We should have some idea about the details of ribosome before we get our study about ribosome structure. Now let us have them:
What is Ribosome?
Ribosomes are small, granular, non-membranous structures that are made up of RNA and protein and concerned with protein synthesis.
Distribution of Ribosome
Ribosome are present in both eukaryotic as well as prokaryotic cells. In the eukaryotic cells, these are found mainly attached on the outer surface of the rough endoplasmic reticulum and outer nuclear membrane. In addition, a few ribosome of relatively smaller size are also present within mitochondria and plastids or freely scattered in the cytoplasmic matrix. The cytoplasmic and mitochondrial ribosome are called cytoribosome and mitoribosome respectively. In the prokaryotic cells, due to the nonappearance of the membranous organelles, ribosomes, remain freely scattered in the cytoplasm; these are called monosomes. Ribosome may remain singly or in clusters. During rapid synthesis of proteins in a cell, a few ribosome become joined together temporarily by means of mRNA to form a cluster called polyribosome or polysome. Polysomes are found in the cytoplasm of prokaryotic and eukaryotic cells as well as within mitochondria. Now it will be very easy to understand the topic called ribosome structure.
Number of Ribosome
In eukaryotic cells, there are millions of ribosome (1-10 million) ; the number is especially higher in cells that are engaged in protein synthesis. Prokaryotic cells like bacterial cells contain relatively much smaller number of ribosome.
Before going to the study of ribosome structure let us have some idea about the origin of ribosome. In eukaryotic cells, ribosomes are formed by the joint action of nucleolus and cytoplasm. The ribosomal RNA (rRNA) is formed in the nucleolus. The protein of ribosome is formed in the cytoplasm and then carried to the nucleus where it combines with rRNA to form the subunits of ribosome. These subunits are then transferred to the cytoplasm, where they are assembled into fully formed ribosomes. In prokaryotic cells, the rRNA is formed by the DNA thread present in the nucleoid region ; thus development of ribosomes wholly occurs in the cytoplasm. Now it is very easy to know about ribosome structure.
Structure and Chemical composition of Ribosome
Ribosomes are spherical or oval bodies. Each ribosome is made up of two unequal subunits that remain bound to each other by magnesium ions. If the concentration of Mg 2+ is reduced, the two subunits become dissociated and they connect again if the Mg2+ concentration is raised. Of these two subunits, the bigger one is spherical and the smaller one is oval. Unlike other organelles, the ribosomes are not enclosed by a membrane. Ribosomes are composed of a special type of RNA called ribosomal RNA and histone protein.
The proportion of rRNA and protein is roughly 1: 1 in eukaryotes, whereas it is nearly 2: 1 in prokaryotic cells. All ribosome are not equal in size. They are distinguished on the basis of their physical property of sedimentation coefficient which is expressed by S unit. The larger and heavier is a body, the more will be the sedimentation coefficient.
Three types of ribosome structure have been identified so far; these are designated as 80 S, 70 S and 55 S ribosome. 80 S ribosomes are largest in size and are found in eukaryotic cells; its subunits are 60 S and 40 S. 70 S ribosomes are relatively smaller and are found in prokaryotic cells; these are made up of 50 S and 30 S subunits. 55 S ribosomes are smallest, containing 35 S and 25 S subunits; these are found within mitochondria of mammalian cells. Ribosomes remain attached to ER by ribophorins through their larger subunits.
Ribosomes are concerned with protein synthesis. Ribosomes jointly with the mRNA form the template on which the amino acids are joined one after one by peptide bonds to form the polypeptide chain of the proteins.
Moreover, certain minute particles composed of RNA and protein have also been found to be present in cytoplasm and nucleus of eukayotic cells. The RNA contained in these is mRNA and hence these are supposed to bear genetic information.
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