A typical nucleus consists of four parts. So for the study of cell nucleus structure we shall go on some details about the following parts:
i) Nuclear membrane
iii) Chromatin reticulum
Nuclear membrane as a Cell Nucleus Structure
The nuclear membrane in cell nucleus structure is the membranous covering of the nucleus. It is a double membrane structure comprising of an outer and an inner membrane, each being a trilamellar (P-L-P) unit membrane. These two membranes remain separated by a space called perinuclear space which is about 10-15 nm in width. The nuclear membrane originates from the endoplasmic reticulum and remains attached with it so that the perinuclear space is unbroken with the space inside the cisternae and tubules of the endoplasmic reticulum. The outer membrane is rough surfaced due to the occurrence of ribosome on it. The outer membrane is uneven surfaced due to the presence of ribosome on it. The nuclear membrane possesses numerous pores called nuclear pores. The pores present in it allow the necessary exchange of materials between nuclear material and cytoplasm. The number of pores is about 40-145/um2 in the nuclei of various plants and animals. So, as a cell nucleus structure, in nuclear membrane the pores are not simple opening but are plugged with a hollow cylinder of proteinaceous materials called annulus which extends both into the cytoplasm and nucleoplasm. The pore and the annulus together make up a pore complex.
Nucleoplasm as a Cell Nucleus Structure
nucleoplasm is called the transparent, finely granular and slightly acidophilic colloidal semifluid ground substance of the nucleus. It remains bounded by the nuclear mambrane. Nuclear machinery such as the chromatin reticulum and the nucleolus remain suspended within this semifluid material. It is composed of RNA, DNA, protein, enzymes, coenzymes and minerals. The nucleoplasm functions as the matrix or ground substance of the nucleus.
Chromatin reticulum as a Cell Nucleus Structure
The chromatin reticulumare the coiled network like structure formed by the nucleoprotein threads present in the nucleoplasm. These nucleoprotein threads are called chromatin fibers. During the inter-phase or resting phase, the chromatin fibers form a coiled network. At the time of cell division, the chromatin fibers become dense and divided into a definite number of short, thick, thread or rod like structures called chromosomes. By DNA and basic histone proteins the chromatin threads are composed of. The DNA is the bearer of hereditary characters. Due to the presence of DNA in the chromatin threads, they readily take basic stains and depending on the staining properties, the chromatin reticulum is divisible into following two parts: -
The region of chromatin reticulum which takes a light stain in resting cells and a comparatively deeper stain during cell division is called euchromatin. Throughout resting phase, it remains thin, less coiled and extended, whereas during cell division, it becomes condensed, tightly coiled and thick.
The portion of chromatin reticulum which is darkly stained in both resting as well as dividing conditions of a cell is called heterochromatin. It remains condensed, tightly coiled and thick in both resting and dividing cells; therefore it always takes a deep stain.
Nucleolus as a Cell Nucleus Structure
Nucleolus is the small, dense, highly stainable and refractive, spherical or oval body present in the nucleus of non-dividing cells. In general, each cell contains a single nucleolus but in certain cells such as, some liver cells, lymphocytes, etc. may have more than one nucleoli. The nucleolus is not externally bound by a membrane. It becomes most conspicuous during protein synthesis. It disappears during cell division and reappears after the division is completed. The nucleolus is composed mainly of RNA and proteins.