Principles of Biochemistry Volume 3 - A. Lehninger 1985

Molecular Mechanisms of Genetic Information Transmission
DNA Replication and Transcription
Heterogeneous nuclear RNAs serve as precursors to eukaryotic messenger RNAs

The Processing of Messenger RNA precursors in eukaryotes is a highly complex process. Eukaryotic messenger RNAs found in the Cytoplasm possess three distinct structural features. The first is that eukaryotic mRNAs are typically monocistronic molecules, whereas many prokaryotic mRNAs are polycistronic. The second characteristic property of most eukaryotic mRNAs is that they contain a “tail” of 100 to 200 consecutively attached A residues at their 3' end—the so-called poly(A) tail. This tail is synthesized separately from ATP molecules by polyadenylate polymerase, which Functions essentially in the same manner as RNA polymerase, catalyzing the reaction

Class="center">nАТР → (АМР)_n + nРР_i

Polyadenylate polymerase does not require a template, but it does require mRNA as a primer. The third distinguishing feature of most eukaryotic mRNAs is the presence of a 5'-terminal “cap” (from the English cap), which is a 7-methylguanosine residue attached to the 5'-terminal residue of the mRNA in a very unusual manner—specifically, via a triphosphate linkage (Fig. 28-21). The Functions of the “cap” and the poly(A) tail are not yet precisely known. The cap may be involved in the binding of mRNA to the ribosome, thereby initiating Translation (Sec. 29.10). It is also possible that the cap and the poly(A) tail protect the messenger RNA from enzymatic degradation.