NUCLEUS Structure and Function

NUCLEUS

Nucleus was first discovered by Robert Brown (1831) in orchid cells. It is the most important part of the cell which directs and controls all the cellular functions. That’s why nucleus is very often regarded as ‘director of the cell’. Presence of true nucleus with nuclear membrane and linear chromosomes is the characteristic of all the eukaryotic cells. However, there are some exceptions viz., mature mammalian RBCs, sieve tubes of phloem, tracheids and vessels of xylem.

As far the number of nucleus in a cell is concerned, most of eukaryotic cells have single nucleus within them. However, the number may vary in some cells.

Depending on the number of nuclei cells may be of following types :

Anucleate (without nucleus)   :           Mammalian RBCs.

Uninuclate                               :           Most of Eukaryotic Cells.

Binucleate                               :           Basidiomycetes, Paramoecium          

Multinucleate                          :           Phycomycetes like Mucor, Rhizopus etc., Red

Algae.

The true Nucleus may be defined as: ‘The cellular structure limited externally by a nuclear membrane surrounded by cytoplasm which contains linear nucleoproteinous chromosomes and carry genetic information’s from generation to generation’. The carrier of genetic information nature of nuclei was established by Hammerling (1953) who worked on the macroscopic unicellular alga, Acetabularia and concluded that the morphology of the plant is solely determined by the type of nucleus contained in the plant body.

STRUCTURE OF NUCLEUS

Study of the cell cycle has revealed that each cell has two phases in its cycle:

Interphase and

Phase of cell division.

In fact, interphase is the phase between two cell divisions. This is much longer than the phase of cell division, structure of nucleus is studied in this interphase only.

The electron microscopic studies of interphase nucleus have revealed that the nucleus may consists of following four parts:

           

1.  Nuclear Membrane: It limits the nucleus externally and also known as karyothica. It is bilayedred, lipoproteineous and trilaminar in nature. Outer membrane is called ectokaryotheca and the inner is endokaryotheca. The outer membrane is studded with ribosomes while the inner is free of that. The two membranes have a thickness of 75-90 Å

 And are apart from each other by a distance of 100-300Å. This space is called perinuclear space.

The nuclear membrane has many pores. Its number may vary from 1000-10000 in a nucleus. Each pore is about 400-1000 Å in diameter. The number and size of pores may depend on the needs of the cell. Each nuclear pore is fitted with a cylindrical structure called annulus. The pore and the annulus both collectively form the pore complex or pore basket

Figure:  nucleus

2. Nucleoplasm: It is transparent semi fluid, homogenous, colloidal ground substance inside the nuclear membrane. It is also called nuclear sap, karyolymph or karyoplasm. Nuclear chromatin and nucleolus are embedded within nucleoplasm, chemically, it is formed of water, sugars, minerals (Mn²⁺, Mg²⁺, etc.), Nucleotides, ribosomes, enzymes, DNA and RNA polymerases, mRNA, tRNA molecules etc. It is alkaline in nature (pH = 7.4  ).

Functions :

Nucleoplasm forms the skeleton of nuclei and helps in maintaining their shape.

The process of transcription takes place in the nucleoplasm in which different molecules of RNA are formed.

It supports nuclear chromatin and the nucleolus.

Ribosomal subunits are synthesized in the nucleoplasm.

3. Chromatin Net or thread : Electron microscopic studies of well stained eukaryotic nuclei have revealed that presence of darkly stained network of long, fine and interwoven threads which is called chromatin net or thread. It is also known as nuclear reticulatum. It was first reported by Fleming in 1882. During the phase of cell division, the chromatin net is transformed into chromosomes due to high condensation of DNA molecules. These chromosomes are rod like and have definite shape and size chracteristic of an organism.

The chromatin is chemically nucleoprotein and formed of nucleic acid (DNA) and base proteins i.e., histones . It may be classified in to two categories:

1. Heterochromatin : It is made of comparatively thick regions which is darkly stained. DNA strands in this chromatin are more condensed. Transcriptionally, it is inactive and late replicative. It does not contain active genes.
2. Euchromatin : It is true chromatin and is formed of thick and less darkly stained areas. It has loose, less condensed DNA which is trancriptionally, inactive and early replicating.

NUCLEOLUS

Nucleolus can be seen as a very conspicuous structure in the interphase nucleus. It disapperar during mitosis and reappears at the next interphase. The process by which the nucleolus is formed, is described as nucleologenesis. During prometaphase to early telophase, when the nucleolus remains disappeared, a number of non-ribosomal nucleolar proteins as well as U3 s- RNA are found in (i) the peripheral regions of chromosomes and in the (ii) nucleolus derived foci (NDF) found as cytoplasmic particles 1-2 in diameter;

  Nucleolus : Within each nucleus, there is a darkly stained, granular, naked and large organelle without limiting membrane. It was discovered by Fontana in 1781. The term nucleolus was coined by Bowman (1840). The size of nucleolus is comparatively larger in those cells which have rapid rates of protein biosynthesis.

The position of nucleolus is generally definite within nucleus. It is associated with nucleolar organizer region (NOR) of nuclear chromosome. It is absent in muscle fibres , RBC, Yeast, sperm and prokaryotes. In general, each nucleus has one or two nucleoli. Its number depends on the number of chromosomes in the species. For each haploid set of chromosomes in the nucleus, there is a single nucleolus. However, a pair of nucleoli may be found in haploid nuclei. In human beings, two pairs of nucleoli are found in each diploid nuclei. In human beings, two pairs of nucleoli are found in each diploid nucleus Xenopus oocytes may contain upto 1000 nucleoli in the nucleoli in the nucleus.

Ultrastructure :  The ultrastructure of nucleolus was studied by Borysko and Bang in 1951 and again by Berhard in 1952. On the basis of electron microscopic studies of the structure of nucleolus, de Robertis et al., (1971) described it to be made up of four parts:

1. Fibrillar regions: This part is made up of ribonucleoprotein fibres. It is also called nucleolemma. Each fibre has a length of around 50-80 Å.

2. Granular regions : This part has many granules each having the diameter of 150-200 Å. These are derived from nucleolar fibres, chemically, these granules are also ribonucleoproteins.

3. Protein region: This proteinous part is also called parsamorpha. This is the fluid part of nucleolus in which other parts are found.

4. Chromatin part:  It is made up of chromatin fibres containing DNA. These DNA molecules function as template for RNA synthesis. The chromatin part may be differentiated into two parts      

 a)      Perinucleolar Chromatin: it forms a covering or envelope around nucleolus. It may have ingrowths at certain places inside the nucleolus, which are called trabeculae.

b)      Intranucleolar chromatin: These chromatin fibres are found in internal protein region. These form many septa like structures.

Each nucleolus has dense fibrillar region due to presence of which it is associated with nuclear organizer region of chromosomes. These region have been reported to contain many copies of  DNA responsible for synthesis of ribosomal RNA. These rRNA molecules are rapidly synthesized in this region. The protein of ribosomes are synthesized in the cytoplasm which is transported to nucleus and finally to nucleolus. The rRNA and protein molecules combine to form complete ribosome molecules. 

These newly synthesized ribosomes are associated with thin fibrils of RNA and look like beaded string. This structure is called nucleonema. On the basis of the presence and structure of nucleonema, following three types of nucleoli may be recognized:

Nucleolus with nucleonema which is more common is all types of cells.

Nucleolus without nucleonema which is commonly found in salivary gland cells.

Ring shaped nucleolus containing ribonucleoprotein granules and RNA fibrils.

This is common in endothelial cells and muscle cells.

Functions:

Nucleoli are the site of rRNA biosysthesis.

It stores rRNA.

It helps in the biogenesis of ribosomes.

It helps in the formation of spindle fibres.

It plays important role in mitosis.

Functions of Nucleus

It controls all the cellular functions.

It controls the synthesis of all the structural and enzymatic proteins.

Synthesis of all the 3 types of RNA (mRNA, tRNA and rRNA) takes place in the nucleus.

It plays important role in cell division.

Cell growth is controlled by nucleus

Nucleus controls cellular differentiation by regulating differential gene expression

It induces genetic variation and thus helps in organic evolution.

Sexual reproduction happens due to fusion of two nuclei gametes of opposite sex.

Due to presence of all these organelles and other structures, a cell functions as self-regulatory

systems and provides a definite set of characteristics to different organisms.

 CHROMOSOMES

Chromosomes are rod like or filamentous bodies, which are typically, present in nucleus and become visible during the stage of cell division. Presence of true chromosomes is the characteristic of eukaryotic cells.

Literally, the term chromosomes have been derived from two Greek words; Chroma and soma meaning by ‘colored body’. This is named because they appear as darkly stained bodies during cell division when stained with a suitable dye and viewed under compound microscope.

Chromosomes can well be defined in following way “Chromosomes are individual protoplasmic entities found in the nuclei of almost all eukaryotic cells which multiply themselves through sequential cell divisions and provide physiological and morphological stability to protoplasm and so to a particular individual”

Some important points to remember about chromosomes.

1. Chromosomes were first observed by Straburger (1875) in mitotically dividing cells and the name ‘chromosome’ was proposed by Waldeyer in 1888.
2. Each species has a definite and constant number of chromosomes in their cells. The chromosome number found in somatic cells of the species is called somatic chromosome number and is usually represented by ‘2n’. This is because, ordinarily, somatic cells contain two copies of each chromosome which are morphologically identical and also have same gene content and gene location. They are known as homologous chromosomes.
3. Chromosomes can darkly be stained by treating the dividing cells by acetocarmine, acetoarcine, feulgan and some other basic dyes.
4. In plant kingdom, lowest number of chromosome is found in Haplopappus gracilis and highest in Ophioglossum reticulatum. In animal kingdom, Ascaris mega-locaphala has been found to have lowest number of chromosome.
5. Chromosomes are chemically nucleio protein consisting of DNA and proteins. The bear genes therefore, regarded as ‘bearer of hearedity.’
6. Each chromosome is made up of two longitudinally held chromatids which are visible during mitotic metaphase.
7. The two chromatids of a chromosome are joined at centromere the main function of which is the formation of spindle fibres during cell division.
8. Nucleolus within nucleus is associated with secondary constriction of chromosomes. Therefore, the later is called ‘nucleolar organiser.