11.14 An mRNA molecule may be translated simultaneously by several ribosomes. (a) Four ribosomes are translating an mRNA molecule; the ribosomes are depicted as moving from the 5′ end to the 3′ end of the mRNA. (b) In this electron micrograph, the long horizontal filament is DNA, the dark-staining spheres are polyribosomes, and the thin filaments connecting the ribosomes are mRNAs. Transcription of the DNA is proceeding from right to left; the mRNAs on the right are shorter than those on the left. Each mRNA is being translated by multiple ribosomes.

[Part b: From O. L. Miller, Jr., B. A. Hamkalo, and C. A. Thomas, Jr. Science 169 (1970):392. Reprinted with permission from AAAS.]