A quantitative electron-microscope analysis of chromatin from Xenopus laevis lampbrush chromosomes

The morphology of the DNP axis and RNP transcripts from Xenopus laevis lampbrush chromosomes has been analysed using a modified Miller spreading technique. Two basictypes of chromatin have been distinguished. (1) Discrete portions of DNP exhibiting high levels of transcriptive activity, with clear initiation and termination points (transcription units). Interspersed with the units are sequences with little or no transcriptive activity (spacer DNP). The combination of transcription unit plus spacer DNP is normally repeared to form a transcriptive array. (2) large aggregates of loosely packed, mainly transcriptively inactive regions (chromomeric DNP). Where the DNP axis is not obscured by tightly packed polymerases it has regular beaded appearance, both in the transcriptive arrays and in chromomeric DNP. The beads are 17-20 nm in diameter and have packing densities of about 40 beads per micron of chromatin in preparations from young females not stimulated by human chorionic gonodotrophin. Most transcription units have polymerase packing densities of 13-20 polymerases per micron, but in some cases the density is much lower (less than 10 polymerases /micron). The lateral RNP transcripts in most units have a complex, bushlike appearance, the axis being beaded and folded to form circles and branches.