Introduction to The Translation

A messenger RNA (mRNA) transcript carries a series of codons that interact with the anticodons of aminoacyl-tRNAs so that a corresponding series of amino acids is incorporated into a polypeptide chain. The ribosome provides the environment for controlling the interaction between mRNA and aminoacyl-tRNA. The ribosome behaves like a small migrating factory that travels along the mRNA template, engaging in rapid cycles of peptide bond synthesis to build a polypeptide. Aminoacyl-tRNAs shoot into the ribosome at an incredibly fast rate to deposit amino acids, and elongation factor proteins cyclically associate with and dissociate from the ribosome. Together with its accessory factors, the ribosomal structure provides the full range of activities required for all the steps of translation.
Figure .1 shows the relative dimensions of the components ofthe  translation apparatus. The ribosome consists of two subunits (“large” and “small”) that have specific roles in translation. Messenger RNA is associated with the small subunit; approximately 35 bases of the mRNA are bound at any time during translation. The mRNA threads its way along the surface close to the junction of the two subunits. Two tRNA molecules are active in translation at any moment, so polypeptide elongation involves reactions taking place at just 2 of the approximately 10 codons associated with the ribosome. The two tRNAs are inserted into internal binding sites that stretch across the two ribosomal subunits. A third tRNA remains on the ribosome after it has been used in translation before being recycled.

FIGURE .1 The ribosome is large enough to bind several tRNAs and an mRNA.
The basic structure of the ribosome has been conserved during evolution, but there are appreciable variations in the overall size and proportions of RNAs and proteins in the ribosomes of prokaryotes and the eukaryotic cytosol, mitochondria, and chloroplasts. Figure .2 compares the components of bacterial and mammalian ribosomes. Both are ribonucleoprotein particles that contain more RNA than protein. The ribosomal proteins are
known as r-proteins.

FIGURE .2 Ribosomes are large ribonucleoprotein particles that contain more RNA than protein and are composed of a large and a small subunit.
Each of the ribosomal subunits contains a major rRNA and a number of small proteins. The large subunit may also contain smaller RNA(s). In Escherichia coli, the small (30S) subunit consists of the 16S rRNA and 21 r-proteins. The large (50S) subunit contains the 23S rRNA, the small 5S RNA, and 31 rproteins.
With the exception of one protein that is present in four copies per ribosome, there is one copy of each protein. The major RNAs constitute the larger part of the mass of the bacterial ribosome. Their presence is pervasive so that most or all of the rproteins actually contact rRNA. Thus, the major rRNAs form what is sometimes considered the “backbone” of each subunit—a continuous thread whose presence dominates the structure and determines the positions of the ribosomal proteins.The ribosomes in the cytosol of eukaryotes are larger than those of prokaryotes. The total content of both RNA and protein is greater, the major RNA molecules are longer (called 18S and 28S rRNAs), and there are more proteins. RNA is still the predominant component by mass.
The ribosomes of eukaryotic mitochondria and chloroplasts are distinct from the ribosomes of the cytosol, and they take varied forms. In some cases, they are almost the size of prokaryotic ribosomes and have about 70% RNA; in other cases, they are only 60S and have less than 30% RNA.
The ribosome possesses several active centers, each of which is constructed from a group of proteins associated with a region of ribosomal RNA. The active centers require the direct participation of rRNA in a structural or even catalytic role (where the RNA functions as a ribozyme) with proteins supporting these functions in secondary roles. Some catalytic functions require individual proteins, but none of the activities can be reproduced by isolated proteins or groups of proteins; they function only in the context of the ribosome.
Two experimental approaches can be taken in analyzing the functions of structural components of the ribosome. In one approach, the effects of mutations in genes for particular ribosomal proteins or at specific positions in rRNA genes shed light on the participation of these molecules in particular reactions. In a second approach, structural analysis, including direct modification of components of the ribosome and comparisons to identify conserved features in rRNA, identifies the physical locations of components involved in particular functions.