AFM images of G1-phase premature condensed chromosomes: Evidence for 30 nm changed to 50 nm chromatin fibers

To gain evidence for 30 nm changed to 50 nm chromatin fibers, we used atomic force microscopy (AFM) to study the ultrastructural organization of G1-phase premature condensed chromosomes (PCC). The surface of early G1-phase PCC is smooth and fibrous structures exist around the chromatids. The height of early G1-phase PCC is about 410 nm and the width is 1.07 ± 0.11 μm (n = 30). At late G1-phase, the surface becomes globular. The height of late G1-phase PCC is about 370 nm and the width is 845.04 ± 82.84 nm (n = 30). Phase image reveals that early G1-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 G1-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 G1-phase PCC. Chromatin fibers revealed by closer view of the end of chromatid are about 50 nm. In late G1-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 change to 50 nm chromatin fibers in G1-phase PCC and suggest that 50 nm chromatin fibers are the basic component of the mitotic chromosomes.