Nucleus is the largest organelle in the eukaryote, which is the place for storage, replication and transcription of genetic materials and the control center for cell metabolism, growth, differentiation and reproduction. Normally, a cell has only one nucleus, but the liver cell, glomerular cell and cartilage cell have dual nuclei, and osteoclast has hundreds of nuclei. The morphology of the nucleus is compatible with the morphology of the cell, showing diversity, such as spherical, oval and rod. The complete cell nucleus can be seen only in the interphase period, the basic structure of which including nuclear envelope, nucleolus, chromatin, nuclear matrix (nuclear skeleton) and so on. After the cell enters the division phase, the nuclear envelope breaks down and the various components of the nucleus are redistributed, so that the intact nucleus is not visible.
Nuclear membrane, also called nuclear envelope, is a double-deck membrane with nuclear inclusions packaged in it. It is a part of the entire inner membrane system and the result of regionalization of cells. The nuclear membrane is one of the major markers of eukaryotic cells.
The main chemical compositions of the nuclear membrane are protein and lipid, among which protein occupies around 65%~75%. In addition, there is also a small amount of nucleic acid.
Under electron microscope, the nuclear membrane is composed of two basic parallel concentric membrane units, that is, the inner nuclear membrane and the outer nuclear membrane. Because of the composition and structure of the inner and outer membranes are not the same, so the nuclear membrane is asymmetric bilayer.
It is a membrane with a thickness of about 4-10 nm, which orientes to cytoplasm and connects to RER (rough endoplasmic reticulum). There are some ribosomes adhered to its surface, so that to promote the protein synthesis.
It is a membrane which orientes to nucleoplasm, while there is no ribosome adhered to its surface. Tightly close to the surface of the inner nuclear membrane is nuclear lamina, which looks like medical gauze, acts as the scaffold for the nuclear membrane.
It is the lacuna between the outer and the inner nuclear membrane with a width of about 40-150 nm. Perinuclear space connects to RER and is filled with liquid amorphous substances containing many proteins and enzymes. It is an active exchange channel between the nucleus and cytoplasm.
Nuclear pores are annular structures arranged by protein granules and fiber, and distribute on all eukaryotic cell membrane with the pore size of about 40-150 nm. Nuclear pore is the structure fused by the inner membrane and outer membrane and cannot connect to the perinuclear space. Nuclear pore has nuclear pore complex.
I Types of nuclear pore complex
A complex formed by the arrangement of a number of protein molecules in a specific arrangement.
1. Fish-trap model: Its shape is similar to fish trap. Cytoplasmic ring is at the side of the cytoplasmic side of the nuclear pore and connects to outer nuclear membrane, on which there are 8 fibrils trended to cytoplasm. Nuclear ring is at the side of the karyoplasmic side of the nuclear pore, connects to inner nuclear membrane, on which there are 8 nuclear basket filaments trended to karyoplasm, and the terminuses of 8 nuclear basket filaments form a terminal ring. Spokes extend from the edge of the nuclear pore to the center of the nuclear pore and show radial symmetry. Central plugis is at the center of the nuclear pore.
2. Granule-fibrils model: There are 8 annular granules on the inner and outer rings of the nuclear pore, respectively, 8 peripheral granules at the joint of the double membranes, and a central granule at the center of the nuclear pore. Fibrils are connected to various granules among themselves in order to keep the stabilization of nuclear pore complex.
II Function of nuclear pore complex
The main function of nuclear pore complex is to mediate the material transcription between nuclear pore and cytoplasm.
1. Passive transport: Water-soluble small molecules, such as water molecule, glucose, amino acid, nucleotide and so on, can go in and out of nuclear freely.
2. Active transport: Big molecules or granular materials undergo active transport, which is bidirectionality. Histone, ribosome protein, DNA polymerase, RNA polymerase karyophilic protein, and other nucleophiles are guided by their specific nuclear localization signal (NLS) carried by themselves to perform the nuclear import. The specific base sequences at RNA molecules and RNA - protein complexes (RNP) are the signals of nuclear export, they can be identified by related receptors of the nuclear pore complex to mediate export.
I The regional segmentation of nuclear envelope makes DNA transcription and translation isolate at space and time. Nuclear envelope segregates the cytoplasm with the karyoplasm. So that, the genetic material is regionalized, this makes the replication, transcription and translation of genetic information be isolated completely at space and time, and ensure accurate and efficient expression of eukaryotic genes. Prokaryotic cells are lack of this kind of space and time segregation, so they are called “nucleoid”.
II Regulation of the material transcription and information exchange between cell nucleus and cytoplasm. As the boundary membrane between cell membrane and cytoplasm, nuclear membrane plays an important role in the exchange of substance between nucleus and cytoplasm and determines the type and mode of exchange substances.
Chromatin and chromosome, composed by DNA, histone, non-histone and a small amount of RNA, are two existence forms of genetic materials in cell. Both chromatin and chromosome can be displayed by staining with basic dyes beyond to the same component in the nucleus. They are two structural forms of the cell cycle at different time. During the interphase period, the chromatin appears fine mesh and irregular shape, and scatters in the nucleus. During mitosis period, chromatin highly concentrates, folds and forms bar or dot structures that can be seen under the light microscope, that is chromosome.
The main compositions of chromatin are DNA and histone, which occupy more than 98% of the total chemical composition in chromatin. In addition, there are some non-histone and a small amount of RNA.
The DNA of eukaryotic cells must contain 3 key sequences:
I Replication origin sequence: It is the starting point of cell DNA replication. In eukaryotic cells, multiple replication initiation point sequences can be activated in tandem, so that the entire DNA molecule can be replicated in different regions. DNA replication can start with multiple spots and process simultaneously.
II Centromere sequence: It is a joint of two replicated sister chromatids. This sequence connects to spindle fiber to ensure that the chromosome can be equally divided to two daughter cells with genetic stability.
III Telomere sequence: It is the simple repetitive sequence and exists at amphi-ends of DNA. The telomere DNA is composed by G-rich short tandem repeats. Its function is to ensure the replication completeness of two telomeres of DNA cells.
I Histone
Histone has five compositions of H1, H2A, H2B, H3 and H4. Almost all of the eukaryotic cells are rich in these five histones. Histones are functionally divided into nucleosome histones (H2A, H2B, H3 and H4) and H1 histone. The nucleosomal histones interact each other to form trend of polymer, so that DNA can be crimped to form nucleosome. The H1 histone can be used as a linker between nucleosomes. When combined with DNA, histone can inhibit the replication and transcription of DNA. But once after the chemical modification, the histone phosphorylation will attenuate the affinity with DNA, then DNA unwinds, increasing the active of replication and transcription. Histone methylation could increase the affinity between the histone and DNA, reducing the active of replication and transcription.
II Non-histone
When non-histone combines with histone, they can specially relieve the inhibition of histone to DNA, and enhance replication, transcription and expression.
The content of RNA is rarely small and the content varies a lot
During the interphase, the chromatin is a looser structure and has lighter basic dye, called euchromatin. Their characteristics are:
1. DNA helix is at a low degree.
2. Participate in the processes of DNA replication and RNA transcription.
3. It locates at the center of nuclear, or extends into the nucleolus in loop domain.
4. During mitosis phase, euchromainlocates locate at the chromosomal arm.
During the interphase, the heterochromatin is a tighter structure and has deeper basic dye chromatin. Their characteristics are:
1. DNA helix is at a high degree.
2. It Generally has no transcriptional activity.
3. It usually locates at the margin of nuclear.
4. During mitosis phase, heterochromatin locates among the centromere and the telomere or the chromosomal arm.
Heterochromatin is also classified as constitutive heterochromatin and facultative heterochromatin (or functional heterochromatin). Constitutive heterochromatin is the chromatin that is always in a state of aggregation and has no transcriptional activity in all kinds of cells and the whole life cycle, and locates at the centromere, telomere or secondary constriction. Facultative heterochromatin is the chromatin which is presented a state of aggregation in certain cell types and certain developmental stages, and it can change to euchromatin and restore transcription activity. The exchange of facultative heterochromatin is relevant to the opening and closing of genes.
During the cell division, the chromosome is formed by fold and compression of chromatin.
I First level structure–nucleosome
Nucleosome is the basic structural unit of chromatin, composed by combination of around 200 bp DNA fragment and a histone octamer. Nucleosome core is composed by histone octamer (2H2A+2H2B+2H3+2H4). Approximately 146 bp of DNA surrounds the histone octamer 1.75 circles (83bp per lap). The connection between the closed nucleosome cores is about 54 bp of the linker DNA. H1 locates on the DNA linker and stabilizes the nucleosomal structure. The beaded chains (around 10 nm diameters) are composed by nucleosomes making the length of DNA compressed around 7 times.
II Second level structure–solenoid
The hollow spiral solenoid is formed by the spiral coils of nuclear small beads, whose outer diameter is 30 nm and inner diameter is 11nm, and each circle helix contains 6 solenoids. The distance of two closed helix pitch is 11nm. Histone H1 locates on the inside of the solenoid and plays a stabilizing role in the solenoid. The H1 molecule can make solenoid form or relax, which can regulate the activity of related genes. And the length of the DNA is compressed 6 times again.
III Third level structure-super solenoid
The super solenoid (diameter around 200 nm-400 nm) is formed by the further spiral coils of solenoid. And the length of DNA is composed 40 times again.
IV Fourth level structure-chromatid
The super solenoid further folds to form a single chromatid. The length of DNA is compressed 5 times again.
In conclusion, from DNA to chromatid, the length of DNA is compressed around 8400 times (7×6×40×5).
After chromatin fiber passes through nucleosome and solenoid, chromatin fiber forms the loop with non-histone proteins as its Axis. Every 18 loops are arranged radially in a circle to form miniband, and around 106 minibands sequence along the vertical axis to form chromatid.
I Chromatid
Each metaphase chromosome consists of two chromatids. Two chromatids (sister chromatids) attach to centromere.
II Centromere and primary constriction
In the place where two sister chromatins connect, there is a dented inward and hypochromatic constriction, called primary constriction. The chromatin site of primary constriction which is just the internal structure of primary constriction is called centromere.
III Secondary constriction and satellite
In addition to the primary constriction, some sunken narrowing areas on the long or short arm of chromosome are known as the secondary constriction. Small spherical or virgate globule connected to secondary constriction is satellite.
IV Telomere
Telomere, composed by telomere DNA and telomere protein, is the specialization structure located at the end sites of chromosome and has polarity. Telomere DNA is composed of G-rich short tandem repeats. The base sequence of human telomere is GGGTTA, and this special DNA sequence is synthesized by telomere enzyme. The normal cell cycle is deficient in telomerase. Each time the replication of chromosome, the DNA sequence of telomere will lose 50~100 bp nucleotides. When the telomere becomes short to a certain degree, it indicates that cells are going to age. Telomerase is activated in tumor cells, which can synthesize telomeres and maintain length of telomere during cell division, showing the characters of unlimited reproductive properties.
V Chromosome arms
The chromosome is divided into two parts by centromere, called chromosome arms. The long arm is marked with “q”, while the short arm is marked with “p”.
According to the locations of centromere in chromosome, chromosome can be classified to four types. However, human chromosome only has the first three types of the following at present.
1. Metacentric chromosome: the centromere locates at 4/8 to 5/8 of chromosome.
2. Submetacentric chromosome: the centromere locates at 5/8 to 7/8 of chromosome.
3. Acrocentric chromosome: the centromere locates at 7/8 to 8/8 of chromosome.
4. Telocentric chromosome: the centromere locates at the ends of chromosome.
The number of chromosomes in each species are constant. The number of human chromosomes is 46. Chromosomes of somatic cells exist in pairs. They are diploids and are marked as “2n”. Chromosomes of germ cells are haploids and are marked with “n”.
Karyotype refers to the expression of a complete set of chromosomes in the metaphase of mitosis, which are consisted of chromosome number, length, size, centromere position, number of secondary constriction and the presence and absence of satellite. Karyotype of normal female number is 46, type XX (46, XX); Karyotype of normal male number is 46, type XY (46, XY). The expression of abnormal karyotype: for example, 47, XXY means that this karyotype has one more X chromosome; 45, X means that this karyotype has one less X chromosome.
Chromosome banding is the vertical axis of chromosome showing different depths and width bands after technical disposal. Because of each chromosome has a specific DNA sequence, so it can show its unique banding pattern. Banding pattern of 1q31.2 stands for the second sub-band of 1 band in 3rd zone of long arm area on the NO.1 chromosome.
Nucleolus is the most obvious structure of eukaryotic nuclear at interphase period. Under the light microscope, it is a refracting, homogeneous, non-enveloped spongy sphere. The major chemical component of the nucleolus is protein, which accounts for about 80% of the dry weight, including ribosomal proteins, histones, non-histone proteins and a variety of enzymes. The nucleolus also contains 10% RNA, which binds to the protein and presents as a ribonucleoprotein (RNP) form. Additionally, 8% DNA exists in the nucleolus.
Under electron microscope, the nucleolus is an unbounded membrane and a reticular formation composed of many components, including fibrillar centers (FC), dense fibrillarcomponents (DFC) and granular components.
Fibrillar center, composed by extended loop of DNA for transcribed rRNA, is a low electronic dense area under electron microscopy and the existence site of the rDNA (rRNA gene).
Nucleolar organizer are DNA loops included the main rRNA genes and relate to the formation of nucleolus. Human nucleolar organizer is located at the short arms' secondary constrictions of 13, 14, 15, 21 and 22 chromosomes.
The nucleolar organizer region (NOR) is a region where all nucleolar organizer groups fuse together during the interphase.
The dense fibrillar component is high electronic dense and deep dyeing area, showing a circular or semilunar distribution. It coils around the fibrillar center. Here exists the on-going transcribed rRNA molecule, ribosomal proteins and some special RNA binding proteins.
This region is the site of further processing and maturation of rRNA gene transcription products. The precursor particles of ribosomal subunits, which are at the state of dense granular, densely distribute outside the DFC. The size of the nucleolus is determined by the composition of the granule.
During the cell cycle, the morphology and structure of the nucleolus undergoes periodic change. Nucleolus is obvious at the interphase. But, at the prophase phase of the mitosis, with the agglutination of chromatin, the DNA loops that containing the rRNA genes contracts back to related chromosome secondary constriction. rRNA genes are switched off, rRNA synthesis stops and the nucleolus increasingly shrinks till to disappear (extinction). At the midst phase of mitosis, the presence of nucleolus is not observed. At the telophase phase of the mitosis, the chromosome unwinds, DNA loops with rRNA genes are released, rRNA are resynthesized, and the nucleolus reappears again.
Nucleolus is the synthesis center of rRNA and the area for processing and maturity of rRNA. It is the assembling factory for large or small ribosome subunit.
In the nucleolus, DNA fiber axis area is a series of highly tandem-arranged arrow-like structural units. Both sides of rRNA chains from different transcribed phases compose the structure like the “christmas tree”, which is called transcription complex. Each arrow points in the same direction, every DNA fiber of sagittal unit is a rRNA gene (rDNA). Under the effect of RNA polymerase, the rDNA transcribes out the rRNA with high efficiency. Neonatal rRNA chain vertically extends out from both sides of the long axis of DNA. They are relative shorts when they close to the original apex and gradually increase along the direction of transcription forming a sagittal structure or “christamas- tree-like” structure. Between the two arrows is naked interval rDNA, which is the non-transcribed DNA partition.
Additionally, the nucleolus is also the place of rRNA processing and maturation. Under the action of RNA polymerase I, each rRNA gene in the fibrillar center generates the same primary product– 45S rRNA, which combines with protein carried by cytoplasm and distributes at dense fibrillar component area. After the composition of 45S rRNA, it is cut and processed into 5.8S rRNA, 18S rRNA, and 28S rRNA.
The 18S rRNA combines with 33 kinds of proteins and coils to become small granules. The rRNA of 28S, 5.8S and 5S combine with 49 kinds of proteins and coils to become big granules. These small or large granules are the precursors of the ribosome and distribute in the granule composition areas. After maturity, these granules go into the cytoplasms through the nuclear pore complexs and are assembled into ribosomes. So the nucleolus is manufacturing “factory” for ribosomes.
Nuclear skeleton, also called nuclear matrix, which is composed by non histone proteins, fills the entire nuclear space, connects together with nuclear lamina and nuclear pore complex, and forms the network frame structure.
The main composition of nuclear matrix is fibrin proteins (over 90%), most of which are non-histones. In addition, there is a small amount of RNA. There are certain numbers of RNP granules combined to the nuclear matrix fiber.
The nuclear skeleton plays an important role in the construction of eukaryotic chromosome, regulation of gene expression, DNA replication, damage repair, and processing and transportation of RNA transcription.
The nuclear skeleton supplies the scaffold system for the chromatin and maintains the morphological structure of the nuclear by wrapping around the nucleolus.
DNA anchors at the nuclear skeleton in the form of loop domain. Only when the replication original site is combined to the nuclear skeleton, the synthesis of DNA begins, and at the same time, the enzyme and protein factors that related to the replication of DNA are combined to the nuclear skeleton, too.
There are RNA polymerase binding sites on the nuclear skeleton, so the nuclear skeleton is the RNA processing place for gene transcription and post transcription.
In the eukaryotic cells, the DNA loop anchors to the nuclear skeleton and participates in the construction of chromosome spatial structure.
The nuclear lamina is a high electron density fiber network layer and closes to the inner nuclear membrane. The nuclear lamina is mainly composed by three kinds of nuclear lamina proteins (lamin A, lamin B and lamin C). The molecular weights of these proteins are around 60,000-75,000, which are the member of the intermediate fiber superfamily.
I At the interphase of the cell division, the nuclear lamina supports the nuclear membrane and supplies the anchoring sites of chromatin around the nucleus to stop helix of the anchored chromatin. When cell division, the nuclear lamina is depolymerized and loses chromatin anchoring, and the chromatin condensates to form the chromosome.
II During the cell division, the nuclear lamina participates in many aspects of activity, such as nuclear membrane reconstruction, chromatin condensation and nuclear construction. At the prophase of the cell division, the phosphorylation of the nuclear lamina protein leads to the disintegration of nuclear membrane into nuclear membrane vesicles. Lamin B binds to nuclear membrane vesicles, while lamin A and lamin C disperse in the cytoplasm. At the telophase of cell division, the dephosphorylation of the nuclear lamina protein reassembles the nuclear lamina, mediates the reconstruction of nuclear membrane, and the nuclear membrane vesicles forms the new nuclear membrane after the confluence of membrane.
Since 2021-09-29, 3836 persons visited this page till now.