A polyribosome (or polysome) is a complex of an mRNA molecule and two or more ribosomes that is formed during active translation. Originally coined "ergosomes" in 1963 they were further characterized by Jonathan Warner, Paul Knopf, and Alex Rich. Polysomes can be directly visualized by electron microsopy because they form very high molecular weight particles. Many ribosomes simultaneously read one mRNA progressing along the mRNA to synthesize the same protein. They may appear as linear polysomes or circular rosettes with microscopy but are mainly circular in vivo. This circularization is aided by the fact that mRNA is able to be twisted into a circular formation, creating a cycle of rapid ribosome recycling and utilization of ribosomes. The 5' 7-methylguanosine cap and 3' poly(A) tail present on eukaryotic mRNA aid in this process.