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Module IV Nucleus Module IV Nucleus Structure and functions of interphase nucleus, Nuclear membrane, pore complex, structure and functions of nucleolus Chromosomes – Structure; Heterochromatin, Euchromatin, Nucleosomes, Nucleus is the most important part of the cell situated in the cytoplasm. All the cellular activities are controlled by it. Nucleus is a directing and organizing unit without which the cell could not exist. It was discovered by Robert Brown (1831) in flowering plants and is now recognized as the structure that contains the hereditary material of the cell. The study of nucleus or karyosome constitutes karyology. The location of nucleus varies in the cell depending upon the species. Usually it is situated in the centre of the cell surrounded on all sides by cytoplasm. In green algae, Acetabularia, it shows various positions, though mainly present in the basal part of cell. Generally the nuclei are scattered in the cytoplasm. Morphology: 1. Shape: The shape of nucleus is variable according to cell type. It is generally spheroid but ellipsoid or flattened nuclei may also occur in certain cells. The nuclear margins are generally smooth but they may be lobulated bearing small infoldings of nuclear membrane as in leucocytes. In certain white blood corpuscles the nucleus is dumbbell-shaped and exhibits variation during life history stages. In human neutrophil, it is trilobed. 2. Number: Mostly cell contains a single nucleus, known as mononucleate cell. Cells containing two nuclei are known as binucleate cells (e.g., Paramecium), and cells of cartilage and liver. Sometimes more than two nuclei (3 to 100 nuclei) are present in a single cell. Such cells are called polynucleate cells. Such cells in animals are called syncytial cells (e.g., osteoblast) and such plant cells are termed coenocytes (e.g., siphonal algae). Cells having distinct nucleus are called eukaryotic types, whereas cells without definite nucleus are called prokaryotic cells, (e.g., bacteria, etc.). The latter possess scattered chromatin material (DNA) in the cytoplasm. The mammalian erythrocytes (eukaryotic cells) possess no nuclei. 3. Size: Size of nucleus is not constant and is generally correlated with DNA contents. The nuclear size is a function of chromosome number, i.e., the size is variable depending upon the number of chromosomes. Usually, there exists a ratio between the nuclear volume (Nv) and the volume of the cytoplasm (Cv) for each cell type. It can be expressed by an equation called nucleoplasmic index (NP) given by R. Hertwig which is as follows: NP = Nv / Cv – Nv This index is fixed for each cell and any disturbance of equilibrium of NP acts as stimulus for cell division. Cells containing more than one set of chromosome are called diploid or polyploid cells, having larger number of chromosomes. Thus, cells with more than the diploid number of chromosomes have usually larger nuclei. The nucleus of resting period, when it does not undergo any division, is called the metabolic nucleus, while dividing nucleus, is called kinetonucleus or arbeitonucleus in German language. The nucleus consists of the following main parts: (1) Nucleolemma or nuclear membrane (karyotheca) (2) Nuclear sap or karyolymph or nucleoplasm (3) Chromatin network or fibres (4) Nucleolus (5) Endosomes. 1. Nuclear membrane (karyotheca): The nucleus is separated from the cytoplasm by a limiting membrane called as karyotheca or nuclear membrane. This membrane plays an important role for the transport of the material between the nucleus and the cytoplasm. Nuclear envelope regulates nucleocytoplasmic exchanges and coordinates gene action with cytoplasmic activity. It has direct connections with the endoplasmic reticulum and during cell division, this nuclear envelope develops as an extension of the endoplasmic reticulum applied to the nucleus and subsequently modified. In the end of mitosis, ie, in telophase, the cisternae of the endoplasmic reticulum gather around the chromosomes and by fusing together they form the nuclear membrane. Structure: The nuclear membrane appears to be a double membrane having interruptions or pores at intervals. The outer one is called ectokaryotheca and inner one is termed endokaryotheca. Each of the membranes is about 75 to 90 A thick and both the membranes enclose an intervening space is called as perinuclear space or cisterna. Outer nuclear membrane is comparatively thicker than the inner membrane and has a rough outline due to the presence of attached RNA particles which may form spirals, parallel lines or crescents. The inner membrane is smooth having no ribosomes. The nuclear membrane is followed by a supporting membrane, the fibrous lamina, of uniform thickness (300 A thick). Nuclear pores: The nuclear membrane possesses a number of nuclear pores or annuli, which vary from 40 to 145 per square micro-meter in nuclei of various plants and animals. Watson (1959) stated the number of pores in mammalian cells as 10 percent of the total surface of the nucleus. In amphibian oocytes, certain plant cells and protozoa the surface occupied by the pores may be as high as 20 to 36 percent. The nuclear pores are octagonal in shape, their diameter varies from 400-1000 A, and they are separated from each other by a space of 1500 A. The nuclear pores are enclosed by circular annuli. At the annulus the inner and outer membranes of the nuclear envelope fuse. The pores and annuli collectively form the pore complex. Each annulus consists of eight granules of about 15 nm, which are present on both the nuclear and cytoplasmic surfaces. Inside the pore is a central granule. Fine fibrils (about 30 A in diameter) extend from central granule to the peripheral granules, forming a cartwheel structure. A less defined amorphous annular material is present in the opening itself. This material is digested by trypsin and remains unaffected when exposed to ribonuclease and deoxyribonuclease. It means that annular material is protein in nature. The pore complex is a rigid structure present in a fixed number according to cell type. In certain physiological stages, however, they may change in number. For example, they are reduced in number in maturing erythroblasts and spermatids and it is due to low transcriptional activity of these cells. In some cases, the pore complex is covered by a thin membrane. It has been suggested that the annulus may serve as a sphincter, alternately decreasing and increasing the size of the pore with varying conditions. Some evidence suggests the presence of myosin in the annulus area (Du Praw, 1970). Annulus is supposed to be a hollow cylinder fitting into the nuclear pore (Witschnitzer, 1958). The lumen of the cylinder is 500 A in diameters representing the nuclear pore. The wall of the cylinder consists of eight evenly placed microtubules or microcylinders. Each microtubule is about 200 A in diameters. According to Viviers, a central microtubule of 150 to 180 A diameters is present with in the lumen of annulus and is attached to its inner wall by fibrous struts. Amorphous annular material extends beyond the pore margins (Franke and co- workers, 1966-74). The materials exchanged between nucleus and cytoplasm must traverse the nuclear pore complexes. Thus, annuli or pores control the passage of some molecules and particles, even some ribosome components, between nucleus and cytoplasm. This exchange is very selective and allows passage of only certain molecules of either low or very high molecular weight. The nuclear envelope is a diffusion barrier for ions as small as K+, Na+ or Сl–. On the other hand, very large structures such as ribosomal subunits, which are assembled in the nucleolus, are able to leave the nucleus through the nuclear pore complexes. The unit membranes of karyotheca are composed of protein and lipid, like plasma membrane. At the margins of these pores, the two unit membranes are continuous and at certain places this nuclear membrane joins the membrane of endoplasmic reticulum. 2. Nuclear sap (karyolymph or nucleoplasm): The nucleus contains a transparent, semi-solid, granular and homogeneous matrix during interphase called as nuclear sap or karyolymph. This karyolymph is a fluid substance containing many particles and network. Primarily it is composed of proteinous material and is the main site for enzyme activity. This nuclear sap also shows variable appearance during different stages of cell division. There is some evidence that karyolymph contributes to the formation of the spindle apparatus in plants. Nuclear constituents: The nucleus contains RNA, DNA, proteins of two kinds, histone and nonhistone; some lipids; various organic phosphorus compounds; and various inorganic compounds, mostly salts (Davidson, 1976). DNA: Generally less than half the dry weight is DNA but the amount varies from species to species. DNA remains constant and do not vary with nutrition or during starvation, while proteins in nuclei vary, presistent (ultra-structure,) RNAs: Three kinds of RNA are present in cells: ribosomal (rRNA), transfer (tRNA) and messenger (mRNA). RNA contains nucleotides of the purines, adenine and guanine and pyrimidines, cytosine and uracil. In any of these RNAs the ratio of the bases to one another is constant for a species. However, rRNA, tRNA and mRNA separated from one another disclose different base ratio, indicating difference^ in their chemical composition. The rRNA is synthesized and assembled in the nucleolus, tRNAs and mRNAs are synthesized on the chromosomes, and all the RNAs enter the cytoplasm through the nuclear pores. Enzymes: The nucleus contains a number of enzymes and performs metabolism, including synthesis of DNA and various RNAs. Nucleus lacks the enzymes for aerobic metabolism that are found in mitochondria, but it contains enzymes for anaerobic metabolism and for formation of high-energy phosphates. It also contains enzymes for coenzyme synthesis (nicotinamide adenine dinucleotide or NAD). Proteins: A variety of proteins is present in the nucleus: nucleoproteins, enzymes and structural proteins. The nucleoproteins form two classes, deoxyribonucleoproteins and ribonucleoproteins. Deoxyribonucleoproteins: These largely form the chromosomes; consist primarily of histories and DNA in about equal amounts.
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