In this article we will discuss about the chemical composition and structure of ribosomes.
Chemical Composition of Ribosomes:
Ribosomes are made of RNA and protein. In E. coli, they contain 34% protein and 66% RNA, while in eukaryotes, they contain 40% protein and 60% RNA. Chemical composition of large subunit is different from that of small subunit. The size and sedimentation coefficients of ribosomes and their subunits and the rRNAs vary in prokaryotic cytoplasm, eukaryotic cytoplasm, mitochondria and chloroplasts.
In prokaryotes, proteins in the small subunit (30S) are designated as S1, S2, …, S21, and each type is present in a single copy. In the large subunit (50S), the 31 specific proteins are designated as L1, L2, …. (numbering exceeds 31 because of mistake by early workers). One protein L7/L12 is present in 4 copies ribosome, while the rest are present in single copies.
Structure of Ribosome:
Electron microscopy and negative staining techniques provided the information that led to the construction of a 3-dimensional model of prokaryotic ribosomes. Both the subunits have their characteristic protuberances and ridges (Fig. 2.15).
The small subunit consists of the regions:
(i) Head,
(ii) Platform, and
(iii) Base.
The large subunit also consists of three regions, namely:
(i) Stalk,
(ii) Ridge, and
(iii) Central protuberance.
When associated together, the head of the small subunit and the central protuberance of the large subunit face each other (Fig. 2.15C).
The small subunit functions to bind the mRNA and tRNA. In the large subunit, 5S rRNA is situated on the central protuberance, while the 3′-end of 23S rRNA is situated below the stalk. The rRNA molecules are partly exposed on surface. The 4 molecules of L7/ L12 proteins are located in the stalk.
The large subunit contains the peptidyl (P) site, aminoacyl (A) site and exit (E) site. Peptide bond formation occurs at the central protuberance. The 3′-end of 16S rRNA in E. coli contains pyrimidine rich (UCCU) short sequences knows as “Shine-Dalgarno sequence.”
The 5′ un-translated segment of mRNA has purine rich short sequences (AGGA). The initial binding of mRNA to ribosome occurs by complementary base paring between these sequences at 3′-end of 16 SrRNA and 5′ un-translated region of mRNA. But in eukaryotes, the binding of mRNA to ribosomes is facilitated by cap structure at 5′-end of mRNA.