The Polyribosome

More than one polypeptide chain is synthesized from an mRNA molecule at any given time. Once a ribosome has begun translocating along the mRNA, the start AUG codon is free, and another ribosome can bind. A second 70S initiation complex forms. Once this ribosome ribosome mRNA

ribosome mRNA

polypeptide chain

Figure 8.8. Many ribosomes reading one mRNA form a polyribosome.

polypeptide chain

Figure 8.8. Many ribosomes reading one mRNA form a polyribosome.

has moved away, a third ribosome can attach to the start codon. This process is repeated until the mRNA is covered with ribosomes. Each of these spans about 80 nucleotides. The resultant structure, the polyribosome or polysome (Fig. 8.8), is visible under the electron microscope. This mechanism allows many protein molecules to be made at the same time on one mRNA.

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