Nucleus

Nucleus is the most important cell organelle of the cell, which is generally present at the centre of the cell. It contains the hereditary or genetic material in the cell and thus plays a vital role in inheritance (transfer of traits from parents to offspring) and also controls the growth and reproduction and various other activities of the cell. Nucleus was discovered by Robert Brown in 1831. He described the structure of the nucleus in orchid cells.

Shape of nucleus

Generally, it is spherical or circular in shape. However, it can have various other shapes like oval shape (in smooth muscles and squamous epithelial cells), bean-shaped, string shaped, and may be lobed such as in different types of white blood cells. For example, acidophils have a bilobed nucleus, basophils have a trilobed nucleus and neutrophils have a multilobed nucleus. Besides this, the nucleus can be horseshoe-shaped, which is found in Vorticella.

Size of nucleus

The size of the nucleus ranges from 0.5 micrometres to 2 micrometres. However, its size may increase when the cell is about to undergo cell division, especially during meiosis.

Number of nucleus

There can be more than one nucleus in a cell. The cells with a single nucleus are called monokaryotic or uninucleated cells, whereas, the cells that contain two nuclei are called dikaryotic or binucleated cells as found in Paramecium. The cells with more than two nuclei are called coenocytic or multinucleated cells, e.g. in Opalina (a protozoan), and in skeletal muscles fibres.

Furthermore, there are also certain cells that are living but do not contain a nucleus such as platelets, sieve tubes in plants and mature red blood cells in humans and most of the animals. The cells that do not contain a nucleus are known as enucleated cells. Besides this, the movement of the nucleus in the cell is not fixed due to the sweeping movement of the cytoplasm.

Position of the nucleus

A nucleus can have different positions in a cell as listed below:

• Centric: A centric nucleus is generally located at the centre of the cell as present in most of the eukaryotic cells.
• Ecentric: In this position, the nucleus is located away from the centre towards one side. It is seen in plant cells and adipocytes.
• Peripheral: The nucleus is present closer to the plasma membrane.
• Basal: The nucleus is present towards the base of the cell. This position of the nucleus is found in columnar cells.

Structure of Nucleus

Nucleus is surrounded or enclosed by a double-layered porous nuclear membrane or nuclear envelope. So, it is a double membrane bound structure. The space between the two membranes (outer nuclear membrane or inner nuclear membrane) is called perinuclear space. The outer nuclear membrane is connected with the endoplasmic reticulum (ER) and is also studded with ribosomes.

It is the largest cell organelle. Except for translation, all steps of gene expression and DNA replication occur here. Only in eukaryotic cells, the nucleus is enclosed by a nuclear membrane, whereas, in prokaryotic cells, it lacks the nuclear membrane.

The nuclear membrane is not continuous as there are protein channels or nuclear pores in the nuclear membrane that allow certain molecules like RNA to move in and out of the nucleus. There can be 1000 to 10000 nuclear pores in the nuclear envelope. The average diameter of the nuclear pore is 800 Angstrom. Furthermore, the number of nuclear pores per unit area is not the same as it changes with the metabolic activities of the cell. The nuclear envelope including both the inner and outer membrane is known as Karyotheca.

Pore complex

The inner and outer nuclear membrane is in continuity at the periphery of the nuclear pores. Each pore is octagonal is shape as a pipe-like octagonal structure with lumen fitted in the pore. This ring or pipe-like structure is called the annulus. The annulus is made of vertical fibres. The lumen of the annulus is filled with nucleoplasmin.

This entire structure or pore area is called the pore complex. It controls the movement or transport of materials from the nucleus to the cytoplasm and vice versa. So, any material that is required to move out or into the cell will move through the pore complex.

The skeletal proteins called fibril lamina or nuclear lamina is present on the inner surface of the inner nuclear membrane. It is a protein mesh made of electron-rich proteins. Its main function is to provide mechanical support to the inner nuclear membrane and the nucleus. The nuclear membrane disappears during mitosis or meiosis. In mitosis, it disappears at the late prophase.

What is nucleoplasm?

The nucleus is filled with a viscous fluid, which is called nucleoplasm. It is also called nuclear sap or karyolymph. It keeps the nucleus turgid and contains all the material or components that are required for DNA synthesis, RNA synthesis and to assemble DNA into chromosomes. It also contains the regulatory proteins that are needed to regulate cellular processes. For example, it has enzymes for DNA replication. The divalent ions are also present in the nucleoplasm such as Mg²⁺ and Mn²⁺ ions. They act as activators for enzymes present in the nucleoplasm.

Nucleoplasm also contains mRNA, tRNA and rRNA as they are produced here. A dense mass is also present in the nucleus which is known as nucleolus. The chromatin material (a complex of DNA and protein) is also suspended in the nucleoplasm. They are thread-like structures that condense to form chromosomes.

Nucleolus

Nucleolus was discovered by Fontana in the cells of Eel. The term Nucleolus was given by Bowman. It is a dense mass present in the nucleus and is associated with the nucleolus organizer region (NOR). It does not have any membrane. However, it appears to have a membrane as its outer part is made of nucleolar chromatin fibres which are arranged in such a way that it appears that the nucleolus has a membrane.

An amorphous material is present inside the nucleoplasm that is called pas amorpha. In the pas amorpha, granules of rRNA are present. So, the major function of the nucleolus is the formation and storage of rRNA. The nucleolus disappears during cell division and is formed again after the cell division is completed. Generally, one cell has one nucleolus, however, cells with more than one nucleoli also have been reported. For example, four nucleoli are found in the nucleus of an onion cell.

Chromosomes

The nucleus also houses the chromosome which is made of DNA and histone proteins. The chromosomes are formed when chromatin is condensed and organized before the start of cell division. When the cell is resting chromosomes remain as entangled structures called chromatin.

Functions of the Nucleus

• It acts as the control centre of the cell as it contains information in the form of genes (segments of DNA) that form the chromosomes.
• It contains the hereditary material (DNA) that carries information from one generation to another generation.
• It directs protein synthesis as it the site of DNA transcription, which is the first step of gene expression or protein synthesis in which mRNA is produced.
• Its membrane helps in cell compartmentalization by separating its contents from that of cytoplasm.
• The processing of newly formed m-RNA (pre-mRNA) occurs in the nucleus. The pre-mRNA undergo post-transcriptional modification in which molecules are added or removed from it before it leaves the nucleus.
• DNA replication also occurs in the nucleus.
• The chromatins are arranged into chromosomes in the nucleus when a cell is about to undergo cell division.
• It has nuclear pores in the membrane that allows selective transport of molecules and regulatory factors and products synthesized in the nucleus such as mRNA.