In this essay we will discuss about:- 1. Definition of Nucleus 2. Number of Nucleus 3. Position 4. Shape 5. Biochemical Analysis 6. Structure 7. Functions.
Contents:
- Essay on the Definition of Nucleus
- Essay on the Number of Nucleus
- Essay on the Position of Nucleus
- Essay on the Shape of Nucleus
- Essay on the Biochemical Analysis of Nucleus
- Essay on the Structure of Nucleus
- Essay on the Functions of Nucleus
Essay # 1. Definition of Nucleus:
Nucleus (L. nucleus- kernel) is a specialized double membrane bound protoplasmic body which contains all the genetic information for controlling cellular metabolism and transmission to the posterity.
A nucleus in the non-dividing or metabolic phase is called interphase nucleus. Like other cellular structures, living unstained nucleus does not show much internal differentiation. For detailed study of nucleus, the cells must be properly killed, fixed and stained.
Nucleus is the largest cell organelle. Though first observed by Leeuwenhoek in red blood corpuscles of fish, nucleus was first studied in orchid root cells by Robert Brown in 1831.
A nucleus is present in all living eukaryotic cells with the exception of mature sieve cells of vascular plants and red blood corpuscles of mammals. Even here a nucleus is present during the early stages of their development. Presence of hereditary information in the nucleus was proved by the work of Joachim Hammerling (1953) on single celled alga Acetabularia (Fig. 8.51).
Essay # 2. Number of Nucleus:
Commonly cells are uninucleate, that is, they possess a single nucleus. The protistan Paramecium caudatum has two nuclei (bi- nucleate), macronucleus for controlling metabolic activities of the organism and micronucleus possessing hereditary information.
Multinucleate or polynucleate condition is found in some cells of bone marrow, striated muscles, latex vessels, several fungi and algae. Multinucleate animal or protistan cells are called syncytial cells (e.g., epidermis of Ascaris) while in plants and fungi they are called coenocytic cells (e.g., Rhizopus, Vaucheria). Acellular slime moulds have a multinucleate protoplasmic body called Plasmodium.
Essay # 3. Position of Nucleus:
Nucleus is usually found in the region of maximum metabolic activity in the cytoplasm. Commonly it is situated in the geometric centre of the cell. In plant cells it is pushed to peripheral position on one side due to the development of a large central vacuole. Nucleus is peripheral in fat-storing cells or adipocytes, and basal in glandular cells. It is suspended in central vacuole by cytoplasmic strands in Spirogyra.
Essay # 4. Shape of Nucleus:
The nuclei are generally rounded in outline. They appear oval or elliptical in plant cells having large central vacuoles. Disc-shaped nuclei occur in the cells of squamous epithelium, lobed in white blood corpuscles and irregularly branched in silk spinning cells of insects.
Essay # 5. Biochemical Analysis of Nucleus:
DNA- 9-12%. RNA- 5%. Lipids- 3%. Basic Proteins- 15%. Acid proteins, neutral proteins and enzymes- 65%. Traces of minerals like Calcium, Magnesium, Potassium and Sodium (Phosphorus is a constituent of DNA, RNA and acid proteins).
Essay # 6. Structure of Nucleus:
A typical interphase nucleus is 5-25 pm in diameter. It is differentiated into five parts— nuclear envelope, nucleoplasm, nuclear matrix, chromatin and nucleolus (Fig. 8.53).
a. Nuclear Envelope (= Karyotheca):
It bounds the nucleus on the outside. The nuclear envelope separates the nucleus from the cytoplasm. It is made up of two lipoprotein and trilaminar membranes, each of which is 60-90A thick. The inner membrane is smooth.
The outer membrane may be smooth or its cytoplasmic surface may bear ribosomes like the rough endoplasmic reticulum. The two membranes of the nuclear envelope are separated by an electron transparent perinuclear space. The space is 100—500 A in width. The outer membrane is often connected to endoplasmic reticulum.
Nuclear envelope contains a large number of pores or perforations (Fig. 8.52). In some cases 10% of the envelope is occupied by pores. The two membranes of the envelope become continuous in the region of pores.
Nuclear pores have complex structure. They may have diaphragm, septum, plug of electron dense material or nucleoplasmin, blebs or annuli. Annuli are circular structures around the pores. The pores and their annuli form a pore complex called annulated pore.
An annulated nuclear pore may possess 9 cylinders, one central and eight peripheral. Instead, there may be a network of granules and filaments. The nuclear pores control the passage of substances to the inside or outside of the nucleus, e.g., RNAs, ribosomes, proteins.
b. Nucleoplasm (Nuclear Sap, Karyolymph, Strasburger, 1882):
It is a transparent, semifluid and colloidal substance which fills the nucleus. It contains nucleosides and a number of enzymes (e.g., DNA polymerase, RNA polymerase, nucleoside phosphorylase) which are required for the synthesis and functioning of DNA, RNA, nucleoproteins, etc. Some of the proteins present in nucleoplasm are essential for spindle formation.
c. Nuclear Matrix:
It is a network of fine fibrils of acid proteins that function as scaffold for chromatin. On the periphery, below the nuclear envelope, nuclear matrix forms a dense fibrous layer called nuclear lamina. Terminal ends of chromatin fibres or telomeres are embedded in nuclear or fibrous lamina. Nuclear matrix consists of two types of intermediate filaments, lamin A and lamin B.
Nuclear matrix and nuclear lamina form:
(i) Scaffold for chromatin,
(ii) Attachment sites to telomeric parts,
(iii) Mechanical strength to nuclear envelope, and
(iv) Components of nuclear pore complex.
d. Chromatin:
It is hereditary DNA-protein fibrillar complex which is named so because of its ability to get stained with certain basic dyes. Chromatin occurs in the form of fine overlapping and coiled fibres which appear to produce a network called chromatin reticulum.
Chromatin fibres are distributed throughout the nucleoplasm. They are differentiated into two regions— euchromatin and heterochromatin, Heitz (1928). Euchromatin is narrow (10-30nm thick) lightly stained and diffused fibrous part which forms the bulk of chromatin.
Heterochromatin is wider (100 nm thick), darkly stained and condensed granular part which is attached here and there on the euchromatin. Depending upon the size of granules formed by heterochromatin they are called chromocentres, karyosomes or false nucleoli.
The whole of chromatin is not functional. Generally only a portion of euchromatin which is associated with acid proteins takes part in transcription or formation of RNAs. During prophase of nuclear division, the chromatin fibres condense to form a definite number of thread-like structures called chromosomes.
e. Nucleolus (plural-nucleoli):
It was first discovered by Fontana in 1781, described by Wagner in 1840 and provided with its present name by Bowman in 1840. Nucleolus is a naked, round or slightly irregular structure which is attached to the chromatin at a specific region called nucleolar organizer region (NOR).
Commonly 1-4 nucleoli are found in a nucleus. Up to 1600 nucleoli are reported in the oocytes of Xenopus. A covering membrane is absent around nucleolus. Calcium seems to be essential for maintaining its configuration. Nucleolus has four components— amorphous matrix, granular part, fibrillar portion and chromatin (Fig. 8.54).
(i) Amorphous Matrix:
It is the homogeneous ground substance of the nucleolus. Matrix is formed of protein.
(ii) Granular Portion:
It consists of granules of the size of 150-200 A which lie scattered in the amorphous matrix. The granules are formed of protein and RNA in the ratio of 2:1. They are believed to be precursors of ribosomes.
(iii) Fibrillar Portion (Nucleolonema):
It is formed of a large number of small fibrils that are 50—so A long. The fibrils are made up of both protein and RNA and are believed to be precursors of granules.
(iv) Chromatin Portion:
It is that part of chromatin which is associated with nucleolus. Depending upon its position nucleolar chromatin is of two types— perinucleolar and intra-nucleolar. The perinucleolar chromatin lies around the periphery of the nucleolus. It gives rise to ingrowths or trabeculae which produce the intra-nucleolar chromatin.
(i) Nucleolus is the principal site for the development of ribosomal RNAs.
(ii) It is the centre for the formation of ribosome components,
(iii) Nucleolus stores nucleoproteins. The same are synthesised in the cytoplasm (over the ribosomes) and transferred to nucleolus,
(iv) It is essential for spindle formation during nuclear division.
Essay # 7. Functions of Nucleus:
Nucleus is an essential and integral part of the eukaryote cell. It stores genetic information in its DNA molecules which can be passed on to daughter cells. It also controls cellular activities.
i. Chromatin:
Nucleus contains hereditary material called chromatin. Chromatin is DNA- protein complex. It is made of a number of fine fibres that condense to form chromosomes. Number of chromosomes is fixed for a species. They bear genes.
ii. Genetic Information:
Chromatin part of nucleus possesses all the genetic information that is required for growth and development of the organism, its reproduction, metabolism and behaviour.
iii. Cellular Activities:
Nucleus controls cell metabolism and other activities through the formation of RNAs (mRNA, rRNA, tRNA) which control synthesis of particular type of enzymes.
iv. Ribosomes:
Ribosomes are formed in nucleolus part of the nucleus.
v. Variations:
All variations are caused by changes in genetic material present in the nucleus.
vi. Cell Growth and Maintenance:
With the help of RNAs, nucleus directs the synthesis of some structural proteins and chemicals required for cell growth and maintenance.
vii. Cell Differentiation:
It directs cell differentiation by allowing certain particular sets of genes to operate.
viii. Cell Replication:
Replication of nucleus is essential for cell replication.