Nucleus is the most prominent and the largest cellular organelle. It has a diameter of 10 μ to 22 μ and occupies about 10% of total volume of the cell. Nucleus is present in all the cells in the body except the red blood cells. The cells with nucleus are called eukaryotes and those without nucleus are known as prokaryotes. Presence of nucleus is necessary for cell division. Most of the cells have only one nucleus (uninucleated cells). Few types of cells like skeletal muscle cells have many nuclei (multinucleated cells). Generally, the nucleus is located in the center of the cell. It is mostly spherical in shape. However, the shape and situation of nucleus vary in some cells.


Nucleus is covered by a membrane called nuclear membrane and contains many components. Major components of nucleus are nucleoplasm, chromatin and nucleolus.

Nuclear Membrane

Nuclear membrane is double layered and porous in nature. This allows the nucleoplasm to communicate with the cytoplasm. The outer layer of nuclear membrane is continuous with the membrane of endoplasmic reticulum. The space between the two layers of nuclear membrane is continuous with the lumen of endoplasmic reticulum. Pores of the nuclear membrane are guarded (lined) by protein molecules. Diameter of the pores is about 80 to 100 nm. However, it is decreased to about 7 to 9 nm because of the attachment of protein molecules with the periphery of the pores. Exchange of materials

between nucleoplasm and cytoplasm occurs through these pores.


Nucleoplasm is a highly viscous fluid that forms the ground substance of the nucleus. It is similar to cytoplasm present outside the nucleus. Nucleoplasm surrounds chromatin and nucleolus. It contains dense fibrillar network of proteins called the nuclear matrix and many substances such as nucleotides

and enzymes. The nuclear matrix forms the structural framework for organizing chromatin. The soluble liquid part of nucleoplasm is known as nuclear hyaloplasm.


Chromatin is a thread-like material made up of large molecules of DNA. The DNA molecules are compactly packed with the help of a specialized basic protein called histone. So, chromatin is referred as DNA-histone complex. It forms the major bulk of nuclear material. DNA is a double helix which wraps around central core of eight histone molecules to form the fundamental packing unit of chromatin called nucleosome. Nucleosomes are packed together tightly with the help of a histone molecule to form a chromatin fiber. Just before cell division, the chromatin condenses to form chromosome.


Chromosome is the rod-shaped nuclear structure that carries a complete blueprint of all the hereditary

characteristics of that species. A chromosome is formed from a single DNA molecule coiled around histone molecules. Each DNA contains many genes. Normally, the chromosomes are not visible in the

nucleus under microscope. Only during cell division, the chromosomes are visible under microscope. This is because DNA becomes more tightly packed just before cell division, which makes the chromosome visible during cell division. All the dividing cells of the body except reproductive cells contain 23 pairs of chromosomes. Each pair consists of one chromosome inherited from mother and one from father. The cells with 23 pairs of chromosomes are called diploid cells. The reproductive cells called gametes or

sex cells contain only 23 single chromosomes. These cells are called haploid cells.


Nucleolus is a small, round granular structure of the nucleus. Each nucleus contains one or more nucleoli.

The nucleolus contains RNA and some proteins, which are similar to those found in ribosomes. The RNA is synthesized by five different pairs of chromosomes and stored in the nucleolus. Later, it is condensed to form the subunits of ribosomes. All the subunits formed in the nucleolus are transported to cytoplasm through the pores of nuclear membrane. In the cytoplasm, these subunits fuse to form ribosomes, which play an essential role in the formation of proteins.


Major functions of nucleus are the control of cellular activities and storage of hereditary material. Several

processes are involved in the nuclear functions.

Functions of nucleus:

1. Control of all the cell activities that include metabolism, protein synthesis, growth and reproduction (cell


2. Synthesis of RNA

3. Formation of subunits of ribosomes

4. Sending genetic instruction to the cytoplasm for protein synthesis through messenger RNA (mRNA) 5. Control of the cell division through genes

6. Storage of hereditary information (in genes) and transformation of this information from one

generation of the species to the next.

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