In the centre of eukaryotic cell, very important cell organelle is located which is named as nucleus. The nucleus is differentiated from the cytoplasm due to the present of a membranous structure called nuclear membrane or nuclear envelope. In prokaryotic cells the nuclear envelope is absent, thus no distinct nucleus is present. The shape of the nucleus is generally spherical but it may slightly irregular. Nucleus contains DNA and is the control and information centre for eukaryotic cell. It has two major functions. The nucleus directs chemical reactions in cells by transcribing genetic information from DNA to RNA, which then translates this specific information into proteins like enzymes that determine the cell’s specific activities. Nucleus also stores genetic information and transfers it during cell division from one cell to the next and from one generation of organisms to the next. Nucleus comprises of nuclear envelope, chromosomes and nucleolus.
Structure of Cell:
Nuclear envelope: It is gateway to nucleus. The nuclear envelope is a structure that separates the nucleus from the cytoplasm that is continuous with endoplasmic reticulum at number of points. It acts as a barrier between the contents of the nucleus and cytoplasm. The nuclear envelope is made up of two layers, other nuclear membrane and inner nuclear membrane. The structure and chemical composition of these membranes is the same as that of the cell membrane. There is present space between two nuclear envelopes layers the cisternae. Outer layer may be continuous with endoplasmic reticulum, cytoplasm and adjacent cells or exterior.
Nuclear pore:
Nuclear membrane has at places small pores called nuclear pores that are formed by the fusion of two layers of nuclear envelope. In addition to fusion of two layers of nuclear envelope, the pore is composed of an ordered array of globular and filamentous granules forming nuclear pore complex. These granules are made up of protein. These nuclear pores control the transport of different molecules into and out of the nucleus.
Size of pore is also important as it allows specific sized molecules to pass through. Generally it presents the movement of DNA but permits RNA to be moved out. These pores also provide direct contact of nucleus to the cytoplasm, endoplasmic reticulum or event to the exterior through endoplasmic reticulum. Number of pores present in the nucleus is variable and depends upon specific function of that particular cell. The nucleus of undifferentiated cells like eggs may have over thirty thousand pores while the differentiated cell like eukaryocytes may have only three or four pores in single nucleus. In majority of cells nuclear pores may exceed over three thousand in single nuclear envelope.
Chromosomes:
Genetic Containers:
Inside the nuclear envelope there is present a fluid material called nucleoplasm. In non dividing cells the nucleoplasm contains nucleoli, chromosomes and enzymes for the synthesis of DNA and RNA. In addition it also performs a number of other functions as well. Genetic material is in the form of network of threads called chromatic or chromatin network. Chromatin consists of uncoiled, tangled mass of chromosomes that are coloured bodies containing hereditary information in segments of DNA called genes chromosomes are self duplicating and carry the hereditary instructions. During cell division each chromosome coils tightly which makes the chromosome visible when viewed through light microscope. Chromosomes are made up of bead like structure, the nucleosomes. Nucleosomes are connected with one another by means of a strand of DNA called the linker DNA or linker that consists of 50 nucleotides. A nucleosome is made up of an octamere of histones surrounded by two turns of DNA ribbon that consists of about 200 nucleotides. The octamere is formed by eight different types of histones called H2A, H2B, H3 and H4. Another histone H1 fixes DNA helix over histone octamere and parents from uncoiling. The number of chromosomes in all individuals of the same species remain constant generation after generation e.g. in man each cell has 46 chromosomes, frog cell has 26, chimpanzee has 48 and fruit fly has 8 chromosomes.
Nucleolus:
It is permeable point for ribosomes. Nucleus contains one or two discrete non membrane bound structure called nucleolus in the nucleoplasm of non-dividing cells. Sometimes the number of nucleoli may be two or more, even in thousands in case of amphibian egg. They can be readily stained with basophilic dyes; chemically the nucleoli are composed of nucleic acids, especially the ribnucleic acid (RNA) and some proteins. Nucleolus is pre assembly points for ribosomes in many stages of synthesis and assembly. Assembly of ribosome is completed after they leave the nucleus through pores of nuclear envelope into cytoplasm where they are helpful in protein synthesis.
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