Transcript Part2
The Transport of Molecules into and out of the Nucleus • Through an aqueous channels whose diamenter is adjustable • Bidirection: import and export • Active and passive transportation • Localization signal dependnet NPC Nuclear Localization Signals,NLS microinjection T-antigen Steroid hormone Nuclear Export Signals, NES mRNP The HIV Rev protein, which contains a NES, can override the restriction against transporting premRNAs with unspliced splice sites • Proteins imported to or exported from the nucleus contain NLS or NES. Proteins that shuttle between the nucleus and cytoplasm contain both signals. • Pre-mRNAs bound by a spliceosome normally are not exported from the nucleus, assuring that only fully processed, functional mRNAs reach the cytoplasm for translation. • Macromolecules are actively transported into and out of the nucleus through nuclear pores. Differences between nuclear pores transportation and transmembrane transportation • It occurs through a large, regulated aqueous pore rather than through a protein transporter that spans one or more lipid bilayers. • It is thought that a nuclear protein is transported through the pores while it is in a fully folded conformation; by contrast, proteins have to be unfolded during their transport into other organelles. • The localization signal will not be cut off after transportation. This is indispensable for nuclear reconstruction following mitosis. Nucleolus • The appearance of the nucleolus changes drammatically during the cell cycle. nucleolus changes rRNA of nucleolus • Nucleolar fusion • The ribosomal RNA gene are located near the tips of each of 5 different chromosomes (NORs) • cell contains multiple copies of the rRNA genes • Because of their repeating arrangement, and because they are transcribed at a very high rate, the tandem arrays of rRNA genes can easily be seen in spread chromatin preparations. • The rRNA genes are transcribed by RNA polymerase I, and each gene produces the same primary RNA transcript. In humans this primary RNA transcript, known as 45S rRNA. • Before it leaves the nucleus in assembled ribosomal particles, the 45S rRNA is cleaved to give one copy each of the 28S rRNA, the 18S rRNA, and the 5.8S rRNA of the final ribosome. • The derivation of these three rRNAs from the same primary transcript ensures that they will be made in equal quantities. • The remaining part of each primary transcript is degraded in the nucleus. • The 5S rRNA genes are transcribed by RNA polymerase III (a number of other genes encoding small stable RNAs, most notably the transfer RNA [tRNA] genes, are also transcribed by RNA polymerase III ) Procedure of ribosome assemblage • the intact 45S transcript is first packaged into a large complex containing many different proteins imported from the cytoplasm. • These components remain in the nucleolus when the ribosomal subunits are exported to the cytoplasm in finished form. • As the 45S rRNA molecule is processed, it gradually loses some of its RNA and protein and then splits to form separate precursors of the large and small ribosomal subunits. • 18S rRNA integrated into small ribosomal subunits, and 28s,5.8S,5S rRNA integrated into large ribosomal subunits. • The last steps in ribosome maturation occur only as these subunits are transferred to the cytoplasm. (This delay prevents functional ribosomes from gaining access to the incompletely processed hnRNA molecules in the nucleus.) 染色体 染色体DNA 复制源：DNA复制起始点 使染色体能够进行复制 着丝粒：姐妹染色单体连接部位 细胞分裂时使两拷贝分离 端 粒：染色体末端 保证DNA复制的完整性 染色体蛋白 组蛋白 H2A H2B H3 H4—核小体 H1 — 染色体高级结构 非组蛋白 染色体组装：不同结构水平 染色质的种类：常染色质和异染色质的区别 总结3 核被膜 外核膜 结构：与内质网膜连续 表面有骨架成分分布 功能：与蛋白合成有关 总结1 内核膜 分布：内核膜内表面 核纤层 组成：核纤层蛋白 功能：间期 染色质锚定 分裂期 调控核膜 核间隙 核孔 结构：核孔复合体 功能：核内外物质转运 大分子 小分子 核基质 结构：核骨架 功能： 核仁 结构：三个特征性功能区 纤维中心 rDNA 总结2 （核仁组织者） 原纤维成分 rRNA 颗粒成分 成熟的rRNA前体 功能：rRNA 合成、加工 核糖体装配