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Tham khảo tài liệu 'atomic force microscopy in cell biology episode 2 part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 386 Zuccheri and Samori itself approximately 18 times on average. When not complexed with proteins like in nucleosomes the supercoiled DNA should be in the interwound form which is called plectonemic. What is almost constantly found in the SFM literature is the imaging of DNA plasmids that simply do not appear plectonemic even if the authors describe them as supercoiled. Among other researchers we have seen and reported plasmids that appeared too loosely coiled for their native superhelical density. A comparison between theoretical and computational evaluations would also suggest that plasmids with a native superhelical density should be significantly more coiled under the conditions of SFM imaging Vologodskii 1992 Vologodskii and Cozzarelli 1994 . The shape of supercoiled plasmids in SFM images is often very open with only one or two apparent crossovers. Sometimes it is difficult to distinguish the loosely coiled plasmids from the relaxed plasmids that can have random flops especially if they are very long. At times strange looking supercoiled plasmids have been presented. The superhelical density should be spread homogeneously along the entire length of the molecule where no extended sections have a significantly looser appearance. On the contrary many reports show plasmids with apparently highly coiled sections and other sections which are completely loose Pope et al. 2000 . It is our understanding that this could be due to local condensation conditions which can drive some strand-to-strand contacts not only due to supercoiling. Under the conditions of DNA deposition and dehydration adopted for SFM imaging some condensation could occur especially if enabled by the superhelical tension. These conditions could even drive the molecules toward unnatural shapes. A seriously limited number of EM studies demonstrate nicely interwound plectonemic plasmids among them the cryo-EM studies demonstrate the nicest ones Adrian et al. 1990 . As mentioned earlier the cryo-EM .
