To gain evidence for 30 nm changed to 50 nm chromatin fibers, we used atomic force microscopy (AFM) to study the ultrastructural organization of G(1)-phase premature condensed chromosomes (PCC). The surface of early G(1)-phase PCC is smooth and fibrous structures exist around the chromatids. The height of early G(1)-phase PCC is about 410 urn and the width is 1.07 +/- 0.11 mu m (n = 30). At I ate G(1)-phase, the surface becomes globular. The height of late G(1)-phase PCC is about 370 nm and the width is 845.04 +/- 82.84 nm (n = 30). Phase image reveals that early G(1)-phase PCC is composed of 50 nm (48.91 +/- 6.63 nm, n = 30) chromatin fibers and these 50 nm chromatin fibers tangle together, while late G(1)-phase PCC is composed of 30 nm (30.96 +/- 4.07 nm, n = 30) chromatin fibers. At high magnification, fibers existing around the chromatids become clear in early G(1)-phase PCC. Chromatin fibers revealed by closer view of the end of chromatid are about 50 nm. In late G(1)-phase PCC, the surface presents globular structures. The shape of these globular structures is regular and the diameter is 118.96 +/- 11.70 nm (n = 30). Our results clearly show that 30 nm chromatin fibers chan-e to 50 nm chromatin fibers in G(1)-phase PCC and suggest that 50 nm chromatin fibers are the basic component of the mitotic chromosomes. (c) 2007 Elsevier B.V. All rights reserved.