their active sites are made of RNA, ribosomes are also referred to as “ribozymes.”
Prokaryotic ribosomes vs. Eukaryotic ribosomes
Ribosomes of prokaryotes (e.g. bacteria) are smaller than most of the ribosomes of eukaryotes (e.g. plants and animals). Ribosomes are formed in the cytoplasm of prokaryotic cells. In eukaryotic cells, the production of ribosome involves both the cytoplasm and the nucleolus.
Both prokaryotic and eukaryotic ribosomes are made up of two ribosomal subunits. The subunits of the ribosomes are identified by their sedimentation rate represented by Svedberg unit (S). The prokaryotic ribosome (70S) is made up of 50S (large subunit) and 30S (small subunit). The eukaryotic ribosome (80S) consists of 60S (large subunit) and 40S (small subunit). N.B. the S units do not add up since they represent measures of sedimentation rate, not mass. In prokaryotes, the 30S ribosomal subunit contains the 16S rRNA whereas the 50S ribosomal subunit contains 5S rRNA and 23S rRNA. In mammals, the 40S ribosomal subunit contains the 18S rRNA whereas the 60S ribosomal subunit contains rRNAs: 5S, 5.8S, and 28S. In eukaryotes, the semi-autonomic organelles like chloroplasts and mitochondria also have 70S ribosomes that resemble those of prokaryotes (e.g. bacteria). Because of this, it is suggested that these eukaryotic organelles have descended from their ancestral bacteria (see Endosymbiotic theory). Below is a table that compares the structural components of prokaryotic and eukaryotic ribosomes.
Prokaryotic vs Eukaryotic Ribosomes
|Ribosomes||70S ||80S |
|Prokaryote: E. coli2|
|Eukaryote: R. norvegicus2|
In eukaryotes, the ribosomes may be classified as either ‘free’ or ‘bound’. Free ribosomes may be found suspended in the cytosol whereas bound ribosomes are attached to endoplasmic reticulum (as such called rough endoplasmic reticulum). Free ribosomes are involved in the synthesis of proteins that will function in the cytosol while bound ribosomes in the synthesis of proteins that are to be exported or used within the cell membrane. The two types of ribosomes have similar function and structure, and in fact, are interchangeable. In fact, the bound ribosomes are attached to the ER transiently. They may come and go. They attach to the endoplasmic reticulum (via the translocon) when a signal peptide is synthesized by protein translation at the ribosome, and then recognized by a signal recognition particle.
The ribosomes are described as the site of protein synthesis. Protein synthesis is a process of creating protein molecules. In biological systems, the major steps are amino acid synthesis, transcription and translation. The ribosomes, though, are involved in translation. During translation, the amino acids are added by tRNAs and then are linked together in a specific order as specified in the mRNA transcript.
Common biological reactions
Common biological reactions
Common biological reactions
Ribosome biogenesis refers to the biosynthesis of ribosomes. In eukaryotes, the sites of ribosome formation are the cytoplasm and the nucleus. The ribosome of eukaryotes is 80S as opposed to the ribosome of prokaryotes, which is 70S. The 80S ribosome is comprised of a large subunit (60S) and a small subunit (40S). Each of these subunits is comprised of ribosomal protein and rRNA(s). The ribosomal proteins are synthesized in the same manner as other proteins are produced, i.e. first by transcription within the nucleus and then moved into the cytoplasm for translation and maturation. Mature ribosomal proteins are moved back into the nucleus, particularly in the nucleolus for ribosomal subunit assemblies, i.e. 60S or 40S assembly. As for the rRNA components of the 60S or 40S, they are produced in the nucleus. In mammals, the rRNAs 18S, 28S, and 5.8S are transcribed in the nucleolus organizer region into a single unit pre-rRNA (referred to specifically as 45S pre-RNA) by the catalytic action of RNA polymerase I. The result is a large pre-rRNA made up of 18S, 28S, and 5.8S, which after processing would be released individually. As for the 5S rRNA, the genes encoding for it are transcribed into pre-5S rRNA by the RNA polymerase III. However, the pre-5S rRNA transcript is produced in the nucleoplasm, outside the nucleolus. Nevertheless, it finds its way to the nucleolus for the assembly. To form the large subunit (i.e. 60S) of the ribosomal complex, 5S rRNA combines with 28S and 5.8S rRNA. 18S, in turn, forms the small subunit (i.e. 40S) by combining with the ribosomal proteins. These subunits would then be moved from the nucleolus into the cytoplasm for the assembly of complete and functional 80S ribosome.
Common biological reactions
Using the mRNA as a template, the ribosome traverses each codon, pairing it with the appropriate amino acid. This is done through interacting with transfer RNA (tRNA) containing a complementary anticodon on one end and the appropriate amino acid on the other.
For proteins that need to be packed for transport (either within the cell or outside the cell), a signal peptide is the first to be synthesized. The signal peptide is produced by protein translation at the ribosome. This signal is an indication that the protein is for further processing in the endoplasmic reticulum (ER). When this signal is recognized by a signal recognition particle the ribosome translating the protein docks to the endoplasmic reticulum via the translocon. The ribosome, then, returns back to the translation of the protein. The chain continues to grow as the mRNA transcript is translated through the docked ribosome. The chain eventually makes its way into the ER through the translocon that spans across the ER membranes. The signal peptide is removed by a signal peptidase in the lumen of the ER. The nascent protein is folded in the ER by the chaperone proteins (e.g. ERp29, protein disulfide isomerase, BiP/Grp78, calnexin, etc.). The properly-folded protein is then packed into a transport vesicle to be shuttled to the Golgi apparatus where it would undergo maturation for transport along the cytoskeleton to other cytoplasmic organelles like lysosomes and peroxisomes or for secretion out of the cell.
- from ribonucleic acid and Greek: soma (body)
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