- It is one the most important and largest cellular organelle which has a diameter of 10 µm to 22µm occupying about 10% of total volume of the cell.
- All the cells in the body possess nucleus except red blood cells.
- On the basis of presence of nucleus in the cells, the cells are classified as eukaryotes and prokaryotes.
- Nucleus having cells are called eukaryotes whereas without nucleus they are called prokaryotes.
- Its presence is required for cell division.
- Mostly cells possess single nucleus so called uni-nucleated cells while in case of cells like skeletal muscle cells many nuclei are present. So, they are called multinucleated cells.
- They are present generally in the centre of the cell and is mostly spherical in shape but the shape and situation of nucleus vary in some cells.
Structure of Nucleus
- Nuclear membrane is the structure which covers or surrounds the nucleus.
- It contains many components where nucleoplasm, chromatin and nucleolus are the major one.
a) Nuclear membrane
- It is double layered structure and is porous in nature also.
- Due to porosity of this membrane, there is communication of nucleoplasm with the cytoplasm.
- There is a continuity of the outer layer of this membrane with the membrane of the endoplasmic reticulum.
- Protein molecules guard the pores of the nuclear membrane and pores are of diameter 80- 100nm.
- The size of the pores may decrease to about 7 to 9 nm because of the attachment of the protein molecules with the periphery of the pores.
- Pores help in the exchange of the materials between nucleoplasm and cytoplasm.
- It forms the ground substance of the nucleus and is a highly viscous fluid which is similar to the cytoplasm present outside the nucleus.
- The structures like chromatin and nucleolus are surrounded by it.
- It also contains dense fibrillar network of proteins called the nuclear matrix and many substances like nucleotides and enzymes.
- The nuclear matrix is responsible in forming the structural framework for organizing chromatin.
- Nuclear hyaloplasm is the soluble liquid part of nucleoplasm.
- It is a thread like material made up of large molecules of DNA.
- A specialized basic protein called histone keeps the DNA molecules in compact form for which it is also called DNA-histone complex which forms the major bulk of nuclear material.
- DNA is a double helix that wraps around central core of eight histone molecules to form the fundamental packing unit of chromatin called nucleosome which are packed together tightly with the help of histone molecule to form a chromatin fiber.
- Chromosome is formed by the condensation of chromatin just before the cell division.
- A rod shaped nuclear structure is present, responsible for carrying a complete blueprint of all the hereditary characteristics of that species called chromosomes.
- A single DNA molecules coiled around histone molecules forms a chromosome where each DNA contains many genes.
- Under microscope the chromosomes are not visible inside the nucleus normally.
- Only during cell division, there visibility is possible because DNA becomes more tightly packed just before when the cell starts dividing.
- Except reproductive cells, all the cells of the body contain 23 pairs of chromosomes where each pair consists of chromosome inherited from the mother and one from father.
- The cells with paired number of chromosomes are called diploid cells.
- The haploid cells contain only 23 chromosomes that are mostly the reproductive cells called gametes or sex cells.
Image source: microbenotes
- A small round granular structure of the nucleus is known as nucleolus which may be one or more in number.
- RNA and some proteins are present in nucleolus which is similar to those found in ribosomes.
- Five different pairs of chromosomes synthesize the RNA which is later on stored in the nucleolus and results in the formation of the subunits of the ribosomes.
- The formed subunits are then transported to the cytoplasm through the pores of nuclear membrane.
- These subunits fuse together to form ribosomes in the cytoplasm and helps in the formation of proteins.
Image source: Biologydictionary
Functions of Nucleus
- The cellular activities are controlled by the nucleus along with the storage of the hereditary material.
- There are several processes involved in the nuclear functions. They are:
- Various activities of cells including metabolism, protein synthesis, growth and reproduction (cell division) are in control of nucleus.
- Synthesis of RNA.
- Formation of subunits of the ribosomes.
- Sending genetic instruction to the cytoplasm for protein synthesis through messenger RNA (mRNA).
- Control of the cell division through genes.
- Storage of the hereditary information (in genes) and transformation of this information from one generation of the species to the next.
Nucleus: Structure and Functions