Ribosomes are complex molecular machines that translate the information
encoded in mRNA into proteins. The translation process consists of three
substeps: initiation, elongation, and
termination. During initiation, the ribosome assembles at the 5' end of the mRNA
and then starts to move along the mRNA, translating one codon after another.
During each elongation step,
the ribosome binds and selects a tRNA molecule, performs some proof reading,
catalyzes the formation of the next peptide bond, and then translocates the mRNA
by one codon.
In general, several ribosomes are simultaneously loaded onto the same mRNA and then form
a polysome.
In a simplified description of this process,
each mRNA represents a one-dimensional lattice of codons, the ribosomes enter this lattice at the 5' end with a certain initiation rate, move processively towards the 3' end, and then dissociate from the latter end. It turns out, however, that the life time of the mRNA is often comparable to the
overall translation time, which implies that the polysome does not attain a steady state
[1].
This aging effect leads to translation rates that decrease with increasing mRNA length, in agreement with experimental data on E. coli
[2].
|
|
