How are ribosomes called
Ribosomes, the organelles occurring in large numbers in prokaryotes and eukaryotes as well as mitochondria and plastids, where protein biosynthesis takes place (translation). R. occur both freely in the cytoplasm and on the outside of the endoplasmic reticulum (rough ER). With a diameter of approx. 25 nm, ribonucleoprotein particles are usually large and appear as well-contrasted particles in electron microscope images (microscope).
construction. Ribosomes always consist of two subunits (UE), the so-called small UE and major UE. These in turn are made up of one to three molecules of ribosomal RNA (rRNA) and several proteins. There are differences between prokaryotes and eukaryotes in terms of the nature of the R. Prokaryotic R. are classified according to their sedimentation coefficient as 70S R. with 50S and 30S UE, eukaryotic R. as 80S R. labeled with 60S- and 40S-UE (see fig.), (see table). According to the endosymbiotic theory, the R. of the mitochondria and plastids belong to the 70S type. In prokaryotes the large ribosomal UE contain two rRNAs, in eukaryotes and plastids three rRNAs and in mitochondria only one rRNA. The small AEs always consist of only one rRNA. As far as the number of proteins is concerned, there are also differences between the different R. types: eukaryotic R.s contain 40-45 proteins in their large UE (prokaryotes: 33) and their small UE 30-35 proteins (prokaryotes: 21). Their exact location was determined by immuno-electron microscopic methods. There are no covalent bonds between the rRNA molecules and ribosomal proteins, only electrostatic and hydrophobic interactions. A number of studies indicate that the two ribosomal UEs form a space inside in which translation takes place. Defined binding sites are for the mRNA, the transfer RNA loaded with an amino acid (A binding site), the growing polypeptide (P binding site) and the E-digit (Exit) through which the unloaded tRNA leaves the ribosome.
R., which are not involved in translation processes, are present in the cytoplasm in their subunits dissociated. During the so-called Ribosome cycle first binds the small UE to the messenger RNA (mRNA), which is followed by the binding of the large UE. After the translation is complete, both UEs detach from the mRNA. Their lifetime in the cytosol is around six hours. Since mRNA molecules are usually relatively long, Polysomes or Polyribosomes are present in which several rows are lined up one behind the other with an average distance of 100 nucleotides. They are characterized by a helical structure (see Fig.). According to their function, they are usually multienzyme complexes that catalyze the individual steps of translation in sequence. A number of antibiotics inhibit certain translation steps by specifically binding to 70S or 80S ribosomes. They include chloramphenicol, cycloheximide, puromycin and streptomycin.
Ribosomes: Schematic representation of a ribosome during translation
Ribosomes: Characteristics of various types of ribosomes
Ribosomes: Structure of a polysome in the scheme. The nascent peptide chain grows with increasing distance from the start codon
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