Transcript Nucleus and Chromosomes

Chapter 9
The cell Nucleus
Xiamixinuer·Yilike
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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.
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1. The nucleus: Nuclear envelope and NPC
A. Structure: Double-membrane nuclear
envelope surrounds the nucleus
Structure of the interphase nucleus
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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.
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 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。
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A typical nonmitotic nucleus includes four
major components.
nuclear
envelope
nucleoluos
chromatin
matrix
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B. The nuclear envelope consists of
two membranes by a perinuclear
space.
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The inner surface of the nuclear envelope is lined by
the nuclear lamina
The nuclear lamina supports the nuclear envelope: Gives
shape and stability of nuclear envelope;
Provides a structure link between chromatin and nuclear
envelope;
The nuclear lamina is composed of lamins.
The integrity of the nuclear lamina is regulated by
phosphorylation and dephosphorylation.
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Breakdown and reformation of nuclear envelope during mitosis
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The structures of nuclear envelope
outer nuclear membrane
inner nuclear membrane
perinuclear space
nuclear pores
nuclear lamina
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C. Nuclear pore complex (NPC)
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The structure of
Nuclear pore
complex (NPC)
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Ring subunit：8 pair
Annular subunit：8
Old
model
structure
Central plug：1
fibril：
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nuclear lamina
pore
outer 蛋
membrane
Perinuclear space
Inner
B
B
membrane
B
B
B
B
B
lamina A.B.C
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D. Molecules enter and exit the nucleus
through nuclear pore complex
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Bidirectional traffic
Molecules enter and exit the nucleus
through nuclear pore complex
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 Passive transport—passively diffuse
3000-4000 NPC/cell(mammalian);
To import about 106 histone/3 mins.(DNA-sythesizing
cell) = 100 histone/ min/NPC
Each NPC contains one or more open aqueous
channels: 9 nm in diameter and 15 nm long
The 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
<60kd globular protein
Able to enter the nucleus
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 Active transport
Pore diameter is about 9nm 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.
Transport of large
proteins into nucleus
needs nuclear
localization signal (NLS)
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Transport of mRNA out of the nucleus
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Chapter 9 section 2
Nucleolus and Chromosomes
Learning Objectives
(1) The components of chromatin and packaging of
chromosome
(2) Nucleolus.
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Chromatin fibe
human nucleus under LM
human lymphocyte nucleus under
SEM
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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 1.5cm2 μ m)
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Chromosomes exist in different states
throughout the life of a cell.
Chromatin: (Interphase)
Fibers, 10-30nm 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.
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Levels of
organization
of chromatin
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B. Nucleosomes are packed together to
form chromatin fibers and chromosomes
 4 degree folding model
Nonhistone proteins provide a
structural scaffold for long
chromatin loops.
2nm DNA –11nm nucleosome –
30nm fiber – Loop– Metaphase
chromosome
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4 degree folding model
 First degree:
Nucleosomes
 Second degree:
Filament 10nm in diameter
 Third level:
Fiber,30nm in diameter
 Fourth level:
Chromosome
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A. DNA packaging:First degree of
packaging is Nucleosomes
Nucleosomes are the basic unit of chromatin structure.
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 Evidence:
(1)Electron micrographs of chromatin fibers
Isolated from interphase nucleus: 30nm thick
Chromatin unpacked, show the nuclesome
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 Evidence:
(3)X-ray diffraction studies
3-D of nucleosome
Nature 389:251, 1997
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Digested briefly:
H1+Octamer+200
bpDNA
Digested longer:
Octamer+146bp
H1 is released
Core particle
Structure of
nucleosome
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Structural
organization of
the nucleosome
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CORE OF PERIPHERAL HISTON
H2B
H3
NUCLEO
SOME
DNA (146bp、1.75
H4
ROUND）
10nm
H2A
H2A
H1
H4
H2B
HISTONE：2（H2A、H2B、H3、
H4）OCTAMER
H3
CORE
LINKER
DNA（60bp）
H3
H4
H2B
H2A
H1
H2A
H4
H2B
DNA MOLECULE：146bp、
1.75 ROUND
NUCLE
OSOME
H3
HISTONE：H1
LINKER
DNA MOLECULE：60bp
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 A histone octamer forms the nucleosome core
Histone octamer:
(H2A-H2B)-(H3-H4)-(H3-H4)-(H2A-H2B)
Where is the histone H1?
H1 molecules are associated with the linker
region. 146+15~50bp linker DNA
Linker DNA:15-50bp
200bpDNA:
Nucleosomal DNA:146bp to wrap
1.75 times around the histone core.
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 Evidence:
(2)Nuclease digestion (Rat liver chromatin)
The basic
repeat unit,
containing an
average of
200bp of DNA
associated with
a protein
particle, is the
nucleosome
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Histone
 Histones:
Number of
Residues
Mass
Arg% Lys%
(KDa)
H1
215
23.0
1
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H2A
129
14.0
9
11
H2B
125
13.8
6
16
H3
135
15.3
13
10
H4
102
11.3
14
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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 H2A, H2B, H3, H4 are
remarkably similar among distantly species
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Second level: nucleosome filament
10nm in diameter
H1
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 Third level:
fiber,30nm in diameter
 Fourth level:
chromosome
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Summary of 4 degree folding
model
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B. Euchromatin
and
Heterochromatin
Euchromatin
 The possibility of transcription;
 The types of chromosomal structure—30-nm fibers
and looped domains;
 Light-staining, less condensed;
 Transcriptional activity
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Heterochromatin:
 Dark-staining, condensed chromatin;
 No transcriptional activity; in a typical
mammalian cell, approximately 10% of
the genome is packaged into
heterochromatins forming CEN and TEL
Example of facultative
heterochromatin:
Random inactivation
of X chromosome in
different cells during
early embryonic
development
 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
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Human mitotic chromosomes and karyotype
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Main structures of chromosome: Including centromere, arm (p and q), secondary constriction, telomere and satellite.
C. Main structures of chromosome

Centromere & Kinetochore
Centromere: Highly repeated
DNA+Kinetochore
The Centromere and Kinetochore:
serve as a site for the attachment of
spindle microtubules during mitosis
and meiosis
The structure of a human CEN
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A typical mitotic chromosome
satellite at metaphase
secondary constriction
nucleolar organizing region,NOR
centromere
telomere
Human chromosome No.14
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Three functional elements are required for
replication and stable inheritance of chromosomes
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 Telomeres:
Functions
Protect the chromosomes from nuclease influence
For the complete replication of chromosome
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 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.
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D. Nucleolus
a. Structure
Fibrillar centers
Dense fibrillar component
Granular component
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b. Functions of nucleolus: Ribosomal Biogenesis
 Ribosomal Biogenesis:
pre-rRNA synthesis
Process
Assembly
FC. DFC
DFC.GC
Cytosol
Directional process
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 Characteristics of RNA transcription
NORs in human
chromosomes:
13\14\15\21\22
Code for (in eukaryotes):
18s, 28s, 5.8s rRNA
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 rRNA genes are tandemly arrayed in genome:
 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 hnRNP
(heterogeneous nuclear
ribonucleoprotein) particles.
These likely associate into
“spliceosomes” involved in processing
Also found in the nucleus are snRNPS
(small nuclear ribonucleoproteins).
These are believed to also contribute
to RNA splicing and to histone mRNA
synthesis
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 Cleavage of pre-rRNA as RNP particles
Mammalia
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 Synthesis and processing of 5s rRNA
5s rRNAs are encoded by a large number genes
(Human, 2000)
5s rRNA gene are located outside the nucleolus.
5s rRNAs are transcibed by RNA poly III.
The 3’ end of 5s rRNA is removed during
processing.
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The
assembly of
ribosomes
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 The nucleolus disappears during mitosis
Nucleolar fusion
of human
fibroblast in
culture
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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.
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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??
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