ADVERTISEMENTS:
This article provides a note on the structure of ribosome.
Ribosome is a key component in the process of translation therefore studied extensively. One bacterial cell contains ~10,000 such structures and a eukaryotic cell contains many times more.
A bacterial ribosome is about 250 nm in diameter and consists of two subunits, one large and one small. Both subunits consist of one or more molecules of rRNA and an array of ribosomal proteins.
ADVERTISEMENTS:
Association of two subunits is called mono-some. The structure of prokaryotic ribosome is given in the figure 8.2 B. The subunit and rRNA components are most easily isolated and characterize on the basis of their sedimentation behavior in sucrose gradient (there rate of migration is called Svedberg’s coefficient ‘S’.
It is a unit of sedimentation velocity; sedimentation in ultracentrifuge depends on both the mass and shape of molecule, and is not a simply a measure of molecular mass) (Fig. 8.2A). The mono-some of prokaryotes is a 70S particle and in eukaryotes it is 80S. The prokaryotic 70S mono-some consists of a 50S and 30S subunit and the eukaryotic 80S mono-some consists of a 60S and 40S subunit. The sum value of ‘S’ is not a simple arithmetic addition.
The large subunit in prokaryotes consists of a 23S RNA molecule, a 5S rRNA molecule and 31 ribosomal proteins. In the eukaryotic equivalent, a 28S rRNA molecule is accompanied by a 5.8S and 5S rRNA molecule and ~50 proteins. The smaller prokaryotic subunits consist of a 16S rRNA component and 21 proteins. Similarly, in eukaryotes smaller subunits consist of a 18S rRNA component and ~33 proteins. These proteins are involved in binding of various molecules involved in translation and precise control of process.
ADVERTISEMENTS:
In eukaryotes, many more copies of a sequence encoding the 28S and 18S components are present. The exact functions of various components are still not very much clear. In bacteria, the mono-some has a combined molecular weight of 2.5 million Dalton.
Structure of tRNA is described in Chapter 5. Transfer RNA has an anticodon loop and a amino acid binding site besides other loops and stems as described earlier. Before translation can proceed, the tRNA molecule must be chemically linked to their respective amino acids. This activation process, called charging (Fig. 8.2), governs by the enzymes called amino acyl tRNA synthetizes. Because there are 20 different amino acids, there must be 20 different tRNA molecules and as many different enzymes.
In theory, because there are 61 triplet codes, there could be the same number of specific tRNAs and enzymes. However, because of the ability of the third member of a codon to wobble, it is now thought that there are at least 32 different tRNAs. It is also believed that there are only 20 synthetases, one for each amino acid.
During the initial step, the amino acid is converted to an activated form, reacting with ATP to create an aminoacylicadenylic acid. A covalent linkage is formed between the 5′-phosphate group of ATP and the carboxyl end of the amino acid. This enzyme-amino acid complex then reacts with a specific tRNA molecule.
In the next step, the amino acid is transferred to the appropriate tRNA and bonded covalently to the adenine residue at the 3′ end. The charged tRNA may participate directly in protein synthesis. Aminoacyl tRNA synthetases are highly specific enzymes because they recognize only one amino acids and only a subset of corresponding tRNAs, called is accepting tRNAs. This is a crucial point in maintaining the correctness of translation.
