10.3 Under the electron microscope, DNA molecules undergoing transcription exhibit Christmas-tree-like structures. (a) Electronic micrograph of Christmas-tree-like structures. (b) The trunk of each “Christmas tree” (a transcription unit) represents a portion of a DNA molecule; the tree branches (granular strings attached to the DNA) are RNA molecules that have been transcribed from the DNA. As the transcription apparatus proceeds down the DNA, transcribing more of the template, the RNA molecules become longer and longer.

[Dr. Thomas Broker/Phototake.]