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Ribosomes are minute particles consisting of RNA and associated proteins that function to synthesize proteins. Proteins are needed for many cellular functions, such as repairing damage or directing chemical processes. Ribosomes can be found floating within the cytoplasm or attached to the endoplasmic reticulum. Their main function is to convert ...
[1] [2] Eukaryotic ribosomes are also known as 80S ribosomes, referring to their sedimentation coefficients in Svedberg units, because they sediment faster than the prokaryotic ribosomes. Eukaryotic ribosomes have two unequal subunits, designated small subunit (40S) and large subunit (60S) according to their sedimentation coefficients.
Ribosomal RNA is the predominant form of RNA found in most cells; it makes up about 80% of cellular RNA despite never being translated into proteins itself. Ribosomes are composed of approximately 60% rRNA and 40% ribosomal proteins, though this ratio differs between prokaryotes and eukaryotes. [2] [3]
The 60S subunit is the large subunit of eukaryotic 80S ribosomes, with the other major component being the eukaryotic small ribosomal subunit (40S). It is structurally and functionally related to the 50S subunit of 70S prokaryotic ribosomes.
Ribosomes in eukaryotes contain 79–80 proteins and four ribosomal RNA (rRNA) molecules. General or specialized chaperones solubilize the ribosomal proteins and facilitate their import into the nucleus. Assembly of the eukaryotic ribosome appears to be driven by the ribosomal proteins in vivo when assembly is also aided by chaperones.
Due to their high importance in the assembly of ribosomes for protein biosynthesis, the rDNA genes are generally highly conserved in molecular evolution. The number of copies can vary considerably per species. [1] Ribosomal DNA is widely used for phylogenetic studies. [2] [3]
Eukaryotic ribosomes are known to bind to transcripts in a mechanism unlike the one involving the 5' cap, at a sequence called the internal ribosome entry site. This process is not dependent on the full set of translation initiation factors (although this depends on the specific IRES) and is commonly found in the translation of viral mRNA. [9]
Ribosome biogenesis is a very tightly regulated process, and it is closely linked to other cellular activities like growth and division. [3] [4] Some have speculated that in the origin of life, ribosome biogenesis predates cells, and that genes and cells evolved to enhance the reproductive capacity of ribosomes. [5]