Ribosomes are defined as ancient molecular machines that are responsible for production of protein in all living cells. They are large (2.5 MD molecular weight or larger) macromolecular complexes composed of RNA (2/3) and protein (1/3).
The ribosomes are large ribonucleoproteins
consisting of RNAs and proteins, ubiquitous in all cells, which translate genetic information stored in the messenger RNA into polypeptides.
In prokaryotes such Escherichia coli
, there are three ribosomal RNAs (16S, 23S and 5S), which are organized as single transcriptional unit. In all eukaryotes studied so far, the organization of the ribosomal RNA genes is recognizably similar to that of prokaryotes, but with major differences; the size of the small subunit RNA has increased from 16S to 18S, and that of the large subunit from 23S and 28S; a new small 5.8S r-RNA has become interspersed between the 18S and the 28S rRNA, and the 5S rRNA has become separated from the other r-RNAs in a different transcription unit. The former transcription unit is generally referred to as the rRNA gene or the ribosomal DNA (r DNA). 5S genes are transcribed by a different RNA polymerase from r-RNA genes.
There are generally more copies of the 5S genes than of the r-RNA genes. The human genome contains about 100 copies of r-RNA genes per haploid set. Many other species, including most plants, have several thousand copies. The r-RNA genes transcribed to give a precursor the 45S pre-r RNA, which is processed in the series of post- transcriptional modifications to the mature r RNA species.
Eukaryotic ribosome structure:
Most eukaryotes contain two distinct types of ribosomes: cytosolic and organellar.
The cytosolic ribosomes of eukaryotic cells (other than mitochondrial and chloroplast) are 80S types.
Organellar ribosomes from mitochondria and chloroplast are similar than cytosolic ribosomes and bear resemblance to the bacterial 70S ribosomes.
There are two different types of ribosomes in the cytosol; one is membrane- bound ribosomes, which are attached to the cytosolic side of the ER membrane and second is free ribosomes which are not attached to any membrane.
Membrane bound and free ribosomes are structurally and functionally indistinguishable. They vary only in the proteins they are manufacturing at any given time.
In cytosol, a single m-RNA usually has a number of ribosomes translocating in 5’to 3’ direction, each making a separate but identical polypeptide chain; the entire structure is known as polyribosomes or polysomes.
In eukaryotic cells, proteins are synthesized in cytosol and within organelles.
In the cytosol, proteins are synthesized on membrane-bound as well as membrane free ribosomes.
Proteins that are imported into organelles such as mitochondria, chloroplasts and peroxisomes are synthesized on membrane free ribosomes in the cytosol, whereas proteins that are imported into ER-golgi system are synthesized on ER membrane bound ribosomes.
Bacterial ribosome structure:
There are about 10,000 ribosomes in a bacterial cell which constitutes 30% of the total mass of the cell.
The bacterial ribosomes occur in free form the cytoplasm. The sedimentation coefficient for bacterial ribosome is 70S which is made of 30S, a smaller unit and 50S, a large subunit.
The function of small subunit bacterial ribosome comes with messenger RNA during translation and deciphering.
The large subunits have the role as peptidyl transferase center and peptide bond forms at this site.
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