False-colour transmission electron micrograph (TEM) of a polyribosome from a human brain cell. Polyribosomes (or polysomes) are beads of individual ribosomes connected by slender strands of messenger RNA (mRNA), one of the three types of ribonucleic acid. They exist as sites of protein synthesis within a cell. In this cell, a neurone, protein is required for internal use in an extensive cytoplasm. mRNA's role is in carrying genetic information from the nucleus to the ribosomes to be translated into an amino acid sequence of specific proteins. Magnification: x240,000 at 6x7cm size.