细胞科学
Ce ll Sc ienc e
（ 11 ）
蔡国平
11.4,The nucleolus
The most prominent substructure within the nucleus
during interphase is the nucleolus,100 years ago,one
found a big spherical structure,called nucleolus within the
nucleus,but up until 1960s,it was not identified that the
nucleolus is a ribosome production factory,designed to
fulfill the need for large scale transcription and processing
of rRNA and assembly of ribosomal subunits,
1.r RNA genes and transcription and processing of rRNAs
The nucleolus is organized around the chromosomal
regions that contain the genes for the 5.8S,18S and 28S
rRNA within a single transcription unit for a 45S pre-rRNA,
which is transcribed within the nucleolus by RNA
polymerase I,The 5S RNA,another component of 60S
(large) ribosomal subunit is transcribed outside of the
nucleolus by RNA polymerase III.
Eukaryotic cells contain multiple copes of rRNA genes (or
called rDNA) to support the production of a large numbers of
ribosomes,For example,the human genome contains about
200 copies of the gene encoding the single transcription unit
for 5.8S,18S and 28S rRNAs,and approximately 2000 copies
of the gene encoding 5S rRNA,The genes for 18S,5.8S and
28S rRNAs are distributed into 10 clusters in tandem array
near the tip of one of the two copies of five different
chromosomes (13,14,15,21,and 22); the 5S rRNA genes
locate in a single tandem array on chromosome 1,Another
typical example is Xenopus oocytes,rRNA genes of mature
oocytes (in diameter 1mm) are amplified about 2000-fold,
resulting in ~1million copies of rRNA gene and nearly 1012
ribosomes per cell,The cells need take ~500 years to produce
such huge numbers of ribosome,if the oocytes were without
t h e s e l e c t i v e a m p l i f i c a t i o n of r R N A g e n e s,
These multiple even amplified rRNA genes are
distributed to different numbers of the nucleolus,from one
to thousands,Namely,following each cell division,nucleoli
form around the chromosomal regions that contain the 5.8S,
18S and 28S rRNA genes,these regions are therefore called
nucleolar organizing regions(NOR),The variety of
numbers of nucleolus of cells is not only due to the
differentiation of species but depending on the
development stages,Many species have two nucleolus
numbers,But some have only one,like human; some have
more than two nucleoli,such as a mature Xenopus oocyte
three years old have thousands of the nucleoli,
The transcription and processing of rRNAs
Each nuclear organizing region contains a cluster of
tandemly repeated large (5.8S,18S and 28S ) rRNA genes
that are separated from each other by nontranscribed spacer
DNA,These genes are very actively transcribed by RNA
polymerase I,In addition,most small nuclear RNAs,snRNA
(and mRNAs) are synthesized by RNA polymerase II,5S
RNA (and tRNA) are synthesized by RNA polymerase III.
As mRNA,rRNAs are transcribed in the initial precursor
RNA form,called primary transcript or pre-RNA,which
derived from corresponding transcription unit segment of
rDNA,The primary transcript of large (5.8S,18S and 28S )
rRNA genes is the large 45S pre-rRNA with two external
transcribed spaces at both 5’ and 3’ ends and two internal
transcribed spaces (at between 18S/5.8S and 5.8S/28S),
These transcribed spaces then are removed by a series of
ordered cleavage steps during the early stages of and after
the transcription,finally resulting in formation of mature
rRNA (5.8S,18S and 28S ),Each of the tandemly arrayed
repeated rRNA genes are very actively transcribed and
processed,Therefore in electron micrographs,the Each of
the tandemly arrayed repeated rRNA transcription units is
surrounded by 100 or so densely packed growing rRNA
fibrils or cleaved intermediate rRNA precursor fibrils,
forming a structure that looks like a Christmas tree,the dark
granule at the base of each fibril is RNA polymerase I,and
the characteristic knob at the 5’end of pre-rRNA fibril is the
attached U3 snRNP required for processing cleavage,The
nontranscribed spacer DNA regions between transcription
units are devoid of nascent RNA fibrils,
In addition to cleavage,the processing of pre-rRNA
involves in the modifying of the pre-rRNA resulting both
from the addition of methyl groups to specific bases and
ribose residues and from the conversion of uridine to
pseudouridine,
Ribosome assembly
The formation of ribosomes involves the assembly of the
ribosomal precursor RNA with both ribosome proteins and 5S
rRNA,Ribosome proteins are imported from the cytosol into
the nucleolus and begin to assemble on the pre-rRNA prior to
its cleavage,As the pre-rRNA is processed,remaining ribosome
proteins and 5S rRNA ( which is synthesized outside of
nucleolus by RNA polymerase III) are then incorporated into
preribosomal particles,Final preribosomal particles are
exported to the cytoplasm,forming the active 40S and 60S
ribosome subunits,Most of the preribosomal particles in the
nucleoli represent the precursor of the larger subunit,because
the smaller ribosome subunit mature more rapidly then the
larger subunit,As above mentioned,during the formation of
ribosomes there is a considerable amount of molecular traffic in
one way or shuttling rapidly of carrier-snoRNPs between the
nucleus and the cytosol.
2) The structure and functions of nucleolus
Unlike other organelles in the cell,nucleolus is not bound by
a membrane; it is a large substructure,which consists of an
aggregate of macromolecules above-mentioned,including the
rRNA genes(rDNAs) themselves,precursor rRNAs,rRNA
processing enzymes,snoRNPs,ribosomal protein subunits and
partly assembled ribosomes.
Morphologically,nucleolus consists of three distinguishable
regions embedded in amorphous proteinous matrix:a pale-staining
component (filament center,fc),nearby which more dense fibrillar
component(dfc),and granular component(gc) around both
filament region,These different regions are thought to represent
the sites of progressive stages of rRNA transcription,processing
and preribosomal particle assembly,rDNA are located and
transcribed in The fc and dfc contains,the dfc contains the nascent
pre-rRNA and associated proteins,processing and assembly of the
pre-rRNA is initiated in the dfc,and continues in the gc,which
contains nearly completed preribosomal units,ready for the
export to the cytoplasm,
During the cell cycle,there is the disassemble and
reassemble cycle of nucleolus,Each time a karyocytic cell
undergoes mitosis,chromosomes disperse and nucleolus
dissociate,at the telophase of mitosis,each nucleolar
organizing center that carry a cluster of rRNA genes
begins to form a tiny prenucleolus bodies,then these
rapidly coalesce as they grow to form one or more larger
nucleolus typical of interphase cells,
Functions of nucleolus Actually,in addition to its
important role in ribosome biogenesis,the nucleolus can be
thought of as a large factory at which many different
noncoding RNAs are processed and assembled with
proteins to form a large of variety of RNP complexes,such
as U6 snRNP for pre-mRNA processing,telomerase RNP
complexes,and the signal recognition particles (SRP),The
tRNA are processed there as well,For multiple functions,
various types of RNA molecules play a central part in
chemistry and structure,suggesting the nucleolus may have
evolved from an ancient structure present in cells
dominated by RNA catalysis.
3) The nuclear bodies
In addition to the nucleolus in the nucleus,there are
several other highly dynamic non-membraneous
subnuclear structures,called as nuclear bodies,which
include Cajal bodies and GEM (Gemini of coiled bodies)
and interchromatin granule cluster.Their appearance is
probably the result of the tight association of proteins and
RNA (and perhaps DNA) components in particular,snRNPs
or snoRNPs,which are involved in RNA processing and
assembled,stored and recycled.
4,The chromatin and chromosomes
1) general concept of chromatin
(1) enchromatin（常染色体） and heterochromatin （异染色体）
heterochromatin is divided into two categories,constitutive(结构）
and facultative（兼性） heterochromatin depending on the permanence
of the compacted state.
(2) Composition of chromatin,DNA,histone,nonhistone,RNA
2 ) From DNA to chromosome
(1) four level structure
DNA 2nm
Nucleasome filament of chromatin 10 nm
Higher orders of chromatin organisation-
25nm chromatin fiber
300- 500nm fold loops
Mitotic chromosome 700-1000nm
DNA,histone,nonhistone,
RNA
weight 1 1 0.5-1.5
0.05
Ce ll Sc ienc e
（ 11 ）
蔡国平
11.4,The nucleolus
The most prominent substructure within the nucleus
during interphase is the nucleolus,100 years ago,one
found a big spherical structure,called nucleolus within the
nucleus,but up until 1960s,it was not identified that the
nucleolus is a ribosome production factory,designed to
fulfill the need for large scale transcription and processing
of rRNA and assembly of ribosomal subunits,
1.r RNA genes and transcription and processing of rRNAs
The nucleolus is organized around the chromosomal
regions that contain the genes for the 5.8S,18S and 28S
rRNA within a single transcription unit for a 45S pre-rRNA,
which is transcribed within the nucleolus by RNA
polymerase I,The 5S RNA,another component of 60S
(large) ribosomal subunit is transcribed outside of the
nucleolus by RNA polymerase III.
Eukaryotic cells contain multiple copes of rRNA genes (or
called rDNA) to support the production of a large numbers of
ribosomes,For example,the human genome contains about
200 copies of the gene encoding the single transcription unit
for 5.8S,18S and 28S rRNAs,and approximately 2000 copies
of the gene encoding 5S rRNA,The genes for 18S,5.8S and
28S rRNAs are distributed into 10 clusters in tandem array
near the tip of one of the two copies of five different
chromosomes (13,14,15,21,and 22); the 5S rRNA genes
locate in a single tandem array on chromosome 1,Another
typical example is Xenopus oocytes,rRNA genes of mature
oocytes (in diameter 1mm) are amplified about 2000-fold,
resulting in ~1million copies of rRNA gene and nearly 1012
ribosomes per cell,The cells need take ~500 years to produce
such huge numbers of ribosome,if the oocytes were without
t h e s e l e c t i v e a m p l i f i c a t i o n of r R N A g e n e s,
These multiple even amplified rRNA genes are
distributed to different numbers of the nucleolus,from one
to thousands,Namely,following each cell division,nucleoli
form around the chromosomal regions that contain the 5.8S,
18S and 28S rRNA genes,these regions are therefore called
nucleolar organizing regions(NOR),The variety of
numbers of nucleolus of cells is not only due to the
differentiation of species but depending on the
development stages,Many species have two nucleolus
numbers,But some have only one,like human; some have
more than two nucleoli,such as a mature Xenopus oocyte
three years old have thousands of the nucleoli,
The transcription and processing of rRNAs
Each nuclear organizing region contains a cluster of
tandemly repeated large (5.8S,18S and 28S ) rRNA genes
that are separated from each other by nontranscribed spacer
DNA,These genes are very actively transcribed by RNA
polymerase I,In addition,most small nuclear RNAs,snRNA
(and mRNAs) are synthesized by RNA polymerase II,5S
RNA (and tRNA) are synthesized by RNA polymerase III.
As mRNA,rRNAs are transcribed in the initial precursor
RNA form,called primary transcript or pre-RNA,which
derived from corresponding transcription unit segment of
rDNA,The primary transcript of large (5.8S,18S and 28S )
rRNA genes is the large 45S pre-rRNA with two external
transcribed spaces at both 5’ and 3’ ends and two internal
transcribed spaces (at between 18S/5.8S and 5.8S/28S),
These transcribed spaces then are removed by a series of
ordered cleavage steps during the early stages of and after
the transcription,finally resulting in formation of mature
rRNA (5.8S,18S and 28S ),Each of the tandemly arrayed
repeated rRNA genes are very actively transcribed and
processed,Therefore in electron micrographs,the Each of
the tandemly arrayed repeated rRNA transcription units is
surrounded by 100 or so densely packed growing rRNA
fibrils or cleaved intermediate rRNA precursor fibrils,
forming a structure that looks like a Christmas tree,the dark
granule at the base of each fibril is RNA polymerase I,and
the characteristic knob at the 5’end of pre-rRNA fibril is the
attached U3 snRNP required for processing cleavage,The
nontranscribed spacer DNA regions between transcription
units are devoid of nascent RNA fibrils,
In addition to cleavage,the processing of pre-rRNA
involves in the modifying of the pre-rRNA resulting both
from the addition of methyl groups to specific bases and
ribose residues and from the conversion of uridine to
pseudouridine,
Ribosome assembly
The formation of ribosomes involves the assembly of the
ribosomal precursor RNA with both ribosome proteins and 5S
rRNA,Ribosome proteins are imported from the cytosol into
the nucleolus and begin to assemble on the pre-rRNA prior to
its cleavage,As the pre-rRNA is processed,remaining ribosome
proteins and 5S rRNA ( which is synthesized outside of
nucleolus by RNA polymerase III) are then incorporated into
preribosomal particles,Final preribosomal particles are
exported to the cytoplasm,forming the active 40S and 60S
ribosome subunits,Most of the preribosomal particles in the
nucleoli represent the precursor of the larger subunit,because
the smaller ribosome subunit mature more rapidly then the
larger subunit,As above mentioned,during the formation of
ribosomes there is a considerable amount of molecular traffic in
one way or shuttling rapidly of carrier-snoRNPs between the
nucleus and the cytosol.
2) The structure and functions of nucleolus
Unlike other organelles in the cell,nucleolus is not bound by
a membrane; it is a large substructure,which consists of an
aggregate of macromolecules above-mentioned,including the
rRNA genes(rDNAs) themselves,precursor rRNAs,rRNA
processing enzymes,snoRNPs,ribosomal protein subunits and
partly assembled ribosomes.
Morphologically,nucleolus consists of three distinguishable
regions embedded in amorphous proteinous matrix:a pale-staining
component (filament center,fc),nearby which more dense fibrillar
component(dfc),and granular component(gc) around both
filament region,These different regions are thought to represent
the sites of progressive stages of rRNA transcription,processing
and preribosomal particle assembly,rDNA are located and
transcribed in The fc and dfc contains,the dfc contains the nascent
pre-rRNA and associated proteins,processing and assembly of the
pre-rRNA is initiated in the dfc,and continues in the gc,which
contains nearly completed preribosomal units,ready for the
export to the cytoplasm,
During the cell cycle,there is the disassemble and
reassemble cycle of nucleolus,Each time a karyocytic cell
undergoes mitosis,chromosomes disperse and nucleolus
dissociate,at the telophase of mitosis,each nucleolar
organizing center that carry a cluster of rRNA genes
begins to form a tiny prenucleolus bodies,then these
rapidly coalesce as they grow to form one or more larger
nucleolus typical of interphase cells,
Functions of nucleolus Actually,in addition to its
important role in ribosome biogenesis,the nucleolus can be
thought of as a large factory at which many different
noncoding RNAs are processed and assembled with
proteins to form a large of variety of RNP complexes,such
as U6 snRNP for pre-mRNA processing,telomerase RNP
complexes,and the signal recognition particles (SRP),The
tRNA are processed there as well,For multiple functions,
various types of RNA molecules play a central part in
chemistry and structure,suggesting the nucleolus may have
evolved from an ancient structure present in cells
dominated by RNA catalysis.
3) The nuclear bodies
In addition to the nucleolus in the nucleus,there are
several other highly dynamic non-membraneous
subnuclear structures,called as nuclear bodies,which
include Cajal bodies and GEM (Gemini of coiled bodies)
and interchromatin granule cluster.Their appearance is
probably the result of the tight association of proteins and
RNA (and perhaps DNA) components in particular,snRNPs
or snoRNPs,which are involved in RNA processing and
assembled,stored and recycled.
4,The chromatin and chromosomes
1) general concept of chromatin
(1) enchromatin（常染色体） and heterochromatin （异染色体）
heterochromatin is divided into two categories,constitutive(结构）
and facultative（兼性） heterochromatin depending on the permanence
of the compacted state.
(2) Composition of chromatin,DNA,histone,nonhistone,RNA
2 ) From DNA to chromosome
(1) four level structure
DNA 2nm
Nucleasome filament of chromatin 10 nm
Higher orders of chromatin organisation-
25nm chromatin fiber
300- 500nm fold loops
Mitotic chromosome 700-1000nm
DNA,histone,nonhistone,
RNA
weight 1 1 0.5-1.5
0.05
