77. Kireeva, M.L., Walter, W., Tchernajenko, V., Bondarenko, V., Kashlev, M., and Studitsky, V.M. (2002). Nucleosome remodeling induced by RNA polymerase II: loss of the H2A/H2B dimer during transcription. *Mol Cell* **9**, 541-552.

78. Klug, A., and Lutter, L.C. (1981). The helical periodicity of DNA on the nucleosome. *Nucleic Acids Res* **9**, 4267-4283.

79. Kossel, A. (1884). Über einen peptoartigen bestanheil des zellkerns. *Z Physio Chem* **8**, 511-515.

80. Le Bret, M. (1988). Computation of the helical twist of nucleosomal DNA. *J Mol Biol* **200**, 285-290.

81. Lermontova, I., Schubert, V., Fuchs, J., Klatte, S., Macas, J., and Schubert, I. (2006). Loading of Arabidopsis Centromeric Histone CENH3 Occurs Mainly during G2 and Requires the Presence of the Histone Fold Domain. *Plant Cell* **18**, 2443-2451.

82. Leuba, S.H., Bustamante, C., Zlatanova, J., and van Holde, K. (1998). Contributions of linker histones and histone H3 to chromatin structure: scanning force microscopy studies on trypsinized fibers. *Biophys J* **74**, 2823-2829.

83. Leuba, S.H., Yang, G., Robert, C., Samori, B., van Holde, K., Zlatanova, J., and Bustamante, C. (1994). Three-dimensional structure of extended chromatin fibers as revealed by tapping-mode scanning force microscopy. *Proc Natl Acad Sci U S A* **91**, 11621-11625.

84. Lewin, B. (2004). Genes VIII (Pearson Prentice Hall).

85. Li, A., Eirin-Lopez, J.M., and Ausio, J. (2005). H2AX: tailoring histone H2A for chromatin-dependent genomic integrity. *Biochem Cell Biol* **83**, 505-515.

86. Liu, L.F., and Wang, J.C. (1987). Supercoiling of the DNA template during transcription. *Proc Natl Acad Sci U S A* **84**, 7024-7027.

87. Liu, S.T., Rattner, J.B., Jablonski, S.A., and Yen, T.J. (2006). Mapping the assembly pathways that specify formation of the trilaminar kinetochore plates in human cells. *J Cell Biol* **175**, 41-53.

88. Lo, A.W., Craig, J.M., Saffery, R., Kalitsis, P., Irvine, D.V., Earle, E., Magliano, D.J., and Choo, K.H. (2001). A 330 kb CENP-A binding domain and altered replication timing at a human neocentromere. *Embo J* **20**, 2087-2096.
89. Lomvardas, S., and Thanos, D. (2002). Modifying gene expression programs by altering core promoter chromatin architecture. *Cell* **110**, 261-271.

90. Lowary, P.T., and Widom, J. (1997). Nucleosome packaging and nucleosome positioning of genomic DNA. *Proc Natl Acad Sci U S A* **94**, 1183-1188.

91. Lowary, P.T., and Widom, J. (1998). New DNA sequence rules for high affinity binding to histone octamer and sequence-directed nucleosome positioning. *J Mol Biol* **276**, 19-42.

92. Lu, X., and Hansen, J.C. (2003). Revisiting the structure and functions of the linker histone C-terminal tail domain. *Biochem Cell Biol* **81**, 173-176.

93. Lu, Z.H., Xu, H., and Leno, G.H. (1999). DNA replication in quiescent cell nuclei: regulation by the nuclear envelope and chromatin structure. *Mol Biol Cell* **10**, 4091-4106.

94. Luger, K., and Hansen, J.C. (2005). Nucleosome and chromatin fiber dynamics. *Curr Opin Struct Biol* **15**, 188-196.

95. Luger, K., Mader, A.W., Richmond, R.K., Sargent, D.F., and Richmond, T.J. (1997). Crystal structure of the nucleosome core particle at 2.8 Å resolution. *Nature* **389**, 251-260.

96. Lusser, A., and Kadonaga, J.T. (2003). Chromatin remodeling by ATP-dependent molecular machines. *Bioessays* **25**, 1192-1200.

97. Malik, H.S., and Henikoff, S. (2003). Phylogenomics of the nucleosome. *Nat Struct Biol* **10**, 882-891.

98. Masumoto, H., Masukata, H., Muro, Y., Nozaki, N., and Okazaki, T. (1989). A human centromere antigen (CENP-B) interacts with a short specific sequence in alphoid DNA, a human centromeric satellite. *J Cell Biol* **109**, 1963-1973.

99. Masumoto, H., Nakano, M., and Ohzeki, J. (2004). The role of CENP-B and alpha-satellite DNA: de novo assembly and epigenetic maintenance of human centromeres. *Chromosome Res* **12**, 543-556.

100. Meneghini, M.D., Wu, M., and Madhani, H.D. (2003). Conserved histone variant H2A.Z protects euchromatin from the ectopic spread of silent heterochromatin. *Cell* **112**, 725-736.
101. Morales, V., and Richard-Foy, H. (2000). Role of histone N-terminal tails and their acetylation in nucleosome dynamics. *Mol Cell Biol* **20**, 7230-7237.

102. Moreno-Moreno, O., Torras-Llort, M., and Azorin, F. (2006). Proteolysis restricts localization of CID, the centromere-specific histone H3 variant of Drosophila, to centromeres. *Nucleic Acids Res* **34**, 6247-6255.

103. Morillon, A. (2006). Is histone loss a common feature of DNA metabolism regulation? *Biochem Cell Biol* **84**, 450-462.

104. Moroi, Y., Peebles, C., Fritzler, M.J., Steigerwald, J., and Tan, E.M. (1980). Autoantibody to centromere (kinetochore) in scleroderma sera. *Proc Natl Acad Sci U S A* **77**, 1627-1631.

105. Noll, M., and Kornberg, R.D. (1977). Action of micrococcal nuclease on chromatin and the location of histone H1. *J Mol Biol* **109**, 393-404.

106. O'Donohue, M.F., Duband-Goulet, I., Hamiche, A., and Prunell, A. (1994). Octamer displacement and redistribution in transcription of single nucleosomes. *Nucleic Acids Res* **22**, 937-945.

107. Okada, M., Cheeseman, I.M., Hori, T., Okawa, K., McLeod, I.X., Yates, J.R., 3rd, Desai, A., and Fukagawa, T. (2006). The CENP-H-I complex is required for the efficient incorporation of newly synthesized CENP-A into centromeres. *Nat Cell Biol* **8**, 446-457.

108. Ouspenski, II, Van Hooser, A.A., and Brinkley, B.R. (2003). Relevance of histone acetylation and replication timing for deposition of centromeric histone CENP-A. *Exp Cell Res* **285**, 175-188.

109. Palmer, D.K., O'Day, K., Trong, H.L., Charbonneau, H., and Margolis, R.L. (1991). Purification of the centromere-specific protein CENP-A and demonstration that it is a distinctive histone. *Proc Natl Acad Sci U S A* **88**, 3734-3738.

110. Palmer, D.K., O'Day, K., Wener, M.H., Andrews, B.S., and Margolis, R.L. (1987). A 17-kD centromere protein (CENP-A) copurifies with nucleosome core particles and with histones. *J Cell Biol* **104**, 805-815.

111. Parseghian, M.H., and Hamkalo, B.A. (2001). A compendium of the histone H1 family of somatic subtypes: an elusive cast of characters and their characteristics. *Biochem Cell Biol* **79**, 289-304.
112. Pehrson, J.R. (2004). Core histone variants. In New comprehensive biochemistry, Volume 39 Chromatin structure and dynamics: State-of-the-art, Elsevier Edition, J. Zlatanova and S.H. Leuba, eds. pp. 181-204.

113. Pehrson, J.R., and Fried, V.A. (1992). MacroH2A, a core histone containing a large nonhistone region. *Science* **257**, 1398-1400.

114. Peterson, C.L. (2000). ATP-dependent chromatin remodeling: going mobile. *FEBS Lett* **476**, 68-72.

115. Peterson, C.L., and Laniel, M.A. (2004). Histones and histone modifications. *Curr Biol* **14**, R546-551.

116. Phair, R.D., Scaffidi, P., Elbi, C., Vecerova, J., Dey, A., Ozato, K., Brown, D.T., Hager, G., Bustin, M., and Misteli, T. (2004). Global nature of dynamic protein-chromatin interactions in vivo: three-dimensional genome scanning and dynamic interaction networks of chromatin proteins. *Mol Cell Biol* **24**, 6393-6402.

117. Prunell, A. (1998). A topological approach to nucleosome structure and dynamics: the linking number paradox and other issues. *Biophys J* **74**, 2531-2544.

118. Prunell, A., and Kornberg, R.D. (1978). Relation of nucleosomes to DNA sequences. *Cold Spring Harb Symp Quant Biol* **42** Pt 1, 103-108.

119. Rajagopalan, H., and Lengauer, C. (2004). Aneuploidy and cancer. *Nature* **432**, 338-341.

120. Ramakrishnan, V., Finch, J.T., Graziano, V., Lee, P.L., and Sweet, R.M. (1993). Crystal structure of globular domain of histone H5 and its implications for nucleosome binding. *Nature* **362**, 219-223.

121. Randall, S.K., and Kelly, T.J. (1992). The fate of parental nucleosomes during SV40 DNA replication. *J Biol Chem* **267**, 14259-14265.

122. Razin, A. (1998). CpG methylation, chromatin structure and gene silencing-a three-way connection. *Embo J* **17**, 4905-4908.

123. Rhodes, D. (1985). Structural analysis of a triple complex between the histone octamer, a Xenopus gene for 5S RNA and transcription factor IIIA. *Embo J* **4**, 3473-3482.
124. Richmond, T.J., Finch, J.T., Rushton, B., Rhodes, D., and Klug, A. (1984). Structure of the nucleosome core particle at 7 Å resolution. *Nature* **311**, 532-537.

125. Riley, D., and Weintraub, H. (1978). Nucleosomal DNA is digested to repeats of 10 bases by exonuclease III. *Cell* **13**, 281-293.

126. Robinson, P.J., Fairall, L., Huynh, V.A., and Rhodes, D. (2006). EM measurements define the dimensions of the "30-nm" chromatin fiber: evidence for a compact, interdigitated structure. *Proc Natl Acad Sci U S A* **103**, 6506-6511.

127. Saenger, W. (1988). Principle of nucleic acid structure, Springer-Verlag Edition (NY).

128. Saha, A., Wittmeyer, J., and Cairns, B.R. (2006). Chromatin remodelling: the industrial revolution of DNA around histones. *Nat Rev Mol Cell Biol* **7**, 437-447.

129. Saragosti, S., Moyne, G., and Yaniv, M. (1980). Absence of nucleosomes in a fraction of SV40 chromatin between the origin of replication and the region coding for the late leader RNA. *Cell* **20**, 65-73.

130. Sarma, K., and Reinberg, D. (2005). Histone variants meet their match. *Nat Rev Mol Cell Biol* **6**, 139-149.

131. Schalch, T., Duda, S., Sargent, D.F., and Richmond, T.J. (2005). X-ray structure of a tetranucleosome and its implications for the chromatin fibre. *Nature* **436**, 138-141.

132. Schwarz, P.M., Felthauser, A., Fletcher, T.M., and Hansen, J.C. (1996). Reversible oligonucleosome self-association: dependence on divalent cations and core histone tail domains. *Biochemistry* **35**, 4009-4015.

133. Schwarz, P.M., and Hansen, J.C. (1994). Formation and stability of higher order chromatin structures. Contributions of the histone octamer. *J Biol Chem* **269**, 16284-16289.

134. Shelby, R.D., Monier, K., and Sullivan, K.F. (2000). Chromatin assembly at kinetochores is uncoupled from DNA replication. *J Cell Biol* **151**, 1113-1118.

135. Shelby, R.D., Vafa, O., and Sullivan, K.F. (1997). Assembly of CENP-A into centromeric chromatin requires a cooperative array of nucleosomal DNA contact sites. *J Cell Biol* **136**, 501-513.
136. Shi, Y., Lan, F., Matson, C., Mulligan, P., Whetstine, J.R., Cole, P.A., and Casero, R.A. (2004). Histone demethylation mediated by the nuclear amine oxidase homolog LSD1. *Cell* **119**, 941-953.

137. Shogren-Knaak, M., Ishii, H., Sun, J.M., Pazin, M.J., Davie, J.R., and Peterson, C.L. (2006). Histone H4-K16 acetylation controls chromatin structure and protein interactions. *Science* **311**, 844-847.

138. Shore, D., and Baldwin, R.L. (1983a). Energetics of DNA twisting. I. Relation between twist and cyclization probability. *J Mol Biol* **170**, 957-981.

139. Shore, D., and Baldwin, R.L. (1983b). Energetics of DNA twisting. II. Topoisomer analysis. *J Mol Biol* **170**, 983-1007.

140. Shrader, T.E., and Crothers, D.M. (1989). Artificial nucleosome positioning sequences. *Proc Natl Acad Sci U S A* **86**, 7418-7422.

141. Shrader, T.E., and Crothers, D.M. (1990). Effects of DNA sequence and histone-histone interactions on nucleosome placement. *J Mol Biol* **216**, 69-84.

142. Shure, M., and Vinograd, J. (1976). The number of superhelical turns in native virion SV40 DNA and minicol DNA determined by the band counting method. *Cell* **8**, 215-226.

143. Simpson, R.T. (1978). Structure of the chromatosome, a chromatin particule containing 160 pb of DNA and all histones. *Biochemistry* **17**, 5524-5531.

144. Simpson, R.T., and Stafford, D.W. (1983). Structural features of a phased nucleosome core particle. *Proc Natl Acad Sci U S A* **80**, 51-55.

145. Sivolob, A., De Lucia, F., Revet, B., and Prunell, A. (1999). Nucleosome dynamics. II. High flexibility of nucleosome entering and exiting DNAs to positive crossing. An ethidium bromide fluorescence study of mononucleosomes on DNA minicircles. *J Mol Biol* **285**, 1081-1099.

146. Sivolob, A., and Prunell, A. (2000). Nucleosome dynamics V. Ethidium bromide versus histone tails in modulating ethidium bromide-driven tetrasome chiral transition. A fluorescence study of tetrasomes on DNA minicircles. *J Mol Biol* **295**, 41-53.
147. Sivolob, A., De Lucia, F., Alilat, M., and Prunell, A. (2000). Nucleosome dynamics. VI. Histone tail regulation of tetrasome chiral transition. A relaxation study of tetrasomes on DNA minicircles. *J Mol Biol* **295**, 55-69.

148. Sivolob, A., Lavelle, C., and Prunell, A. (2003). Sequence-dependent nucleosome structural and dynamic polymorphism. Potential involvement of histone H2B N-terminal tail proximal domain. *J Mol Biol* **326**, 49-63.

149. Sivolob, A., and Prunell, A. (2003). Linker histone-dependent organization and dynamics of nucleosome entry/exit DNAs. *J Mol Biol* **331**, 1025-1040.

150. Sivolob, A., and Prunell, A. (2004). Nucleosome conformational flexibility and implications for chromatin dynamics. *Philos Transact A Math Phys Eng Sci* **362**, 1519-1547.

151. Smith, S., and Stillman, B. (1991). Stepwise assembly of chromatin during DNA replication in vitro. *Embo J* **10**, 971-980.

152. Strahl, B.D., and Allis, C.D. (2000). The language of covalent histone modifications. *Nature* **403**, 41-45.

153. Strick, T.R., Allemand, J.F., Bensimon, D., Bensimon, A., and Croquette, V. (1996). The elasticity of a single supercoiled DNA molecule. *Science* **271**, 1835-1837.

154. Studitsky, V.M., Clark, D.J., and Felsenfeld, G. (1994). A histone octamer can step around a transcribing polymerase without leaving the template. *Cell* **76**, 371-382.

155. Studitsky, V.M., Kassavetis, G.A., Geiduschek, E.P., and Felsenfeld, G. (1997). Mechanism of transcription through the nucleosome by eukaryotic RNA polymerase. *Science* **278**, 1960-1963.

156. Subirana, J.A. (1990). Analysis of the charge distribution in the C-terminal region of histone H1 as related to its interaction with DNA. *Biopolymers* **29**, 1351-1357.

157. Sullivan, B., and Karpen, G. (2001). Centromere identity in Drosophila is not determined in vivo by replication timing. *J Cell Biol* **154**, 683-690.

158. Sullivan, B.A., and Karpen, G.H. (2004). Centromeric chromatin exhibits a histone modification pattern that is distinct from both euchromatin and heterochromatin. *Nat Struct Mol Biol* **11**, 1076-1083.
159. Sullivan, K.F., Hechenberger, M., and Masri, K. (1994). Human CENP-A contains a histone H3 related histone fold domain that is required for targeting to the centromere. *J Cell Biol* **127**, 581-592.

160. Swigon, D., Coleman, B.D., and Tobias, I. (1998). The elastic rod model for DNA and its application to the tertiary structure of DNA minicircles in mononucleosomes. *Biophys J* **74**, 2515-2530.

161. Tagami, H., Ray-Gallet, D., Almouzni, G., and Nakatani, Y. (2004). Histone H3.1 and H3.3 complexes mediate nucleosome assembly pathways dependent or independent of DNA synthesis. *Cell* **116**, 51-61.

162. Takami, Y., and Nakayama, T. (1997). One allele of the major histone gene cluster is enough for cell proliferation of the DT40 chicken B cell line. *Biochim Biophys Acta* **1354**, 105-115.

163. Thoma, F., Koller, T., and Klug, A. (1979). Involvement of histone H1 in the organization of the nucleosome and of the salt-dependent superstructures of chromatin. *J Cell Biol* **83**, 403-427.

164. Tomonaga, T., Matsushita, K., Yamaguchi, S., Oohashi, T., Shimada, H., Ochiai, T., Yoda, K., and Nomura, F. (2003). Overexpression and mistargeting of centromere protein-A in human primary colorectal cancer. Cancer Res 63, 3511-3516.

165. Toth, K., Brun, N., and Langowski, J. (2006). Chromatin compaction at the mononucleosome level. *Biochemistry* **45**, 1591-1598.

166. Travers, A. (1999). The location of the linker histone on the nucleosome. *Trends Biochem Sci* **24**, 4-7.

167. Travers, A.A., and Owen-Hughes, T. (2004). Nucleosome remodeling. In New comprehensive biochemistry, Volume 39 Chromatin structure and dynamics: State-of-the-art, Elsevier Edition, J. Zlatanova and S.H. Leuba, eds. pp. 421-466.

168. Trifonov, E.N., and Sussman, J.L. (1980). The pitch of chromatin DNA is reflected in its nucleotide sequence. *Proc Natl Acad Sci U S A* **77**, 3816-3820.

169. Tsunaka, Y., Kajimura, N., Tate, S., and Morikawa, K. (2005). Alteration of the nucleosomal DNA path in the crystal structure of a human nucleosome core particle. *Nucleic Acids Res* **33**, 3424-3434.
170. Turner, B.M. (1993). Decoding the nucleosome. *Cell* **75**, 5-8.

171. van Holde, K.E. (1988). Chromatin (NY: Springer Verlag).

172. Verheijen, R., van Venrooij, W., and Ramaekers, F. (1988). The nuclear matrix: structure and composition. *J Cell Sci* **90** (Pt 1), 11-36.

173. Verreault, A. (2000). De novo nucleosome assembly: new pieces in an old puzzle. *Genes Dev* **14**, 1430-1438.

174. Vicent, G.P., Nacht, A.S., Smith, C.L., Peterson, C.L., Dimitrov, S., and Beato, M. (2004). DNA instructed displacement of histones H2A and H2B at an inducible promoter. *Mol Cell* **16**, 439-452.

175. Vila, R., Ponte, I., Collado, M., Arrondo, J.L., and Suau, P. (2001). Induction of secondary structure in a COOH-terminal peptide of histone H1 by interaction with the DNA: an infrared spectroscopy study. *J Biol Chem* **276**, 30898-30903.

176. Walter, W., and Studitsky, V.M. (2001). Facilitated transcription through the nucleosome at high ionic strength occurs via a histone octamer transfer mechanism. *J Biol Chem* **276**, 29104-29110.

177. Wang, J.C. (1996). DNA topoisomerases. *Annu Rev Biochem* **65**, 635-692.

178. White, J.H. (1969). Self-linking and the Gauss integral in higher dimensions. *Am J Maths* **91**, 693-728.

179. Wisniewski, J.R., Zougman, A., Krueger, S., and Mann, M. (2006). Mass spectrometric mapping of linker histone H1 variants reveals multiple acetylations, methylations and phosphorylation as well as differences between cell culture and tissue. *Mol Cell Proteomics*.

180. Woodcock, C.L., Grigoryev, S.A., Horowitz, R.A., and Whitaker, N. (1993). A chromatin folding model that incorporates linker variability generates fibers resembling the native structures. *Proc Natl Acad Sci U S A* **90**, 9021-9025.

181. Wu, H.M., and Crothers, D.M. (1984). The locus of sequence-directed and protein-induced DNA bending. *Nature* **308**, 509-513.

182. Yoda, K., Ando, S., Morishita, S., Houmura, K., Hashimoto, K., Takeyasu, K., and Okazaki, T. (2000). Human centromere protein A (CENP-A) can replace histone H3 in nucleosome reconstitution in vitro. *Proc Natl Acad Sci U S A* **97**, 7266-7271.
183. Zhou, Y.B., Gerchman, S.E., Ramakrishnan, V., Travers, A., and Muyldermans, S. (1998). Position and orientation of the globular domain of linker histone H5 on the nucleosome. *Nature* **395**, 402-405.

184. Zinkowski, R.P., Meyne, J., and Brinkley, B.R. (1991). The centromere-kinetochore complex: a repeat subunit model. *J Cell Biol* **113**, 1091-1110.

185. Zivanovic, Y., Goulet, I., Revet, B., Le Bret, M., and Prunell, A. (1988). Chromatin reconstitution on small DNA rings. II. DNA supercoiling on the nucleosome. *J Mol Biol* **200**, 267-290.

186. Zlatanova, J., and Leuba, S.H. (2003). Chromatin fibers, one-at-a-time. *J Mol Biol* **331**, 1-19.

187. Zofall, M., Persinger, J., Kassabov, S.R., and Bartholomew, B. (2006). Chromatin remodeling by ISW2 and SWI/SNF requires DNA translocation inside the nucleosome. *Nat Struct Mol Biol* **13**, 339-346.
<figure><img src="image_120.png" /></figure>
<figure><img src="image_121.png" /></figure>
# RESUME

Ce mémoire présente les résultats que j’ai obtenus pendant ces années passées au laboratoire de *Biochimie de la Chromatine*, le plus souvent en collaboration avec d’autres laboratoires en France, en Ukraine et aux Etats-Unis.

Deux questions m’ont particulièrement intéressée :

*Structure et dynamique du nucléosome centromérique.* Le centromère, qui permet la ségrégation des chromatides sœurs pendant la mitose, est un des plus gros complexes nucléaires. Alors qu’on connaît maintenant beaucoup des protéines qui le constituent, on ne savait pratiquement rien sur la structure et la dynamique du nucléosome centromérique, qui a la particularité de contenir un variant de H3 : CENP-A. La technique originale des minicercles, développée au laboratoire depuis de nombreuses années, a été ici irremplaçable. Les résultats, inattendus, nous ont permis de donner un élément de réponse à la question centrale du mécanisme de l’adressage de ce variant d’histone au centromère (article I).

*Réponse en rotation d'une fibre de chromatine.* La réponse en traction d'une fibre de chromatine est relativement bien connue. Mais nous sommes les premiers à avoir étudié sa réponse en rotation, grâce à un système de pinces magnétiques. Les résultats ont permis de généraliser à la fibre de chromatine les notions d'états conformationnels du nucléosome (article II) et de transition chirale du tétrasome (un constituant du nucléosome), initialement mises au jour grâce au système des minicercles. On a ainsi montré que la transition chirale pouvait concerner le nucléosome entier, et pas seulement le tétrasome (article III).

# ABSTRACT

In this report, I presented the results I obtained during the time I spent in the laboratory of *Biochimie de la Chromatine*, often in collaboration with other laboratories in France, in Ukraine and in the United States.

Two questions interested me most:

*Structure and function of the centromeric nucleosome.* The centromere, which allows chromatids segregation during mitosis, is one of the biggest nuclear complexes. Many of the centromere component proteins have been identified but little is known about the structure and dynamics of the centromeric nucleosome. The original “minicercle approach”, developed in this lab in the last 20 years, was invaluable in this study. The results were unexpected and gave an answer to the question of the exclusive localization of CENP-A in the centromere (article I).

*Torsional manipulation of chromatin fibers.* The response of chromatin fibers under traction has been investigated for some time. But we are the first to have studied their response under torsion in a magnetic tweezers set-up. The notions of nucleosome access to different conformational states (article II) and of tetrasome chiral transition, initially described using the “minicercle approach”, could be extended to the chromatin fiber. Moreover, we showed that the chiral transition concerned the whole nucleosome, and not only the tetrasome (article III).