Chapter 9 The cell Nucleus Xiamixinuer Yilike 1

Chapter 9 The cell Nucleus Xiamixinuer·Yilike 1

Chapter 9 Nucleus Learning Objectives 1. Mastering: ultrastructure of nuclear envelop; nuclear pore complex; composition and four levels organization of chromatin; packaging of chromatin; types of chromatin. 2. Comprehending: structure and function of nucleolus; function of nuclear pore complex; process of RNA processing. 3. Understanding: basic function of nucleus; nuclear matrix. 2

1. The nucleus: Nuclear envelope and NPC A. Structure: Double-membrane nuclear envelope surrounds the nucleus Structure of the interphase nucleus 3

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The main functions of nucleus 1. Carry genetic information（DNA）; 2. Duplicate, transcript of genetic information and control protein synthesis; 3. Regulation and control centre of living action of cells. 5

¡ NP(nucleoplasmic index) NP= Vn/(Vc－Vn) ¡ Usually size of nucleus can be estimated by calculate the NP. ¡ Normal cell NP≈0. 5， ¡ Dividing cell NP>0. 5， ¡ Aging cell NP<0. 5。 6

A typical nonmitotic nucleus includes four major components. nuclear envelope nucleoluos chromatin matrix 7

B. The nuclear envelope consists of two membranes by a perinuclear space. 8

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v. The inner surface of the nuclear envelope is lined by the nuclear lamina ØThe nuclear lamina supports the nuclear envelope: Gives shape and stability of nuclear envelope; Provides a structure link between chromatin and nuclear envelope; ØThe nuclear lamina is composed of lamins. ØThe integrity of the nuclear lamina is regulated by phosphorylation and dephosphorylation. 11

Breakdown and reformation of nuclear envelope during mitosis 12

The structures of nuclear envelope outer nuclear membrane inner nuclear membrane perinuclear space nuclear pores nuclear lamina 13

C. Nuclear pore complex (NPC) 14

The structure of Nuclear pore complex (NPC) 15

Ring subunit： 8 pair Annular subunit： 8 Old model structure Central plug： 1 fibril： 16

nuclear lamina pore outer 蛋 membrane Perinuclear space Inner membrane B B B B A C A C A C lamina A. B. C A C 17

D. Molecules enter and exit the nucleus through nuclear pore complex 18 Bidirectional traffic

Molecules enter and exit the nucleus through nuclear pore complex 19

v Passive transport—passively diffuse Ø 3000 -4000 NPC/cell(mammalian); To import about 106 histone/3 mins. (DNA-sythesizing cell) = 100 histone/ min/NPC ØEach NPC contains one or more open aqueous channels: 9 nm in diameter and 15 nm long ØThe effective size of these channels has been determined by injecting various sizes of colloidal gold particles and examined by electron microscopy. <10 nm in diameter <60 kd globular protein Able to enter the nucleus 20

v Active transport Pore diameter is about 9 nm but much larger objects, including ribosomal subunits can pass through (albeit slower than smaller molecules). This implies that recognition of appropriate signal allows temporary stretching of the pore. Energy is required. ØTransport of large proteins into nucleus needs nuclear localization signal (NLS) 21

Transport of m. RNA out of the nucleus 22

Chapter 9 section 2 Nucleolus and Chromosomes Learning Objectives (1) The components of chromatin and packaging of chromosome (2) Nucleolus. 23

Chromatin fibe human nucleus under LM human lymphocyte nucleus under SEM 24

Eukaryotes package DNA in Chromatin and chromosomes ¡ Each human cell contains about 2 m of DNA within nucleus if stretched end-toend, yet the nucleus of a human cell itself is only about 6 μm in diameter. ¡ Compaction ratio=nearly 10000 -fold. ¡ (Chromosome 22: DNA 1. 5 cm 2 μ m) 25

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q. Chromosomes exist in different states throughout the life of a cell. Chromatin: (Interphase) Fibers, 10 -30 nm in diameter, dispersed through the nucleus. DNA+Proteins+non-Proteins+RNA. Chromosomes: (M phase) Cell division, these fibers condense and fold into larger, compact structure. 27

Levels of organization of chromatin 28

B. Nucleosomes are packed together to form chromatin fibers and chromosomes v 4 degree folding model Nonhistone proteins provide a structural scaffold for long chromatin loops. 2 nm DNA – 11 nm nucleosome – 30 nm fiber – Loop– Metaphase chromosome 29

4 degree folding model ¡ First degree: Nucleosomes ¡ Second degree: Filament 10 nm in diameter ¡ Third level: Fiber, 30 nm in diameter ¡ Fourth level: Chromosome 30

A. DNA packaging: First degree of packaging is Nucleosomes are the basic unit of chromatin structure. 31

Ø Evidence: (1)Electron micrographs of chromatin fibers Isolated from interphase nucleus: 30 nm thick Chromatin unpacked, show the nuclesome 32

Ø Evidence: (3)X-ray diffraction studies 3 -D of nucleosome Nature 389: 251, 1997 33

Digested briefly: H 1+Octamer+200 bp. DNA Digested longer: Octamer+146 bp H 1 is released Core particle Structure of nucleosome 34

ØStructural organization of the nucleosome 35

CORE OF PERIPHERAL HISTON H 2 B H 3 NUCLEO SOME DNA (146 bp、1. 75 H 4 ROUND） 10 nm H 2 A H 1 H 2 B H 4 HISTONE： 2（H 2 A、H 2 B、H 3、 H 4）OCTAMER H 3 CORE LINKER DNA（60 bp） H 3 H 4 H 2 B H 2 A H 1 H 2 A H 4 H 2 B DNA MOLECULE： 146 bp、 1. 75 ROUND NUCLE OSOME H 3 HISTONE：H 1 LINKER DNA MOLECULE： 60 bp 36

v A histone octamer forms the nucleosome core ØHistone octamer: (H 2 A-H 2 B)-(H 3 -H 4)-(H 2 A-H 2 B) ØWhere is the histone H 1? H 1 molecules are associated with the linker region. 146+15~50 bp linker DNA Linker DNA: 15 -50 bp 200 bp. DNA: Nucleosomal DNA: 146 bp to wrap 1. 75 times around the histone core. 37

Ø Evidence: (2)Nuclease digestion (Rat liver chromatin) The basic repeat unit, containing an average of 200 bp of DNA associated with a protein particle, is the nucleosome 38

Histone Number of Residues Mass (KDa Arg% Lys% ) H 1 23. 0 1 29 H 2 A 129 14. 0 9 11 H 2 B 125 13. 8 6 16 H 3 135 15. 3 13 10 H 4 Ø Histones: 215 102 11. 3 14 11 The most abundant proteins associated with eukaryotic DNA Rich in positively charged basic amino acids, which interact with the negatively charged phosphate groups in DNA The amino acid sequences of histones H 2 A, H 2 B, H 3, H 4 are remarkably similar among distantly species 39

Second level: nucleosome filament 10 nm in diameter H 1 40

¡ Third level: fiber, 30 nm in diameter ¡ Fourth level: chromosome 41

Summary of 4 degree folding model 42

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B. Euchromatin and Heterochromatin v. Euchromatin Ø The possibility of transcription; Ø The types of chromosomal structure— 30 -nm fibers and looped domains; Ø Light-staining, less condensed; Ø Transcriptional activity 44

v. Heterochromatin: Ø Dark-staining, condensed chromatin; Example of facultative heterochromatin: Random inactivation of Ø No transcriptional activity; in a typical X chromosome in mammalian cell, approximately 10% of the different cells during genome is packaged into heterochromatins early embryonic development forming CEN and TEL Ø Divided into two classes: Constitutive & facultative Compacted state at all time: Centromere Inactivated at certain phase of life Barr body in a woman’s 45 cell

3. Chromosome number, size, and shape at metaphase are species specific Karyotype Banding Chromatids 46

Human mitotic chromosomes and karyotype 47

Main structures of chromosome: Including centromere, arm (p and q), secondary constriction, telomere and satellite. C. Main structures of chromosome v Centromere & Kinetochore Centromere: Highly repeated DNA+Kinetochore v. The Centromere and Kinetochore: serve as a site for the attachment of spindle microtubules during mitosis and meiosis The structure of a human CEN 48

A typical mitotic chromosome satellite at metaphase secondary constriction nucleolar organizing region, NOR centromere telomere Human chromosome No. 14 49

Three functional elements are required for replication and stable inheritance of chromosomes 50

Ø Telomeres: Functions Protect the chromosomes from nuclease influence For the complete replication of chromosome 51

Ø Telomere, telomerase and cellular aging, cancer cell §Telomerase is found in germ cells, not in somatic cells. §The telomere length of adult is shorter than that of younger. §Telomere shortening is thought to activate a suicide program. So, telomere shortening plays a key role in protecting the body from cancer. 90% of human tumors contain an active telomerase. 52

D. Nucleolus a. Structure Fibrillar centers Dense fibrillar component Granular component 53

b. Functions of nucleolus: Ribosomal Biogenesis v Ribosomal Biogenesis: pre-r. RNA synthesis Process Assembly FC. DFC. GC Cytosol Directional process 54

v Characteristics of RNA transcription NORs in human chromosomes: 1314152122 Code for (in eukaryotes): 18 s, 28 s, 5. 8 s r. RNA 55

v r. RNA genes are tandemly arrayed in genome: Ø Christmas-Like tree when transcripted Primary transcripts condense immediately with a number of proteins to form particles about twice the size of nucleosomes. These particles are called hn. RNP (heterogeneous nuclear ribonucleoprotein) particles. These likely associate into “spliceosomes” involved in processing Also found in the nucleus are sn. RNPS (small nuclear ribonucleoproteins). These are believed to also contribute to RNA splicing and to histone m. RNA synthesis 56

Ø Cleavage of pre-r. RNA as RNP particles Mammalia 57

Ø Synthesis and processing of 5 s r. RNA § 5 s r. RNAs are encoded by a large number genes (Human, 2000) § 5 s r. RNA gene are located outside the nucleolus. § 5 s r. RNAs are transcibed by RNA poly III. §The 3’ end of 5 s r. RNA is removed during processing. 58

The assembly of ribosomes 59

v The nucleolus disappears during mitosis Nucleolar fusion of human fibroblast in culture 60

Summary (1). The components of the typical nonmitotic nucleus (2). The components of chromatin and packaging of chromosome: Scaffold radial loop structure model. (3). Nucleolus components and functions. 61

Homework ¡ Nucleolus Structure includes which parts? ¡ What are the functions of nucleolus? ¡ Describe briefly the difference between euchromatin and heterochromatin. ¡ Describe briefly structure of nucleosome? ¡ Any Questions? ? 62