A ribosome is a cytoplasmic nucleoprotein structure that acts as the machinery for the synthesis of proteins from the mRNA templates. On the ribosomes, the mRNA and tRNA molecules interact to translate the information transcribed from the gene during mRNA synthesis into a specific protein. During periods of active protein synthesis, many ribosomes can be associated with any single mRNA molecule to form an assembly called the polysome.

The components of the mammalian ribosome, which has a molecular weight of about 4.2 × 10⁶ and a sedimentation velocity coefficient of 80S (S = Svedberg units, a parameter sensitive to molecular size and shape) are shown in Table 1. The mammalian ribosome contains two major nucleoprotein subunits— a larger one with a molecular weight of 2.8 × 10⁶ (60S) and a smaller subunit with a molecular weight of 1.4 × 10⁶ (40S).

Table1. Components of Mammalian Ribosomes

The 60S subunit contains a 5S rRNA, a 5.8S rRNA, and a 28S rRNA; there are also more than 50 specific polypeptides. The 40S subunit is smaller and contains a single 18S rRNA and 30 distinct polypeptide chains. All of the rRNA molecules except the 5S rRNA, which is independently transcribed, are processed from a single 45S precursor RNA molecule in the nucleolus. rRNAs are highly methylated post transcriptionally, and are packaged in the nucleolus with the specific ribosomal proteins. In the cytoplasm, the ribosomes remain quite stable and capable of many cycles of translation. The exact functions of the rRNA molecules in the ribosome, over and above their scaffold functions are not yet fully understood. It is clear though that rRNAs are necessary for ribosome assembly, and also play key roles in the binding of mRNA to ribosomes and mRNA translation. Recent studies indicate that the large rRNA component performs the peptidyl transferase activity and thus is a ribozyme. The rRNAs (28S + 18S) represent roughly 70% of total cellular RNA.

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مواضيع ذات صلة

Small RNA

Messenger RNA

Identification of the src and sis Gene

Cellular Counterparts of Retroviral Oncogenes

Retroviruses and Cancer

Identifying a Nucleotide Change Causing Cancer

Transformation and Oncogenesis by Damaging the Chromosome

Bacterially Induced Tumors in Plants

Phosphorylation and the Rapid Response

Chemotaxis: Methylation and Adaptation

The Energy for Chemotaxis

The Mechanism of Chemotaxis