nucleolus

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The most prominent structure in the interphase nucleus of eukaryotic cells

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The nucleolus usually presents as a single or multiple homogeneous spherical bodies Eukaryotic cell The most prominent structure in the interphase nucleus. The size, shape and number of nucleoli depend on the species, cell type and Cellular metabolism State. protein synthesis Vigorous and actively growing cells such as Secretory cell 、 oocyte The nucleolus is large, accounting for 25% of the total nuclear volume. Cells without protein synthesis ability are as follows Muscle cell , dormant plant cell The nucleolus is very small.^[1]

Chinese name: nucleolus
Foreign name: nucleolus

Period of existence: Cell division interval
Presence location: Intranuclear
Form: Homogeneous sphere

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▪ Nucleolar development

Introduction to nucleolus

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Nucleolar interior

Nucleolus is Eukaryotic cell The most obvious structure in interphase nucleus. It is located in the nucleus. Has made contributions to reproduction and gathered ribosome 。 Once assembled ribosomes from nucleus Come out and enter cytoplasm 。 The study of nucleolus structure, dynamics and function is not only for the early cytology Since the discovery of the important function of nucleolus in the 1960s, researchers in various fields have been paying close attention to it.

In light microscopic stained cells, or Phase contrast microscope Lower Living cell The nucleolus can be easily seen in the middle, or in the nucleus of the separated cell. It is usually a single or multiple homogeneous spherical bodies, which are medium round or oval Granular Structure, no adventitia 。

nucleolus ultrastructural yes Fiber center （FC）、 Dense fiber component (DFC)、 Particle composition ( GC ）Three characteristic areas.

U Fiber center (FC): one or several low points surrounded by DFC electron density The main components are rDNA , can be seen as rRNA The place where genes are stored.

u Dense fiber component (DFC): It is composed of dense fibers and has the highest electron density in nucleolus Neosynthesis RRNA. and Binding protein Place of existence, rRNA. Cutting and processing place.

u Particle composition (GC): By Ribonucleoprotein granule Composition is being processed and matured Ribosome subunit The precursor particles of protease And RNase（ Ribonuclease ）Hydrolysis.

Nucleolar components include rRNA ， rDNA and Ribonucleoprotein 。 Nucleolus is RRNA gene Storage, rRNA synthesis, processing and Ribosome subunit Assembly site.

The size and shape of nucleolus depend on the species, cell type and Cellular metabolism State. protein synthesis Vigorous and actively growing cells, such as Secretory cell 、 oocyte The nucleolus of the nucleus is large, accounting for 25% of the total nuclear volume; Cells without protein synthesis ability, such as Muscle cell , dormant plant cell The nucleolus is very small.

The nucleolus number of various organisms is generally certain, such as Africa Rana clawed There are two nucleoli. Human cells have only one nucleolus.

stay cell cycle In the process, nucleolus is a highly dynamic structure mitosis During this period, it showed periodic disappearance and reconstruction. cell division The nucleolus disappears when the cell division ends, and two daughter cells produce new nucleoli respectively. Nucleolus is a kind of dynamic structure, which changes with the change of cell cycle, that is, formation disappearance formation. This change is called nucleolar cycle 。 During the cell cycle, nucleoli are separated and reassembled. When cells enter mitosis, nucleoli first deform and become smaller; later Chromatin Agglutination and stop Ribonucleic acid (RNA) synthesis, consisting of Ribosomal RNA (rRNA) gene DNA The loop gradually shrinks back to the corresponding chromosome Nucleolar organizer region ； nuclear membrane The nucleolus disappears when the rupture enters the metaphase; stay mitosis At the end of the phase, the DNA in the nucleolar tissue region is de agglutinated and the rRNA synthesis starts again. The smallest nucleolus reappears near the nucleolar tissue region of the chromosome. The process of nucleolar reconstruction is related to the assistance and participation of the original nucleolar components. The molecular mechanism of nucleolus formation is unclear, but it needs RRNA gene Activation of.

Nucleolar organizer region (nucleus organizer region): rRNA sequence region, which is related to the formation of interphase nucleolus, and constitutes one or several specific chromosome segments of nucleolus. This segment of DNA is transcribed into rRNA.

Human nucleolar tissue region is located at one end of 10 (5 pairs) chromosomes. There were 10 newly born nucleoli, but they were all very small, and soon fused into a large nucleolus.

Function introduction

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nucleolus

The shape, size and number of nucleoli vary with biological species, cell type and physiological status, but the function of nucleoli is the same. The main function of nucleolus is to ribosome And RNA（ rRNA ）Synthesis of.

Transcription of RNA gene

RNA Gene mapping stay Nucleolar organizer region The gene in this region encodes 18S, 5.8S and 28S RNA, and these three genes constitute one Transcription unit 。 RRNA gene stay Chromatin Axoneme Up in series Repeating arrangement ； Along the transcriptional direction, the new RNA chain starts from DNA The two sides of the long axis stretch out vertically, and gradually grow regularly from one end to the other, forming an arrow shaped shape like a "Christmas tree". Each arrow like structure represents the RNA gene transcription unit, and there are naked DNA spacer segments that are not transcribed between the arrow like structures.

Processing and maturation of RNA precursors

DNA transcription units transcribe 45S RNA precursors, which are soon methylated and spliced into 41S RNA precursors; 41S RNA in the same Splice site Different intermediate precursor RNA can be generated according to different splicing order, and 41S RNA precursor can be spliced into 28S, 18S and 5.8S RNA finally.

Ribosome subunit Assembly of

One of the main functions of nucleolus is assembly protein synthesis 's machine—— ribosome 。 The biogenesis of ribosomes includes RNA synthesis, processing and Ribosome subunit The assembly process of is a vector process. Starting from the nucleolar fiber component Particle composition Continuation. Ribosome small Subunit Maturity is earlier, and the large subunit matures later. Ribosomal Maturation Only when their subunits are transferred to cytoplasm Later, the two subunits only passed through Nuclear pore Only when entering the cytoplasm can functional units be formed. This can prevent functional ribosomes and incomplete nuclear processing hnRNA Molecular proximity. The 5S RNA gene in the large subunit is not located in the nucleolus, and its location is different in different species.

Microscopic observation

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Nucleolar nature

Eukaryotic cell nuclear inclusion Ribonucleic acid (RNA) structure, participating in ribosome Synthesis of ribonucleic acid (rRNA) and formation of ribosome. stay optical microscope Under observation, the nucleolus is generally spherical or ovoid Basophil Structure, often located in the center of the nucleus. Its size varies greatly with cell type and physiological state. stay protein Cells with exuberant synthesis usually have large nucleoli, otherwise they are small in size; The former is as follows Secretory cell 、 neuron and tumor cell Etc., the latter such as Split ball and muscle cells Etc. Because ribosomes are intracellular protein synthesis The function of nucleolus is to synthesize rRNA, so nucleolus is often called "ribosome factory". RNA contained in nucleolus accounts for about dry weight 5~10%, formed by combining with protein Ribonucleoprotein (RNP)。 In addition, it also contains a small amount of DNA, mainly in the nucleolus Chromatin part.

Microstructure

The nucleoli of many types of cells are Nucleolar filament It is composed of a mesh structure, in which the gaps are filled with amorphous stroma. stay electron microscope Under observation, nucleolar filaments are composed of tightly interwoven 5 nm filaments, with dense particles of 15-20 nm in diameter scattered among them. A part of the periphery of the nucleolar region surrounds the nucleolus Chromatin (Figure 1). Nucleolar associated chromatin yes Heterochromatin However, some of them extend into the nucleolus in the form of loops Euchromatin 。 Composed of the above three components three-dimensional structure , even in the same cell, it will change due to its different physiological status. Many evidences indicate that nucleolar filaments contain precursors of granular components. For example, the results of autoradiography combined with biochemical analysis showed that the initial product (45S RNA) of the rRNA encoded gene (rDNA) first appeared in the filament component, then was processed in the nucleolus, and then located in the particle component. Using the three-dimensional reconstruction technology of electron microscope to discover the nucleolus and nuclear membrane There are direct or indirect connections. Indirect connection Including a variety of derivative structures with nuclear membrane, such as ring Lamella , nuclear bud, nuclear channel and Intranuclear small Connection of pipes, etc. The above phenomenon is particularly easy to be seen in cells with vigorous growth, so it may be conducive to accelerating the exchange of cytoplasmic and nucleolar materials.

Nucleolar tissue

stay Cell mitosis During the process, nucleolus appears Periodic change 。 Generally Prophase It gradually disappears, and its filament and particle components are lost in Nucleoplasm in stay Anaphase It reappears. The formation of nucleolus is often closely related to a certain region of a specific chromosome. After the nucleolus disappears, this region forms the Secondary constriction 。 The secondary constriction is generally on Nucleolar organizer region Role of. Many activities plant cell There is only one pair of such chromosomes, called Satellite chromosome 。 However, there are several pairs of many kinds of cells, even many pairs of such chromosomes are involved in nucleolar formation. For example, nucleolar tissue regions exist on chromosome 13, 14, 15, 21 and 22 pairs of human cells. In fact, nucleolus can be regarded as an extension of these chromosomes, often showing strong fusion tendency, so there are many pairs Nucleolar chromosome Only one nucleolus is usually formed in the nucleus of. For example, people Lymphoblast In prophase of division, several nucleolar chromosomes Nucleolar organizer region Attached to the same nucleolus that is about to disappear.

There is rRNA gene rDNA in nucleolar tissue region; At all Eukaryote There are about 50~1000 identical copies of this gene in 30% of cells, so the synthesis of an appropriate amount of rRNA can be maintained. Eukaryotic cell In addition to the transcriptional region, each unit of rDNA also has many non transcriptional regions, called the interval sequence (Figure 2). The length of this interval sequence often varies greatly between different species, or different individuals of the same species, or even different rRNA units of the same individual cell. The significance of this difference and the function of interval sequence.

Nucleolar development

nucleolus

Amphibian oocyte Early stage of development（ Meiosis Prophase Pachytene stage ）, rDNA mass replication, this process is called Gene amplification 。 These newly copied rDNA are not integrated with chromosomes, but exist in hundreds to thousands of extra chromosomal nucleoli in the form of circular molecules embryonic development Synthesize a large amount in the process ribosome Prepare the required rRNA. E.g. slip Xenopus laevis Of oocytes Bivalent Chromosome (4C) contains 12pg DNA [ haploid (1C) genome 3 pg), while oocytes contain about 30 pg rDNA. Therefore, 1~2 × 106 additional RRNA gene It exists in about 1000 free nucleoli. In the nucleus of a Xenopus oocyte, these actively transcribed genes can produce about 300000 rRNA precursor molecules per second, but an individual cell can only produce 10 to 100 such molecules at the same time. 　 fish 、 insect and Molluscs The amplification of rRNA gene was also found in 30% of oocytes, especially in amphibian oocytes. In addition, only in one Transcription unit 18S and 28S were amplified. most Eukaryotic cell in ribosome The other structural component 5SRNA of Chromosome gene Coded. There are two types of Xenopus 5SRNA genes, one is in the Oogenesis The other is expressed in somatic cell Internal expression. Somatic 5SDNA copy number Nearly 450, and nearly 24000 oocytes. Therefore, 5SRNA gene does not need to occur during oogenesis In vitro amplification of staining Phenomenon.

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