Robert Brown identified and described nuclei in plant cells in 1833 and they were immediately recognised as a continuous characteristic of all animal and plant cells. M.J. Schleiden mentioned nucleoli in 1838, but Fontana was the first to notice those (1781). Bowman invented the word nucleolus in 1840.

The nucleus is the cell’s heart. Nearly all the cell’s DNA is contained, duplicated and transcribed here. As a result, the nucleus is in charge of the cell’s metabolic and hereditary operations. The Greek word Karyon is a synonym for this organelle. The true nucleus, as contrasted to the nuclear region, prokaryote, or nucleoid of prokaryotic cells, acts as the principal defining factor of eukaryotic cells.

The nucleus can be observed in all eukaryotic cells, including those of plants and animals. Several eukaryotic cells, however, such as:

1. Numerous eukaryotic cells, including higher plant’s mature sieve tubes and mammalian erythrocytes, do not have a nucleus. Although during the initial phases of development, nuclei are contained in such cells.
2. Bacterial prokaryotic cells lack a true nucleus, which is a single, circular, and massive structure.

The Structure of Nucleus:

1. The form of the nucleus is generally related to the cell’s shape
2. The nucleus typically takes up roughly 10% of the entire cell volume. Nuclei range in size from 3 to twenty-five um in diameter
3. When the cell is not dividing, it is the largest organelle visible
4. The DNA molecule is still in touch with the cytoplasm
5. It stains intensely and is usually spherical; the nuclei of WBC are lobed
6. There is usually just one nucleus in each cell; these cells are known as uninucleate; however, some cells have multiple nuclei and are known as multinucleate
7. Nucleoplasm, which contains a chromatin network and a nucleolus, is enclosed by a double-layered nuclear membrane with fine nuclear pores

The following structures are the primary components of the nucleus:

The nuclear membrane:

1. It is also known as karyotheca or nuclear envelope. Thus, the nuclear membrane and nuclear envelope are the names of the same structure.
2. The nuclear envelope covers the nuclear material. It is made up of two membranes that are parallel with a thickness of 5 nm to 10 nm in thickness.
3. The membrane’s outer layer is usually constant with the endoplasmic reticulum. It is also a carrier for ribosomes.
4. Certain proteins are within the nuclear membrane’s inner layer. They act in the role of binding spots for fibrous sheath that forms part of filaments intermediate, also known in the name of the nuclear lamina.
5. The layers are separated by a cisterna perinuclear that is narrow. These membranes function as a shield to protect the nucleus.
6. In certain places, the inner and outer membranes fuse, creating cracks, which are known as nuclear pores in the envelope of the nuclear. These pores allow for the passage of certain molecules, like RNA and protein molecules, into as well out from the nuclear nucleus.
7. There is a space formed between these two layers called perinuclear space. It is about 10 nm to 50 nm in size.

The nuclear sap or nucleoplasm:

1. It is a protoplasm within the nuclear envelope containing chromosomes and nucleoli.
2. It is a viscous matrix composed mostly of water that also contains various macromolecules as well as ions.
3. It has Structural components, such as fibrillar elements, functional components, such as transcription of mRNA, and nucleoplasmic reticulum, which is continuous with the endoplasmic reticulum.
4. It contains nuclear particles such as inter-chromatin granules and perichromatin granules.

The chromatin fibres:

1. Chromatin consists of double helix DNA. The histones and non-histone are found complexed with DNA.
2. It is located within the nucleus as euchromatin, as well as heterochromatin.
3. It performs a number of functions like folding, protecting, repairing, and controlling the activity of the DNA

The nucleolus:

1. It is a nucleolus that is not enclosed by an outer membrane.
2. It is mainly composed of rRNA as well as proteins such as fibrillarin, DNA, nucleostemin and nucleolin.
3. Their number can vary from one to many in a nucleus.
4. It contains fibrillar centers, pars fibrosa, pars granulosa, and nucleolar matrix.
5. It is involved in the synthesis of rRNA and assembly.

The function of the nuclear membrane in the animal cell:

1. The pores allow big molecules to enter and exit the nucleus, and they also allow small molecules to enter and exit the nucleus.
2. The hereditary material is kept in touch with the rest of the cell through membranes.
3. The outer nuclear membrane contains ribosomes, which create proteins that infiltrate the perinuclear cisterna.
4. The inner layer, which is in charge of structuring the nuclear envelope during mitotic division, is critical.
5. The interior content of the nucleus is protected by these two layers, which act as a protective barrier.

The Function of the nucleus:

1. It is in charge of regulating an individual’s hereditary features.
2. It is in charge of protein synthesis, cell division, and differentiation.
3. In the nucleolus, it stores genetic material in the form of DNA strands, as well as RNA and proteins.
4. It is a section dedicated to the transcription process, which results in the production of mRNA, which is then used to make proteins.
5. It aids in the transfer of RNA and DNA from the cell to the nucleus.
6. Ribosomes, often known as protein factories, are produced by the nucleolus.
7. It is in charge of maintaining the integrity of gene expression and genes.
8. It maintains the cell’s functionality.
9. This organelle coordinates other cell organelles’ activities.
10. It takes care of the repairs.
11. Directly participates in cell division, resulting in genetically identical daughter cells. Mitotic cell division is the name for this process.
12. Through a different type of cell division known as meiotic cell division, contributes to the creation of meio-gametes and meiospores.