In this article we will discuss about the structure of nucleus with the help of suitable diagrams.
The nucleus (Fig. 1.11) is generally a round body occupying the centre of the cell. Its shape, size, position and number vary. The nucleus may contain many lobes. Usually most mature cells possess a nucleus, but there are certain larger cells in the body which may contain more than one nucleus. If the nucleus is removed, the cell dies. The nuclear material differs from the cytoplasm in several respects.
First, it is more opaque to the ultraviolet rays.
Secondly, it shows many selective staining reactions but usually takes basic stain, while cytoplasm may take neutral, basic or acid stain.
Thirdly, the nucleus is very rich in deoxyribonucleic acid (DNA), while cytoplasm is rich in ribonucleic acid (RNA).
The nuclear membrane is also a unit membrane. Surrounding the nucleus, there is a lipoprotein nuclear envelope. This envelope is double layered and the spaces between two folds are known as perinuclear cisterns. In the apparently quite permeable membrane of the nucleus, pores (areas of discontinuity) of about 6 mn in diameter are closed by a thin homogeneous membrane which permits passage of molecules from the nucleus to the cytoplasm (Fig. 1.12). This indicates connecting link between the genes and ribosomes, the site of cytoplasmic protein synthesis.
According to staining reactions, two types of nucleoli are found. Those taking basic stain are called karyosome, and those taking acid stain, plasmosome. The body of the nucleus is made up of a fine network of a particular substance, called linin.
The meshes of this network are filled up with clear protoplasm – the nucleoplasm (karyoplasm, karyolymph or nuclear sap). In unstained specimens nothing more can be seen. But in stained specimen, numerous particles of blue-staining materials of irregular shape but smaller than nucleoli are found in the nucleus. This material is generally described as chromatin.
It was believed for many years that chromatin was the breakdown products of chromosome during the interval between successive cell division (resting stage or more correctly the interphase stage) and chromosome is the recollection of chromatin into visible rod-like structure during cell division, but very recent observations hold just the opposite views. Because if chromatins contain different genes which determine the heredity of the cell and again the reassembly of different chromatins to form chromosome then it would invite different accidents.
Because in such case the genes belong to one chromosome would have been incorporated into another chromosome. Recent observations thus claim that chromatins seen in interphase nucleus are nothing but densely stained certain scattered portions of chromosomes which are visible in the microscope. This visibility of the chromosome mainly depends upon the coiling and uncoiling of the chromosomes.
During cell division the chromosomes become tightly coiled and this coiled chromosomes or the coiling portions of the chromosomes are stained deeply. But following cell division or in interphase stage the coiled chromosomes become uncoiled all over its length but some portions still remain coiled. These tightly coiled portions become visible as granules or granular mass in the interphase nuclei.
Chromatin can thus be described as heteropyknotic, because of having one or two densities. On the basis of density in staining, the chromatin can be grouped into two types (flow chart 1.2). The coiled portions of the chromosomes are called the positively heteropyknotic and the uncoiled (expanded) portion is called the negatively heteropyknotic.
But on the genetic basis, there is another terminology for chromatin. The extended portion of the chromosome is genetically active and is called euchromatin and the coiled portion (positively heteropyknotic) is genetically inactive and is termed heterochromatin (Fig. 1.15). The euchromatin is actively engaged in the synthesis of specific messenger RNA (mRNA). It is believed that the heterochromatic portion is concentrated with DNA and RNA while the euchromatic region contains DNA and histone.
It has recently been possible by Miller and Beatty (1969) to study under electron microscope the structure and function of isolated living genes in the amphibian egg cells. The presence of extrachromosomal nucleoli in amphibian oocytes has permitted isolation and electron microscopic observation of the genes coding for ribosomal RNA (rRNA) precursor molecules.
Visualisation of these genes was possible because many precursor molecules are simultaneously synthesised on each gene. During early growth of the amphibian oocyte, the chromosomal nucleolus organiser is multiplied to produce about a thousand extrachromosomal nucleoli within each nucleus.
These extrachromosomal nucleoli have been claimed to function similarly to chromosomal nucleoli in the synthesis of rRNA precursor molecules. The figure shows the nuclear genes from an amphibian oocyte (Fig. 1.16). Extrachromosomal molecules are composed of a compact, fibrous core and a granular cortex.
Each fibrous core consists of an axial fibre or core axis that is covered with matrix material. Each matrix unit is composed of 80-100 fibrils. The core axis is DNA coated with protein and the fibrils of matrices are ribonucleoprotein (RNP). It has been shown that RNA synthesis occurs within matrix units. It is believed that each matrix-covered DNA axis is a gene coding for rRNA precursor molecules.
Each matrix segment along an axis is separated from its neighbour by a matrix-free axis segment. These genes which code for ribosomal RNA, repeat along the DNA axis and are visualised because approximately 100 enzymes are simultaneously transcribing each gene. The gradient of fibrils that extend from each gene contains rRNA precursor molecules in progressive stages of completion.
Inside a nucleus there is usually single or may be from two to five smaller bodies known as nucleolus or nucleoli which lie among nuclear sap (karyoplasm) and among the pale-staining karyoplasm chromatin granules lie. The nucleolus comprises the irregular network or rows of fine granules, nucleolonema as seen in E.M. The nucleolus loses its identity during cell division. The nucleolus contains still smaller nucleus known as nucleololus or nucleolinus or nucleolonucleus.
The nucleus is responsible for the synthesis of messenger RNA (mRNA) which carries the genetic information in code through the pores in the nucleus. Recently it has been studied that mRNA is formed in the strands of DNA within the nucleus and actually the strands of DNA (Fig. 1.13) direct the synthesis of specific mRNA. The mRNA thus formed in the nucleus comes out of the nucleus for carrying the DNA-message to the protein-synthesising centre (ribosome) of the cytoplasm.
Here it is attached to the ribosome and stretched out (Fig. 1.14) on its surface to direct the protein synthesis. The amino acid sequences in the protein are determined by the transfer or soluble RNA (tRNA or sRNA) which recognises the code for the amino acids the tRNA is carrying to the particular spot of the ribosomal surface where the mRNA is already attached. A main function of tRNA is to transfer the specific amino acid to the template of mRNA for correct amino acid sequence.
There are 20 specific tRNA for 20 specific amino acid. With the help of these tRNA the protein is synthesised with proper sequences at the template of the mRNA and are stretched on the surface of the ribosome. After completion of protein synthesis, the protein molecules become detached from ribosomal particles and pass into the canal of the endoplasmic reticulum. From here it passes into the Golgi complex.