The following points highlight the five main components of the nucleus. The components are: 1. Nuclear envelope 2. Nuclear Sap or Nucleoplasm 3. Nucleolus 4. Chromatic Reticulum 5. Chromosomes.
Nucleus: Component # 1. Nuclear Envelope:
The nuclear membrane or karyotheca is the outer boundary of the nucleus. It is found in the nuclei of all the eukaryotic cells. Electron microscopy has revealed that it is composed of two concentric unit membranes, an outer membrane and an inner membrane of lipoproteins (Fig. 8.2). According to Du Praw, each membrane is about 80-100 Å thick. The outer and inner membranes are separated by a clear space of 100 to 150 Å.
The space between the two unit membranes is known as perinuclear space or perinuclear cisterna. Outer nuclear membrane is rough surfaced due to attached ribosomes. Sometimes it becomes out-folded to give rise to endoplasmic reticulum. The inner nuclear membrane is free from ribosomes and sometimes it is associated with chromatin.
In many vertebrate cells there is a layer of densely packed fibres just inside the inner nuclear membrane forming a third concentric sheath of uniform thickness (about 300 Å). This is known as fibrous lamina. It is not known whether these layers have any special function other than mechanical support (Fig. 8.3).
The nuclear envelope possesses special circular structures, the annuli (singular-annulus) through which certain materials can pass. The presence of annuli is the most characteristic structural feature of the nuclear envelope (Fig. 8.2).
Afzelius (1963) found that the nuclear envelope in luminescent marine dinoflagellate noctiluca has a single membrane only which is 150 Å thick and completely lacking annuli. Annuli can be observed only with electron microscope.
They were first discovered by H.G. Callan and Tomlin in 1950. Some authors call the entire annular structure a ‘pore’ some others call the entire structure an annulus and still some others use both terms. The annulus is a thickened ring surrounding a central pore.
Since they almost do not represent permanent openings in the nuclear envelope, the term ‘pore’ and ‘pore complex’ are misleading. Thus, it seems preferable to use the term ‘annulus’, in precise sense, signifying a thickened, electron dense ring of material set into the nuclear envelope and functioning by some unknown mechanism to control the passage of particles in and out of the nucleus.
It allows the passage of materials but only through a restricted region. That is not simply a pore but a directional mechanism of transport.
Some authors have suggested that typical nuclear envelopes are penetrated by permanent pores of500-800 A diam., but some kinds of protein molecules only 100 Å or less in diameter are unable to enter the nucleus from the cytoplasm. This clearly indicates that there is no existence of permanent openings of about 800 A diam.
On the outer extreme the nuclear envelope is not completely closed to passage of large particles, but this maintains a very sensitive control over which particles pass and in which direction. One of the best established features of annulus ultrastructure is that inner and outer nuclear membranes come together in the margin of each annulus.
The middle of the annulus in many but not all nuclei is also covered by a thin single membrane, which evidently checks the free passage of ions or other molecules across the envelope. Annuli have an outer diameter of 1200 A and inner diameter between 0 and 500 Å (Fig. 8.3).
The structure of each annulus is eight-partite. Somewhat more specifically, 8 spheroid particles have been found to be arranged around the circumference of each annulus in the nuclear envelope of amoeba (Fig. 8.4) and of pea seedlings. Annulus shows contraction and relaxation.
In unstretched annuli a thin membrane is present in the centre. A relaxed annulus would contain a temporary central opening large enough to allow passage of resolvable particles as ribosomes. Different annuli in the same nuclear envelope can exist in an expanded or open configuration.
The nuclear membrane can easily be observed in the metabolic nucleus. It disappears at certain stage of nuclear division and reappears on the completion of the division process.
There is no well -defined nuclear envelope in the prokaryotic cells.
Nucleus: Component # 2. Nuclear Sap or Nucleoplasm:
The undifferentiated protoplasm or ground substance present inside the nuclear envelope is called nuclear sap or nucleoplasm or karyolymph. The nucleoplasm seems to contain granules of various sizes and densities and is yet of unknown composition and function.
During nuclear division, the nucleoplasm is of course continuous with cytoplasmic matrix. The other nuclear components such as the chromatic reticulum and nucleolus remain suspended in the nucleoplasm.
Nucleoplasm contains nucleoproteins and many other inorganic and organic substances, such as, nucleic acids, proteins, enzymes, lipids and minerals.
Nucleus: Component # 3. Nucleolus:
It is an intranuclear organelle of eukaryotes. The term nucleolus was coined by Bowman (1840). It occurs as a spherical, acidophilous structure suspended in the nucleoplasm either in central or eccentric position. The nucleolus is found in close association of a specific chromosome at a fixed point called nucleolus-organising region (Fig. 8.5).
The number of nucleoli present in each nucleus is fixed for the cells of a particular species of plant or animal and it depends upon the number of chromosomes or set of chromosomes. In many plants and animals, there is only one nucleolus for each haploid set of chromosomes. In vicia faba, for example, each diploid nucleus contains two nucleoli.
In some organisms there may be two or more nucleoli for each haploid set of chromosomes and in some others the number of nucleoli within the nucleus changes with the advancement in the age of the cell. Many lower organisms, such as bacteria, yeasts and some algae, undifferentiated embryonic cells of amphibians, and even certain mammalian cells like erythrocytes, reticulocytes and spermatozoa lack nucleoli.
The nuclei of polyploids may have more nucleoli than those of diploids.
Structure:
The ultrastructure of nucleolus has been reviewed by Day (1968), Bernhard and Granboulan (1968) and Bush and Smetana (1970).
Four major components are normally observed in nucleoli of all organisms [Fig. 8.5 (B)]. These are:
(i) Particulate or granular, the pars granulosa,
(ii) Fibrillar component, the pars fibrosa,
(iii) Nucleolar associated chromatin, the pars chromosoma,
(iv) Amorphous part, the pars amorpha.
The pars granulosa. The particulate component is made up of ribonucleoprotein granules, organised as ribosome like particles, 150 to 200 A in diameter. Although these particles are similar in from, chemical content and staining reaction to cytoplasmic ribosomes, they are slightly smaller than ribosomes.
These granules contain 25 S RNA and protein in 1:2ratio. Birnstiel et al. (1963) have referred these particles as nucleolar ribosomes.
The pars fibrosa comprises of the protein containing fibrils, 50 to 150 Å in diameter.
The granules of particulate components are connected together by protein fibrils of pars fibrosa and may exhibit tortuously coiled filamentous structure. Thus pars granulosa and pars fibrosa make up the microscopically identifiable permanent structure called nucleolonema.
The nucleolonema is considered as one of the important and persistent spongy components of nucleolus. It never disappears completely in course of any metabolic change. Its existence has been noted even during cell division. Thus, there is no question of it being developed de novo.
The pars amorpha or amorphous part is mostly protein and consists of tightly packed fibrils and particles in some organisms.
In animal nucleolus, the nucleolonema generally appears embedded in pars amorpha (i.e., intermingled), whereas in plants the peripheral nucleolonema surrounds the centrally located pass amorpha. In either case, the nucleolonema tends to be associated with chromosomal heterochromatin.
The pars chromosoma or nucleolus associated chromatin is a network penetrating the body of nucleolus, but attached to the nucleolus organising region of chromosome. The DNA of nucleolar chromatin serves as a template for the synthesis of RNA.
Functions of Nucleolus:
The main functions of nucleolus are as follows:
1. RNA production. The nucleolus is one of the most active sites of RNA synthesis. It produces 70-90 per cent RNA in most of the cells. Ribosomal RNA (rRNA) is synthesised and stored in nucleolus. Ribosomal RNA is transcribed by rDNA.
The tailoring of 45S rRNA into 5.8S rRNA, 18 S rRNA and 28 S rRNA and their subsequent methylation occur in nucleolus. The rRNA and ribosomal proteins become complexed to form granules which are precursors of ribosomes.
rDNA-> Fibrils (Containing rRNA) + ribosomal proteins —> Granules (ribosomal sub-units)
The nucleolus thus makes ribosomal sub-units rather than whole ribosomes. In addition to rRNA, nucleolus also produces some sorts of mRNA (messenger RNA) and at least one type of RNA of low molecular weight.
2. Protein synthesis. According to Maggio (1960) and some other workers protein synthesis takes place in nucleolus, especially that of ribosomal proteins, but others believe that ribosomal proteins are synthesized in the cytoplasm.
3. Ribosome formation. In eukaryotes, rDNA serves as template for 45 S rRNA. Half of the 45 S rRNA is broken down to form 28 S and 18 S rRNAs and the other half is degraded to nucleotides within the nucleolus. 28 S rRNA combines with intranucleolar proteins to form 60 S sub-units of ribosomes and the 18 S rRNA combines with extra nucleolar proteins to form 40 S sub-units of ribosomes.
The 60 S and 40 S sub-units of ribosome combine to form ribosome granules (Fig. 8.6).
Nucleus: Component # 4. Chromatic Reticulum:
In the nucleoplasm a dark staining network is seen which is termed nuclear reticulum or chromatic reticulum. The threads of reticulum are formed of chromatin. The chromatin network can easily be seen in the interphase nucleus under compound microscope. During the cell divisions chromatin fibres become thick ribbon-like structures and are called chromosomes.
The network can readily be stained with basic dyes, such as acetocarmine, haematoxylene, crystal violet and Feulgen stain. The chromatin is a characteristic substance in the nucleus which is of great importance to the cell.
The fibres of chromatin are twisted, uniformly distributed and anastomosed. Two types of chromatin have been recognised.
These are:
(i) Heterochromatin; and
(ii) Euchromatin.
Heterochromatin is darkly stained, condensed and well delimited region in the chromatin thread.
The light stained and diffused regions of the chromatin is called euchromatin. The heterochromatin and euchromatin both contain DNA which is the basic genetic material, and thus do not differ in this regard. Thus heterochromatin and euchromatin are two different states of chromosomes rather than substances.
Spectrophotometric measurements have disclosed that the quantity of DNA per unit area may be two or three times higher in the heterochromatin than in the euchromatin in the same nucleus.
Nucleus: Component # 5. Chromosomes:
The term chromosome refers only to the deeply staining DNA containing bodies observed in dividing cell of nucleate organisms. Chromosomes are nuclear components possessing a special organization, individuality and function. They are capable of reproducing themselves without change of morphology and physiologic behaviour in successive generations.
A detailed account on the morphology of chromosomes is given in a separate chapter.
A nucleus of a cell which is not in the process of division is known as interphase or metabolic nucleus. A cell without nucleus cannot survive for long. Unicellular organisms like Amoeba and Acetabularia from which the nuclei were removed were unable to survive for long unless new nuclei from other cells were transplanted thus the nucleus is essential for the survival of the cell.
There are three important functions of the nucleus:
(i) Cell maintenance,
(ii) Cell replication, and
(iii) Control of cytoplasmic activities.
Most of these functions are due to the presence of DNA in the nucleus.