Visualization of rRNA transcription and processing in eukaryotes. Transcription of rRNA and its assembly into precursor ribosomes can be visualized by electron microscopy. The structures resemble Christmas trees: the trunk is the rDNA and each branch is a pre-rRNA transcript. Transcription starts at the top of the tree, where the shortest transcripts can be seen, and progresses down the rDNA to the end of the gene. The terminal knobs visible at the end of some pre-rRNA transcripts likely correspond to the SSU processome, a large ribonucleoprotein required for processing the pre-rRNA.