Ribosomes, their role. The characteristic of ribosomal RNA.
The ribosome (from ribo nucleic acid and the Greek soma, meaning "body") is a large and complex molecular machine, found within all living cells, that serves as the primary site of biological protein synthesis (translation). Ribosomes link amino acidstogether in the order specified by messenger RNA (mRNA) molecules.
In molecular biology, Ribosomal ribonucleic acid (rRNA) is the RNA component of the ribosome, and is essential for protein synthesis in all living organisms. It composes the predominant material within the ribosome, which is ca. 60% rRNA and 40% protein by weight. Ribosomes contain two major rRNAs and 50 or more proteins. The LSU and SSU rRNAs are found within the large and small ribosomal subunits, respectively. The LSU rRNA acts as a ribozyme, catalyzing peptide bond formation. rRNA sequences are widely used for working out evolutionary relationships among organisms, since they are of ancient origin and are found in all known forms of life.
The ribosomal RNAs form two subunits, the large subunit (LSU) and small subunit (SSU). mRNA is sandwiched between the small and large subunits, and the ribosome catalyzes the formation of a peptide bond between the 2 amino acids that are contained in the rRNA.
A ribosome also has 3 binding sites called A, P, and E.
· The A site in the ribosome binds to an aminoacyl-tRNA (a tRNA bound to an amino acid).
· The amino (NH2) group of the aminoacyl-tRNA, which contains the new amino acid, attacks the ester linkage of peptidyl-tRNA (contained within the P site), which contains the last amino acid of the growing chain, forming a new peptide bond. This reaction is catalyzed by peptidyl transferase.
· The tRNA that was holding on the last amino acid is moved to the E site, and what used to be the aminoacyl-tRNA is the peptidyl-tRNA.
A single mRNA can be translated simultaneously by multiple ribosomes.
Ribosomal RNA characteristics are important in medicine and in evolution.
· rRNA is the target of several clinically relevant antibiotics: chloramphenicol, erythromycin, kasugamycin, micrococcin, paromomycin, ricin, sarcin, spectinomycin, streptomycin, andthiostrepton.
· rRNA is one of only a few gene products present in all cells. For this reason, genes that encode the rRNA (rDNA) are sequenced to identify an organism's taxonomic group, calculate related groups, and estimate rates of species divergence. As a result, many thousands of rRNA sequences are known and stored in specialized databases such as RDP-II and SILVA.
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