(i) Nuclear Envelope (Karyotheca):
The nucleus is surrounded by a nuclear envelope, consists of two parallel membranes, separated by a fluid filled perinuclear space (10-15 nm width).
The outer nuclear membrane is continuous with the membrane of endoplasmic reticulum at several places and its outer face is studded with ribosomes.
The nuclear envelope is supported by nuclear lamina from inside and a network of intermediate filaments (IF) from outside. Nuclear lamina (10-20 nm thick), composed of lamins A, B and C which are the specialized types of IF proteins.
The nuclear envelope is perforated by minute nuclear pores of about 20 – 80 nm in diameter. Around the margin of the pores both the membranes are continuous. Each nuclear pore is plugged by nuclear pore complex (NPC) which consists of three parallel rings or annuli connected to the central barrel-shaped transporter by radial spokes.
The aqueous channels (9 nm wide) between the spokes are the actual passages through which RNA and ribosomal subunits exist the nucleus. Recently, a basket or cage of 100 nm long found extended into the nucleoplasm.The nuclear envelope disintegrates during the division and reappears after telophase.
Functions:
(a) Provides a definite shape to the nucleus
(b) Resists the crushing pressure of cytoplasmid fluid.
(ii) Nucleoplasm (Nuclear Sap, Karyolymph or Nucleochylema):
It is the transparent semi-fluid, colloidal ground substance of the nucleus. It is composed of nucleic acids, proteins, enzymes’ and minerals.
Functions:
(a) Provides turgidity to the nucleus
(b) Holds chromatin, nucleolus
(c) Contain raw materials for synthesis of nucleic acids, and ribosomal subunits.
(iii) Nuclear matrix:
It is a network of proteinous fibrils that traverse the whole nucleus. Towards the periphery, below the nuclear envelope nuclear matrix forms a dense mesh called nuclear lamina (nuclear cortex or lamina densa).
Functions:
(a) Maintains nuclear shape
(b) Nuclear lamina provides attachment sites to terminal ends chromatin fibers or telomeres of chromosomes.
(iv) Chromatin:
It is the hereditary DNA-protein complexes that make up chromosomes. In an interphase nucleus the chromatin occurs as fine filaments called the chromatin fibers which distributed in form of a diffused network called chromatin reticulum. In interphase (non-dividing) nucleus, two types of chromatin are visible i.e. euchromatin (90%) and hetero-chromatin (10%).
The diffused, lightly stained and transcriptionally active form of chromatin is called euchromatin while the highly compact, darkly stained and transcriptionally inactive form of chromatin is called netero-chromatin. Only 10% of euchromatin is transcriptionally active at a time.
Chromatin consists of DNA (-31%), histones (-36%), non-histone proteins (-28%) and RNA (-5%). Histones are the major proteins of chromatin, rich in basic amino acids (arginine and lysine) that facilitate binding to the negatively charged DNA. There are five types of histone proteins- H1, H2A, H2B, H3 and H4. During spermiogenesis, the spermatid’s chromatin is remodeled where histones are replaced by protamines.
Changes in chromatin structure are affected by chemical modifications of histone proteins, such as acetylation and methylation. Acetylation results in the loosening of chromatin and lends itself to replication and transcription. When certain residues are methylated they hold DNA together strongly and restrict access to various enzymes.
Functions of chromatin:
(a) Package DNA into smaller volumes to fit inside the cell
(b) Strengthen the DNA to allow mitosis and meiosis
(c) Control gene expression and DNA replication.
Organization of chromatin:
The chromatin and chromosomes are inter-convertable. During cell division, the chromatin condenses almost 10,000 times to give the structure of a metaphase chromosome. Again at interphase, the chromosome decondense to become chromatin
The linear length of all the Chromosomal DNA within of any eukaryotic cell is usually million times greater than the diameter of the nucleus in which it is found. For example, the 46 human chromosomal DNAs taken together will measure up to 2.2 meters a length which is far greater than the dimension of a typical nucleus (about 1-5 µm). The DNA must therefore be organized into chromatin to accommodate inside a nucleus.
The packaging of chromosomal DNA into chromatin and subsequently, into chromosome is highly organized and achieved through several hierarchial orders of structure Fig. 3.18.
They are as follows:
DNA →10 nm chromatin → solenoid →looped domain → chromatid → metaphase chromosome
(2nm) (30wm) (300 nm) (700 ran) (1400 wot)
There are three levels of chromatin organization:
i. 10/nm Chromatin fiber:
The packaging of DNA with histones yields a l0nm chromatin fiber. Under electron microscope (EM) the 10 nm fiber appears like a “beads-on-a-string” structure. The 10 nm chromatin composed of repeating structural units called nucleosomes connected by linker DNA segments (10-80bp long). The winding of DNA into nucleosomes causes 5- 7 fold reduction in the length of linear DNA.
Each nucleosome is a disc-shaped particle with a diameter of about 10-11 nm. A nucleosome consists of two full turns of DNA (146bp long) coiled around a histone octamere (Nu body) containing 2 copies of each of the histones, H2A, H2B, H3 and H4. A nucleosome and a linker are together known as a chromatosome.
This description of chromatin structure is often called as nucleosome model (T. Kornberg, and Thomas, 1974. It was supported by Woodcock).DNA is wrapped around each nucleosome with 1 1/4 turn that consists of 146 base pairs but one chrometosome consists of 166 base pairs.
ii. 30nm Chromatin fiber:
According to A. Klug (1982) the 10 nm chromatin tightly coiled into a left handed helix called 30nm chromatin or solenoid with six nucleosomes per turn. The structure of 30nm fiber is stabilized by histone H1 (linker histone). The packing of nucleosomes into a solenoid causes 40 fold reductions in the length of linear DNA.
iii. Higher-level chromatin packaging:
Further folding and packing of 30 nm fiber require some additional proteins called non-histone chromosomal (NHC) proteins. The NHC proteins form a central scaffold (l00nm diameter) around which 30 nm fiber fold radially in form of looped domains (300nm diameter). As a result a chromotid of 700 nm diameter can be formed. Each metaphases chromosome has two chromatids connected together at the centromere.
(v) Nucleolus:
Discovered by Fontane and, studied by Wagner. The nucleolus is a membrane less, spherical, dense colloidal body present in the interphase nucleus. Usually nucleolus found attached to chromatin at a specific region called nucleolar organizer region (NOR) which represents secondary constriction of the nucleolar organizing chromosomes, e.g., in maize thromosome 6 and 9; in human chromosome 13,14,15,20 and 21 contain NOR.
Generally a nucleus contains 1 -4 nucleoli. However, cells of bacteria and yeast lack nucleolus. Smaller or no nucleoli found in the cells with little or no synthetic activities, e.g., sperm cells, blastomeres, muscle cells etc., while the oocytes of Xenopus contain up to 16OO nucleoli.
The ultrastructure of nucleolus has four parts:
(i) Amorphous matrix,
(ii) Granular area,
(iii) Fibrillar portion and
(iv) Chromatin region.
(i) Amorphous Matrix (Pars amorpha):
It is the homogenous ground substance of the nucleolus which composed of proteins.
(ii) GranularArea (Parsgranulose):
It consists of granules of the size of 150-200 A which lie scattered in the amorphous matrix. These granules contain protein and RNA in the ratio of 2:1. These are the precursors of ribosomes.
(iii) Fibrillar Portion (Pars fibrillosa):
In the centre of the nucleoli, there is a region of much denser fibrous network surrounded by granular area. It composed of a large number of small fibrils that are 50-80 A long. The fibrils contain both protein and RNA and are the precursors of granules.
(iv) Chromatin Region:
The chromatin (Feulgen positive) present in the nucleoli is of two types –
(a) Perinucleolar chromatin – it surrounds nucleolus like a shell, and
(b) Intranucleolur chromatin – its tubules project into nucleolus.
Functions of Nucleolus:
1. It is the principal site for the formation of ribosomal RNAs.
2. Nucleolus is the centre for the formation of ribosomes.
3. It stores nucleoproteins. These are synthesized in the cytoplasm (over the ribosomes) and transferred to nucleolus
4. It is essential for spindle formation during nuclear division.