FIGURE 8-36 Model for mitotic chromosome condensation. (a) Stages of chromosome condensation during mitosis. Changes in large-scale chromatin folding (blue) versus distribution of Smc2, a subunit of condensin (red), from early prophase 1 to middle prophase 2 to late prophase 3 to metaphase 4. (b) Transmission electron micrograph of immunogold staining of Smc2 in a section through a metaphase chromosome reveals axial staining of Smc2 of about 0.15–0.2 µm in width. (c) “Hierarchical folding, axial glue” model of metaphase chromosome structure. (Left) 30-nm fiber folds into 100–130-nm chromonema fiber, which folds into 200–250-nm middle prophase chromatid, which folds into 500–750-nm metaphase chromatid. Only one chromatid is shown. (Right) Axial condensin distribution (red) occupies approximately one-third of the chromatid diameter, acting as a cross-linking “glue” to stabilize the structure of the metaphase chromosome.