EXPERIMENTAL FIGURE 10-39 Electron micrograph of pre-rRNA transcription units from the nucleolus of a frog oocyte. Each “feather” represents multiple pre-rRNA molecules associated with protein in a pre-ribonucleoprotein complex (pre-rRNP) emerging from a transcription unit. Note the dense “knob” at the 5′ end of each nascent pre-RNP, which is thought to be a processome. Pre-rRNA transcription units are arranged in tandem, separated by nontranscribed spacer regions of nucleolar chromatin.

[Courtesy of Y. Osheim and O. J. Miller, Jr.]