北京大学细胞生物学、细胞核课件.ppt

Nucleus and Chromosomes C. Nuclear pore complex (NPC) D. Molecules enter and exit the nucleus through nuclear pore complex Passive transport—passively diffuse Active transport NLS are present only in nuclear proteins A typical NLS contains several consecutive basic amino acids (the positively charged Aa: Lys, Arg) Nuclear import and export Nuclear import receptors bind NLS and Nucleoporins The Ran GTPase drives directional transport through NPC Nuclear export works like nuclear import, but in reverse hnRNP proteins contain a nuclear-export signal (NES) Reference: Cell 92: 327, 1998 2. Eukaryotes package DNA in  Chromatin and chromosomes Chromosomes exist in different states throughout the life of a cell Chromatin: (Interphase) Fibers, 10-30nm in diameter, Dispersed through the nucleus. DNA+Proteins+RNA. Chromosomes: (M phase) Cell division, these fibers condense and fold into larger, compact structure A. The complexity of eukaryotic genomes DNA sequencing is being applied to whole genomes as well as to genes Protein-coding sequences (Nonrepeated) Tandemly repeated genes encode rRNAs, tRNAs, and Histones (Moderately repeated) Tandemly repeated DNA: Simple-sequence , Highly repeated , Noncoding DNA fingerprinting depends on differences in length of minisatellite or microsatellite DNA Interspersed moderately repeated DNA B. DNA packaging Nucleosomes are the basic unit of chromatin structure Evidence: (1)Electron micrographs of chromatin fibers Evidence: (2)Nuclease digestion (Rat liver chromatin) Evidence: (3)X-ray diffraction studies A histone octamer forms the nucleosome core Histones: Localization of nucleosome is affected by two factors: C. Nucleosomes are packed together to form chromatin fibers and chromosomes Scaffold radial loop structure model Nonhistone proteins