中文摘要：

真核生物的基因组以染色质的形式存在，染色质在真核生物的基因表达调控及胚胎发育过程中起重要作用，为表观遗传提供一个重要的信息整合平台．染色质的高级结构，特别是30nnl染色质的动态变化在基因转录沉默和激活过程中起着重要的调控功能．但是目前对30am染色质纤维的组装及其精细结构的认识还十分有限．本文通过体外表达系统，表达未经修饰的组蛋白，并利用克隆构建的601DNA均一重复序列，通过逐步降低盐离子浓度并加入组蛋白H1或镁离子的方法，体外重组均一的30nm染色质纤维．并利用镀金属、负染色制样和冷冻电镜制样等手段通过透射式电子显微镜（TEM）对30nm纤维结构的形成原因、组蛋白H1的作用和核小体重复单位（nucleosomerepeatlengths，NRLs）长度对30nm染色质纤维的影响进行研究．研究结果显示在组蛋白H1或二价镁离子存在的情况下，均可形成30nm染色质纤维．其形成的染色质拓扑结构有所不同．统计分析表明，不同长度核小体重复单位（NRLs）形成的染色质纤维直径有所不同（P〈0．05）．同时，我们得到了较为均一的冷冻电镜样品，为进一步研究30nm染色质纤维的高级结构及理解体内染色质存在的形式及动态过程打下了较好的基础．

英文摘要：

Genomic DNA in the eukaryotic nucleus is hierarchically packaged by histones into chromatin. The plasticity and dynamics of higher-order chromatin fiber have been widely thought as the key regulators of transcription and other biological processes inherent to DNA. Elucidating how nucleosomal arrays can be folded into higher-order chromatin fibers is essential to understand the dynamics of chromatin structure. Although the structure of nucleosomes, the fundamental repeating unit of chromatin, which comprises 147 base pairs of DNA wrapped in 1.7 superhelical turns around an octamer ofhistones, has been solved at the atomic resolution, there is still much controversy over the chromatin structure at the higher-order level. Here, we built an in vitro chromatin reconstitution system which adopts histone octamers and arrays of 177 bp and 200 bp repeat of the Widom 601 DNA sequence. Taking advantage of this system, we have obtained highly regular spaced and soluble nucleosome arrays, and folded the arrays into 30 nm chromatin fibers with the existence of linker histone H1 or MgC12 respectively. Several electron microscopic techniques, including metal shadowing, negative staining and Cryo-EM, have been used to investigate the morphology of the reconstituted 30 nm chromatin fibers. Our results suggest that both histone H1 and divalent Mg2~ can help the formation of 30 nm chromatin fibers, but the resulted chromatin fibers display different topologically architectures. To investigate how the length of linker histone may affect the architecture of chromatin, we measured the diameters of the reconstituted 30 nm chromatin fibers with different nucleosome repeat lengths （NRLs） of 177 and 200 bp and found that these two classes of chromatin fibers present different diameters （P 〈 0.05）.