Cell nucleus and Chromosomes
Zhaojian Liu(刘招舰)
2013-04
山东大学医学院
细胞生物学研究所
History
-Discovered in 1831 by
Scottish botanist Robert Brown
-Suggested the nucleus played a
key role in fertilization and
development of the embryo in
plants
-Name (nucleus) derived from
the Latin word for kernel/nut
Robert Brown
1773-1858
Introduction of nucleus
Introduction of nucleus
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The nucleus is the most obvious organelle in any
eukaryotic cell.
Nucleus is the storage of the genetic message of the
cell in which the DNA replication, transcription occurs.
It is surrounded by a double membrane.
It communicates with the surrounding cytosol via
numerous nuclear pores.
The nucleus is, therefore, the control center of the cell.
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The prominent structure in the nucleus is the
nucleolus.
The nucleolus produces ribosomes, which move out
of the nucleus to rough endoplasmic reticulum.
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There are three main types of instructions the
nucleus does.
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The first is that it directs cellular reproduction.
The second is that the nucleus controls a cell's
differentiation.
The third type of instruction of the nucleus is that it
regulates the metabolic activities of the cell.
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Generally, nucleus is spherical and centrally located in
the cell.
Its volume is about 10% of that of the cell and its
diameter is 5-10um.
Usually each cell has a single nucleus, whereas some
cells such as osteoclasts possess several nuclei. Still
other cells as red blood cells have extruded their nuclei.
Constituents of the nucleus:
DNA
<20%
histones
nucleoproteins
nonhistones
enzymes
RNA
mRNA ,tRNA, rRNA
Structure of the nucleus:
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nuclear envelope: two lipid membranes
chromatin: the genetic material of the cell
Nuclear matrix
nucleolus: the center for rRNA synthesis
Section I Nuclear envelope
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The nuclear envelope, composed of two parallel unit
membranes, separated from each other by a 10-30nm
space, the perinuclear cisterna.
The nuclear envelope is perforated at various intervals
by nuclear pores, which permit the communication
between the cytoplasm and the nucleus.
The nuclear envelope helps control the movement of
macromolecules between the nucleus and the cytoplasm
and assist in organizing the chromatin.
(I) structure of nuclear envelope
Two-membrane structure
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The nuclear envelope has two membranes, each with
the typical unit membrane structure.
They enclose a flattened sac and are connected at the
nuclear pore sites.
(I) structure of nuclear envelope
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Outer nuclear membrane
Inner nuclear membrane
Perinuclear space
Nuclear pore comlex
1. Outer nuclear membrane
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The outer nuclear membrane faces the cytoplasm.
It is continuous with the rough endoplasmic reticulum.
Its cytoplasmic surface usually possesses ribosomes actively
synthesizing transmembrane proteins.
Its cytoplasmic surface is surrounded by a thin loose meshwork of
intermediate filaments, vimentin.
2. Inner nuclear membrane
The inner nuclear membrane faces the nuclear contents.
It is in close contact with the nuclear lamina, an
interwoven meshwork of intermediate filaments, 80100nm thick.
The nuclear lamina help to organize and provide support
to the bilayer nuclear membranes and the perinuclear
chromatin.
3. Perinuclear space
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perinuclear space ---the space between
the outer and inner membranes
• 20～40nm
• continuous with rER space.
4 Nuclear pore
 Found by H.G.Callan S.G.Tomlin in 1949.
Named by M.L.Waston in 1959.
Nuclear pores are formed at sites where the inner and outer
membranes of the nuclear envelope are joined, leaving a space
filled with filamentous material.
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Pore size is variable according to the cell types
and tissue
In general 40 - 100 nm
Numbers of pores in given area
• low in cells with slow metabolism and at times
of low activity during cell cycle
• high in cells after cell division and with higher
activity of RNA transport and protein
synthesis
Nuclear pore density of numbers of the nuclear
pores per nucleus
Type of cells
Human lymphocyte
Embryo of leopard frog
Lung cell of human embryo
Kidney cell of African Green
Monkey
Heart cells of Salamander
Embryo of Xenopus laevis
Embryo of Salamander
Density of Nuclear Pore
(pores/m2)
Number of nuclear pores per
nucleus
3.2
5.6
8.5
8.6
405
1,729
2,788
4,277
7.6
51
50
12,707
37.7  106
57  106
Nuclear pore complex (NPC)
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The nuclear pore complex is about 80-100nm in
diameter and spans the two nuclear membranes.
It's thought to be composed of four elements:
Structure of Nuclear pore complex
I
Cytoplasmic ring
II
nuclear ring
III Spoke
 column subunit
 luminal subunit
annular subunit
IV Central plug
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i Cytoplasmic ring
Because it face to cytoplasm It was also called outer
ring. There are 8 symmetric thick filaments distribute
on the ring. It is suggested that these filaments may
act as a staging area for the binding of the proteins
that are to be transported into the nucleus.
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ii nuclear ring
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It face to the inner of the nucleus, and was also called inner
ring. Nuclear ring was more complex than outer ring. There
are also 8 filaments extend inside to 50-70nm. It formed a
little ring on the bottom of the filaments which composed of 8
particles. Thus the whole nuclear ring looks like a fish trap. So
some people called it muclear basket.
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iii Spoke
The spoke fixed on the nuclear membrane and
face to the center of the nuclear pore. It is also
symmetric and complex. In detail, spoke may part
into three subunit.
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① column subunit Distribute at the edge of nuclear pore.
It connect the outer and inner ring and support the whole
nuclear pore.
② luminal subunit
The domain which linked the nuclear membrane was called
luminal subunit. It perforate the nuclear membrane and
extend into the perinuclear cisterna.
③ annular subunit is composed of 8 particles and formed
a channel to exchange substances.
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iv
Central plug
The particle that lied in the center of the nuclear pore. It was
also termed central granule. The current understanding is that
the transporter functions in transport of material into and out of
the nucleus.
But not all the nuclear pores could be seen this structure. So
some scientists think that central plug is not a structure part of
nuclear pore but a particle that just being transported through
the nuclear pore.
35 nm in diameter, transporter
Nuclear pore transport
Due to the structural conformation of the subunits of the nuclear
pore complex, there are several 9-11nm wide channels available
for simple diffusion for ions and small molecules.
Substances larger than 11nm are unable to reach or leave the
nucleus via simple diffusion; instead they are selectively
transported via a receptor-mediated transport process.
Molecules enter and exit the nucleus
through nuclear pore complex
Bidirectional traffic
9.1.3.1 Nuclear protein transport mechanism
basic definition
◆nuclear protein
◆nuclear localization signals ,NLS
◆nuclear export signals, NES
◆importin
◆exportin
Materials exchange
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Transport in and out of the nucleus can occur in
several ways.
• Passive transport-diffusion
• Active Transport
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Passive transport—passively diffuse
3000-4000 NPC/cell(mammalian);
To import about 106 histone/3 mins.
Each NPC contains one or more open aqueous channels: 9
nm in diameter and 15 nm long
<10 nm in diameter
<60kd globular protein
Able to enter the nucleus
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Active transport
Transport of large proteins into nucleus needs nuclear
localization signal (NLS)
Nuclear Localization Signals(NLS)
basic or classic NLS
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Nuclear import and export
Nuclear import receptors bind NLS and Nucleoporins
The Ran GTPase drives directional transport through
NPC
The compartmentalization of Ran-GDP and Ran-GTP.
A model for how GTP hydrolysis by Ran
provides directionality for nuclear
transport
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Nuclear export works like nuclear
import, but in reverse
hnRNP proteins contain a nuclearexport signal (NES)
Reference:
Cell 92: 327, 1998
Getting material out
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mRNA, tRNA, subunit of ribosome
could transport out of the nucleus to
cytoplasm.
Transport of mRNA through a pore
Imported many types of proteins
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DNA polymerase, RNA polymerase
and other proteins are imported
through nuclear pore.
5. Structure and function of
nuclear lamina
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Underlying the inner nuclear membrane is the nuclear lamina.
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The nuclear lamina is a dense (~30 to 100 nm thick)
fibrillar network inside the nucleus of a eukaryotic cell.
It is composed of intermediate filaments and
membrane associated proteins.
Composition of nuclear lamina
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Lamins A: next to Nuclear skeleton
lamins C: next to Nuclear skeleton
lamins B: near the inner nuclear membrane. They may
bind to integral proteins inside that membrane.
• lamins B1
• lamins B2
Functional of nuclear lamina
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(1) providing structural support to the
nucleus by binding to inner nuclear
membrane proteins
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(2) Playing a role in nucleus assembly and
disassembly before and after mitosis.
Nuclear lamina disassembly in M phase
(3) Serve as a site of chromatin
attachment
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Chromatin organization
DNA replication
Structure of the nucleus:
nuclear envelope
chromatin
nucleolus
nucleoplasm
Section II Chromatin and Chromosome
History
1Walther Flemming
named chromatin (strongly absorbed
basophilic dyes) in 1879.
Flemming surmised for the first time
that all cell nuclei came from another
predecessor nucleus
2.Wilhelm von Waldeyer-Hartz
 Named the chromosomes (meaning
coloured body) in 1888
What is chromatin and chromosome?
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Chromatin:
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Chromatin carry the genetic information of organism which
lies in interphase.
The complex of DNA and protein which make up the content
of the nucleus of a cell.
Cromatin is composed of:
• DNA
• Histones
• Nonhistones
• RNA
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What is chromatin and chromosome?
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Chromosome:
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In M-phase, the chromatin fibers are extensively
condensed to form chromosomes, structures that are
visible with the light microscope.
Three important DNA sequences of chromatin
Replication origin
 Centromere
Telomere
Histones
◆The histones are small proteins containing a high proportion of
basic amino acids with positive charge that facilitate binding to
the negatively charged DNA molecule.
Histones are Rich in Lysine and Arginine
◆There are two classes of histones:
Nucleosomal histones
H2A, H2B, H3, and H4 which are highly conserved.
H1-like Histone
Variable
Histones are bound with DNA in a non-specific way.
Structural Organization of the Core Histones
Nonhistones
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Nonhistones: This DNA-associated proteins
bind DNA in a specific way, therefore named
sequence specific DNA binding protein, which
can form several specific structures and may
also involved in the regulation of DNA activities.
Nonhistones
Euchromatin and Heterochromatin
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Depending on its transcriptional activity, chromatin
may be condensed as heterochromatin or extended
as euchromatin.
Euchromatin: scattered throughout the nucleus and
not visible with light microscope, which is the active
form of chromatin where the genetic material of the
DNA molecules is being transcribed into RNA.
 Lie is the center of the nucleus
 Replication at early stage of S phase.
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Heterochromatin:
it is visible under light microscope, which located
mostly at the periphery of the nucleus but also
forms irregular clumps throughout the nucleus. It is
condensed inactive form which means it is not
active in RNA synthesis. It can be divided into
Constitutive heterochromatin and Facultative
heterochromatin.
No transcriptional activity
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Constitutive heterochromatin:
Except for replication, this type of heterochromatin
is always in a condensed inactive form during the
whole cell cycle.
Facultative heterochromatin:
In some types of cells or in certain developmental
phases, some euchromatin becomes condensed,
losing their transcriptional function and turns into
heterochromatin. It is a way to close gene activity.
For example: X Chromosome
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Barr Body（ X Chromosome ）: microscopic study of
interphase nuclei of cells from female displays a very
tightly coiled clump of chromatin, the sex chromatin (Barr
Body), the inactive counterpart of the two X
Chromosomes. It is a facultative heterochromatin, which
is one of the two X chromosomes becoming condensed
randomly at embryonic 16 days of female.
X chromosome inactivation in mammals
Dosage compensation
X
X
X
X
Y
Xist (X-inactive specific transcript)
The first lncRNA(long non-coding RNA)
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Avner and Heard, Nat. Rev. Genetics 2001 2(1):59-67
Chromatin packaging
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Each human cell contains about 2
m of DNA within nucleus if
stretched end-to-end, yet the
nucleus of a human cell itself is
only about 6 um in diameter.
Compaction ratio=nearly 10000fold.
From DNA to chromosome needs fourstep packaging.
Step 1 Nucleosome
The basic Unit of Chromatin
The first step of
condensation, from 2 nm to
11nm.
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Evidence:
(1)Electron micrographs of chromatin fibers
Isolated from interphase nucleus:
30nm thick
Chromatin unpacked, show the
unclesome
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Evidence:
(2)Nuclease digestion (Rat liver chromatin)
What is Nucleosome ?
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Nucleosome is the basic unit of chromatin. Each
nucleosome is made up of an octomer of
proteins, duplicates of each of four types of
histones (H2A, H2B,H3 and H4).
The nucleosome is also wrapped with 1.75 turns
of the DNA molecule that continues as linker
DNA extending to the next “bead”. The spacing
between each nucleosome is about 200 base
pairs.
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 1.65
times around the histone core.
Cyclic Diagram for
nucleosome formation
and disruption
Chromatin Remodeling
Covalent Modification of core histone tails
Acetylation of lysines
Mythylation of lysines
Phosphorylation of serines
Histone acetyl transferase (HAT)
Histone deacetylase (HDAC)
Step II Solenoid
Solenoid: (Chromatin fiber of packed nucleosomes)
The second step of condensation, 30nm.
Packaging of chromatin into 30nm is believed to
occur by helical coiling of consecutive nucleosomes
at six nucleosomes per turn of the coil and
cooperatively bound there with Histone H1.
Step III Supersolenoid
Supersolenoid: The third step of condensation, 300nm.
Tighter condensing of the chromatin material is accomplished
by looping the coiled 30nm fibers into 300nm loops held
together by specific protein/DNA bound complexes located at
their bases.
Radical loop model
Step IV metaphase chromosomes
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Chromosomes: The forth step of condensation,
700nm.
Further coiling of the 300nm loops into tightly
woven 700nm helical loops forms the maximally
condensed chromosomes observed in the
metaphase stage of mitosis or meiosis.
chromatid
chromosome
DNA
nucleosome
solenoid
supersolenoid
chromosome
Chromosome in metaphase
Karyotype is the number and appearance of
chromosomes in the nucleus of a eukaryotic cell.
Human mitotic chromosomes and karyotype
Chromsomes can be divided into four
types depend on Centromere positions
There is no telocentric chromosome in human cells
A typical mitotic chromosome at metaphase
satellite
secondary constriction
nucleolar organizing region,NOR
centromere
telomere
Human chromosome No.14
Copyright © The McGraw-Hill Companies, Inc. Permission required for
reproduction or display.
Main structures of chromosome
Cohesin
proteins
Chromatid
Centromere
region of
chromosome
Kinetochore
Kinetochore
microtubules
Metaphase
Main structures of chromosome
The Centromere and Kinetochore: serve as a site
for the attachment of spindle microtubules during
mitosis and meiosis
The centromere is the part of a chromosome that links
sister chromatids. During mitosis, spindle fibers attach to
the centromere via the Kinetochore.
The kinetochore is the protein structure on chromatids
where the spindle fibers attach during cell division to pull
sister chromatids apart.
Main structures of chromosome
Centromere
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Kinetochore domain
Central domain
Pairing domain
Main structures of chromosome
Kinetochores Power Chromosome
Movements in Mitosis
Main structures of chromosome
END REPLICATION PROBLEM
Telomere
Main structures of chromosome
Telomere
To overcome end-replication problem.
Prevent random Binding between chromosomes.
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TTAGGG in Human.
Between 3 and 20 Kb in length
Repetitive sequences at the ends of
chromosomes.
Centromere
Telomere
Telomere
Main structures of chromosome
Telomere
FISH (Human Probe ):TTAGGG
Main structures of chromosome
Telomere
Telomerase is an enzyme
which adds DNA sequence
repeats ("TTAGGG" in all
vertebrates) to the 3' end of
DNA strands in the telomere
regions, which are found at
the ends of eukaryotic
chromosomes.
Main structures of chromosome
Telomere & Ageing
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In humans, telomeres in most
somatic cells shorten with age.
These cell types do not have
enough telomerase activity to
maintain their original telomere
length.
Main structures of chromosome
Role of Telomeres in Cancer
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cancerous tumors acquire
indefinite replicative
capacity by overexpressing telomerase.
telomeres in human
tumors were shorter than
telomeres in the normal
surrounding tissue
Hayflick limit is believed
to help prevent cancer.
A typical mitotic chromosome at metaphase
satellite
secondary constriction
nucleolar organizing region,NOR
centromere
telomere
Human chromosome No.14
Secondary constriction
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Secondary constriction is seen at
the chromosome in addition to
primary constriction/centromere.
Some parts of these constrictions
indicates sites of nucleolus
formation.
Satellite chromosome
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Satellites
A small terminal segment of the chromosome
Connected to its end by a secondary
constriction is known as satellite and
chromosomes with satellites are known as
satellite chromosomes or SAT chromosomes.
Giant chromosome
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Polytene chromosome
Lamp brush chromosome
Polytene chromosome
Lamp brush chromosome
Nucleolus
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The most prominent substructure within the nucleus
is the nucleolus , which is the site of rRNA
transcription and processing, and of ribosome
assembly.
The nucleolus is observed only during interphase
because it dissipates during cell division.
Structure of the nucleolus
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Fibriller center
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Dense fibrillar component
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Granular component
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Fibrillar center
The rRNA genes are located in the fibrillar centers. (NOR)
Nucleolus organizer region (NOR) or nucleolar organizer is a
chromosomal region around which the nucleolus forms. The
region contains several tandem copies of ribosomal DNA genes.
In humans, the NOR contains genes for 5.8S, 18S, and 28S
rRNA clustered on the short arms of chromosomes 13, 14, 15,
21 and 22
NORs in human
chromosomes:
13\14\15\21\22
Localization of NORs in Metaphase
Chromosomes
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In metaphase chromosomes the NORs appear in many
species as particularly thin regions, the "secondary
constrictions" (Heitz 1931; McClintock 1934).
Matsui and Sasaki, using a staining procedure known as
N banding, concluded that the satellites, not the
centromeres, or stalks(secondary constrictions), are the
NORs.
Dense fibrillar component
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Surrounding FC
Containing nucleolar RNAs being transcribed .
Compostion
• r RNA
• RNP
Granular component
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Maturing ribosomal subunits are assembled in GC;
Particle diameter: 15-20nm
RNP: protein+rRNA
Funtion of Nucleolus
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rRNA transcription and processing
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Ribosome Assembly
rRNA transcription and processing
rDNA ---rRNA gene
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rDNA in nucleolus( NORs) yielding a 45S ribosomal
precursor RNA (RNA polymerase I )
rDNA outside of the nucleolus yielding 5S rRNA (RNA
polymerase III)
rRNA transcription and processing
Processing of rRNA
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The 45S pre-rRNA is processed to 5.8S 18S and 28S
rRNAs.
• 18S rRNA of the 40S (small) ribosomal subunit
• 5.8S and 28S rRNAs and 5S rRNA(Chr 1) from of
the 60S (large) ribosomal subunit.
Ribosome Assembly
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5.8S 18S and 28S rRNAs: inside nucleolus
ribosomal protein：cytoplasm→nucleolus
5S rRNA : outside of nucleolus → in nucleolus
small and large ribosomal subunits： nucleolus → cytoplasm
Ribosome Assembly
Nucleoskeleton
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The nucleoskeleton is composed of many interacting
structural proteins that provide the framework for
DNA replication, transcription and a variety of other
nuclear functions.

The lamins and their associated proteins play important roles
in DNA replication and transcription.
Furthermore, actin, actin-related proteins are components of
chromatin-remodeling complexes and are involved in mRNA
synthesis, processing and transport.
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Function of nucleus
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Store genetic information.
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DNA Replication
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DNA transcription
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Produce ribosomes in the nucleolus
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Transport regulatory factors & gene products via nuclear pores
Nucleus and diseases
Chromosome abnormality
Three major single chromosome
mutations; (1)deletion, (2)duplication
and (3)inversion.
Nucleus and diseases
Nucleus and apoptosis
Nucleus and diseases
Nucleus and cancer
Summary
复习要点

第五章
细胞的内膜系统与囊泡转运
1. 内膜系统的概念。
2.内质网
• 内质网的形态结构、类型、标志酶
• 信号假说
• 内质网的功能

3.高尔基体的形态结构特征及其在糖蛋白的合成、加工、溶酶体形成中的作
用；高尔基体的化学组成及标志酶；两种糖基化方式。
4. 溶酶体的形态结构及酶的特点，溶酶体的形成过程、类型及功能；溶酶体
与某些疾病的关系。
5. 过氧化物酶体的形态和酶类特点；过氧化物酶体的功能和发生。
6.掌握囊泡转运的类型，囊泡转运的方向。

第六章 线粒体与细胞的能量转换

掌握线粒体的超微结构特点，外膜和内膜。

线粒体基因组的的特点及核编码蛋白向线粒体的转运。

线粒体与细胞的能量转换，反应发生部位及产生能量方式。

线粒体疾病的特点
第七章 细胞骨架与细胞的运动

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细胞骨架、微管组织中心的概念
三种细胞骨架的化学组成、组装过程
影响微管和微丝组装的药物
微管、微丝的结合蛋白及其与功能的关系；
三种细胞骨架的功能
中间纤维的种类及其与医学关系
第八章 细胞核

概念：细胞核、染色质、常染色质、异染色质（2种）、
组蛋白、非组蛋白、核小体核仁组织区等

细胞核的基本结构组成（超微结构）

染色质的化学组成、超微结构及组装。

核孔复合体的结构与功能；染色体的结构特征。
核仁的化学组成、超微结构与功能动态关系

核仁的功能。