The 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 the nucleus may be irregular or spherical. It controls the life and activities of the cell. It is generally present in the center of 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 mononucleates.
The cells with two nuclei are called binucleates. The cells with more than two nuclei are called multinucleate. It is visible only during the non-dividing stage. During the non-dividing stage, the nucleus is composed of a 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: The nuclear membrane, nucleoli, nucleoplasm, and chromosomes.
The nucleus is surrounded by a 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, differentiated cells like erythrocytes (RBC) have only 3 to 4 pores. Each pore has a definite structure. It controls the movement of substances, passing through it.
It is a 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. The nucleolus synthesizes and stores ribosomal RNA (rRNA). The 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 them. These ribosomes are exported to the cytoplasm through nuclear pores.
The nucleus contains chromatin material. This chromatin material is deeply stained with basic dye. This chromatin material is changed into chromosomes during cell division.
Structure of chromosome
The chromosome is made up of two arms and a centromere. Each chromosome has two identical chromatids at the beginning of cell division. The chromatids are exact replicas (copies) of each other. These chromatids are attached to each other at the centromere. The chromosome is also attached to the spindle fibers at the centromere during cell division.
Chemical composition of chromosome:
Each chromosome is composed of DNA and proteins. A small part of DNA, which control specific character, is called Gene. Genes control all the information and activities of the cell. It also controls the transfer of character from one generation to the other.
Numbers of chromosomes:
- Man has 46 chromosomes.
- The frog has 26 chromosomes.
- Chimpanzee has 48 chromosomes.
- The fruit fly (Drosophila melanogaster) has 8 chromosomes.
- Onion has 16 chromosomes.
- The potato has 48 chromosomes.
- Garden pea has 14 chromosomes.
The number of chromosomes in the normal body cells is diploid (2n). On the other hand, the germ cells (egg and sperm) have a haploid (In) number of chromosomes. Human sperm and eggs have 23 chromosomes (1n), while the sperm and egg of Drosophila have 4 chromosomes.
It forms soluble cell sap. This fluid-like matter is present inside the nucleus.
Functions of nucleus
It performs the 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, and rRNA are synthesized in the nucleus. These RNAs are involved in the synthesis of proteins.