Nucleus was discovered by Robert Brown in 1831. It is a prominent body in many cells. It looks dark than the surrounding cytoplasm. The shape of nucleus may be irregular or spherical. It controls the life and activities of the cell. It is generally present in the center in animal cells. The plant cells have a large vacuole. So their nucleus is pushed towards the periphery. Generally, the cells have one nucleus. Such cells are called mononucleate. The cells with two nuclei are called binucleate. The cells with more than two nuclei are called multinucleate.
It is visible only during non-dividing stage. During non-dividing stage, the nucleus is composed of chromatin network and nucleoplasm. The nucleus disappears and the chromatin material is replaced by chromosomes in the dividing cells. Chemically the nucleus is composed of DNA, RNA and proteins including enzymes. It is composed of four parts: Nuclear membrane, nucleoli, nucleoplasm and chromosomes.
The nucleus is surrounded by nuclear membrane. It separates the nuclear material from the cytoplasm. The nuclear membrane is actually a nuclear envelope as it is composed of two membranes. The outer membrane is continuous with the endoplasmic reticulum at certain points.
The inner membrane encloses the nuclear contents. The outer and inner membranes are discontinuing at certain points. So they form pores, called nuclear pores. The nuclear pores allow the exchange of material between the nucleus and cytoplasm. The number of pores is different in different cells.
The undifferentiated cells have numerous pores. These may be about 30,000 per nucleus. On the other hand, the differentiated cells like erythrocytes (RBC) have only 3 to 4 pores. Each pore has definite structure. It controls the movement of substances, passing through it.
It is darkly stained body within the nucleus. It is without any membranous boundary, which separates it from the other nuclear material. The nucleus may contain one or more nucleoli. Nucleolus synthesizes and stores ribosomal RNA (rRNA). Nucleolus is composed of two regions:
- Peripheral granular region: It is composed of precursors of ribosomal subunits.
- Central fibrillar region: It is composed of large molecular weight RNA and rRNA.
The ribosomes are assembled in the them. These ribosomes are exported to cytoplasm through nuclear pores.
The nucleus contains chromatin material. This chromatin material is deeply stained with basic dye. This chromatin material is changed in to chromosomes during cell division.
Structure of chromosome
The chromosome is made up of two arms and centromere. Each chromosome has two identical chromatids at the beginning of cell division. The chromatids are exact replica (copy) of each other. These chromatids are attached with each other at centromere. The chromosome is also attached with the spindle fibers at centromere during cell division.
Chemical composition of chromosome:
Each chromosome is composed of DNA and proteins. Small part of DNA, which control specific character, is called Gene. Genes controls all the information and activities of the cell. It also controls the transfer of character form one generation to the other.
Numbers of chromosomes:
- Man has 46 chromosomes.
- Frog has 26 chromosomes.
- Chimpanzee has 48 chromosomes.
- Fruit fly (Drosophila melanogaster) has 8 chromosomes.
- Onion has 16 chromosomes.
- Potato has 48 chromosomes.
- Garden pea has 14 chromosomes.
The number of chromosome in the normal body cells is diploid (2n). One the other hand, the germ cells (egg and sperm) have haploid (In) number of chromosomes. Human sperm and eggs have 23 chromosomes (1n), while the sperm and egg of Drosophila has 4 chromosomes.
It forms soluble cell sap. This fluid like matter present inside the nucleus.
Functions of nucleus
It performs following functions:
- It controls all the activities of the cells.
- It controls the transfer of hereditary characters from parents to offspring.
- The three types of RNAs, i.e. mRNA, tRNA, rRNA are synthesized in the nucleus. These RNAs are involved in the synthesis of proteins.