Nucleus is the control center of cells, which plays an important role in cell metabolism, growth and differentiation, and is the main place of genetic material. Generally speaking, eukaryotic cells will die soon after losing their nuclei, but red blood cells can survive for 120 days after losing their nuclei. Sieve cells, a plant, can live for several years after losing their nuclei. Although the shape of the nucleus is varied, its basic structure is basically the same, that is, it is mainly composed of nuclear membrane, chromatin, nucleolus and nuclear skeleton.
The nuclear membrane consists of two unit membranes, which separate the nucleus from the cytoplasm. There is a gap between the two membranes, which is called perinuclear cavity. There are perforations on the membrane, accounting for more than 8% of the membrane area.
The nuclear membrane makes the nucleus a relatively independent system in the cell, forming a relatively stable environment in the nucleus. At the same time, the nuclear membrane is a selectively permeable membrane, which plays a role in controlling the exchange of substances between the nucleus and cytoplasm.
Chromatin: Chromatin is a rosary complex composed of DNA, histone, non-histone and a small amount of RNA in interphase nuclei of eukaryotic cells. It is the formation of genetic material between cells. Easily colored by alkaline substances.
Nuclear fluid: It is the matrix without obvious structure in the nucleus.
Nucleoli often appear in interphase nuclei and are homogeneous spheres. Their shape, size and number vary with biological species, cell formation and physiological state. The main function of nucleoli is to synthesize ribosomal RNA.
From its structure, we can get the function of the nucleus: controlling the inheritance, growth and development of cells. Hamlin's umbrella grafting experiment in Algology proved that the nucleus is the carrier of genetic material.
1, nuclear shell
Nuclear membrane (including nuclear pore complex) is a ubiquitous structure in eukaryotic cells. They are not only the dividing line between cytoplasm and nucleus, but also control the exchange of materials and information between nucleus and cytoplasm. The nuclear membrane is a bilayer membrane with a thickness of about 7 ~ 8 nm, and there is a perinuclear gap with a width of 10 ~ 50 nm between the membranes.
The nuclear shell can be divided into three areas:
Outer nuclear membrane: facing cytoplasm, with ribosome particles attached and connected with endoplasmic reticulum.
Endonuclear membrane: Facing the nuclear substance, there are no ribose particles on the surface, and there are specific protein on the membrane, which provides a binding site for the nuclear fiber layer.
Nuclear pores: Annular openings are formed at the junction of the intima and adventitia, also known as nuclear pore complex, with a diameter of 50 ~ 100 nm, and there are generally thousands. The nuclear pore structure is complex, containing more than 100 kinds of protein, which is closely combined with the nuclear fiber layer to form a nuclear pore complex. This is a selective two-way channel. Its functions are selective macromolecular pathway (active transport), enzyme, histone, mRNA and tRNA；; There is a potential difference, which has a certain regulatory effect on the entry and exit of ions.
The nuclear fiber layer is a layer of fibrin sheet or fiber net with a thickness of 20 ~ 50 nm, which is close to the nuclear intima, and its component is intermediate fibrin, which is called laminin. The nuclear fiber layer is composed of cytoplasmic skeleton and nuclear skeleton. It is generally believed that the nuclear fiber layer provides a structural scaffold for the nuclear membrane and chromatin.
2. Chromosomes and chromatin
Chromatin and chromosome are not different in chemical composition, but have different configurations at different functional stages. Chromatin refers to the linear composite structure composed of DNA, histone and non-histone and a small amount of RNA in interphase cells, and it is the existing form of interphase cell genetic material. After fixed staining, under the optical microscope, we can see that many thick or thin filaments interweave into a net in the nucleus, which can be divided into euchromatin and heterochromatin in morphology. Euchromatin is filamentous and is a part of long-chain DNA molecules. It is very slender and light in color. Heterochromatin is a large dark mass that often adheres to the inner surface of nuclear membrane, and long-chain molecules of DNA contract and curl. Chromosome is a rod-shaped structure formed by chromatin concentration during mitosis or meiosis.
The main components of chromatin are DNA, protein (histone and non-histone) and a small amount of RNA. There are progenitor cells and non-progenitor cells in protein. Histones are rich in lys, Arg and alkalinity, and can bind with negatively charged DNA. They can be divided into five types: H 1, H2A, H2B, H3 and H4. Nonhistones are enzymes involved in DNA replication and transcription.
The structural monomer of chromatin is nucleosome with a diameter of about 10nm, which is connected by filaments of 1.5 ~ 2.5 nm. The core consists of four groups of histones (H2A, H2B, H3, H4), and DNA is wound around the core. Nucleosomes are connected DNA with 1 and 1.
3, nucleolus
The round or oval granular structure in the nucleus has no outer membrane. In protein, cells with strong synthesis often have large or multiple nucleoli, which are rich in protein and RNA molecules. The nucleolus consists of three parts: particle component, fiber center and dense fiber component. Nucleolar components include rRNA, rDNA and ribonucleoprotein. Nucleoli are places where rRNA genes are stored, rRNA is synthesized and processed, and ribosome subunits are assembled.
Nucleolar organizer region: that is, rRNA sequence region, which is related to the formation of nucleolus between cells and constitutes one or several specific chromosome fragments of nucleolus. The DNA of this fragment is transcribed into the location of rRNA, rRNA and rrna.
4. Nuclear skeleton
The nuclear skeleton is a reticular structure composed of fibrin, and its protein component is said to exist in the cytoplasmic skeleton, so the nuclear skeleton should also exist. But now only keratin and myosin are found in the nuclear skeleton, and microtubules are also found in the nuclear skeleton of some protozoa. At the same time, there is a small amount of RNA in the nuclear skeleton, which is necessary to maintain the structural integrity of the nuclear skeleton three-dimensional network. In the evolutionary trend, the nuclear skeleton components changed from diversification to simplification and specialization.