Model of precursor messenger RNA metabolism in a mammalian cell nucleus. Polyadenylated RNA (red) concentrates in topologically arranged centers, which have discrete cores of spliceosome assembly factor SC-35 (yellow). Closely associated with these centers are specific RNA transcript tracks (exons, green; introns, white; and fibronectin gene, lavender) within which both transcription and splicing occur. The dark blue area represents the nucleolus. See the Carter et al. report on page 1330 and the Xing et al. report on page 1326; also see the News story on page 1257. [Art: Ken Carter and John McNeil]

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