Fig. 9. A model of the three-dimensional organization of an active rDNA gene within a fibrillar center. (A) Spread of a `Christmas tree' drawn at scale. (B) For simplicity, only the rDNA gene covered with 180 RPI molecules is presented; it is folded into four identical loops. (C) Each loop (boxed in B) folds into a separate coil. It consists of two identical rows (200 nm in length) of small loops (60 nm in length) (brackets) that are covered with 3-4 RPI molecules. (D,E) A coil is obtained by bending the small loops on the matrix of a cylinder, 60 nm in diameter and 200 nm in length. In this case, it is composed of a stack of six open rings, 30 nm in thickness (brackets on D). (F,G) Upper and side views of a ring: each ring is composed of two twines, 60 nm long; the arrows in (F) point to the two openings of a ring. (H,I) For convenience, the whole length of a rDNA gene has been wrapped into four identical coils (H, side view; I, perspective view). Experimentally, each cluster was composed of three to five coils, whose lengths were variable. In our model, this disparity could be taken into account by placing more (or less) rings in each coil. (J) A classically stained, ultrathin section was merged with a cross-section of the model shown in (H and I) at the same scale. Cross-sections of the coils, localized within the cortex of FC, appear as open rings without rRNA molecules (on the left) or with elongating rRNAs molecules emerging from the convex face of the coil and entering the surrounding DFC. Bar, 400 nm (A), 100 nm (B) and 50 nm (C-J).