If a living being's body is a machine, then the brain is the processing unit or the control center of this machine. We know that the smallest unit of life is a cell, which can be imagined to be a machine as well. So then, what is the processing unit of a cell?
The control center of a cell is the nucleus, also known as kernel of the cell. In the following sections, we shall explore the structure and functioning of a nucleus, and try to understand the working of this smallest processing unit in nature.
Structure of a Nucleus
All the genetic contents of a cell are enclosed in the nucleus, organized as multiple long chains of linear Deoxyribonucleic acid (DNA) molecules along with various proteins, such as histones, which form chromosomes. Within these chromosomes there are genes enclosed, which are nothing but the cell's nuclear genome. Very briefly, the function of a nucleus is to preserve the integrity of these genes and to preside over the activities in the cell by regulating gene processing and other functionalities.
There's no fixed shape to the nucleus. Though in most cases they're round or elliptical, oval shaped or disc-shaped nuclei have also been observed. The nucleus is enveloped by a double layered membrane, known as the nuclear membrane. A clear fluid called nucleoplasm or karyolymph is enclosed in the membrane.
There is a network of fine thread-like matter, called the chromatin network, which contains the DNA molecules and histones arranged in a particular manner. At the time of cell division, condensation of chromatin takes place in the nucleus, resulting in formation of distinct structures known as the chromosomes.
Inside the nucleus, there also exists a round spherical body called nucleolus, amidst the chromatin mesh, and it has the Ribonucleic Acid (RNA) and the ribosomes in abundance, that are involved in protein synthesis. In plants, nucleus also controls the process of photosynthesis which is one of its main functions in the plant cells.
A nucleus is generally present only in eukaryotic (membrane bound) cells. Most cells have just one nucleus and are called uninucleate, however, some can have two or even more nuclei and are known as binucleate and multinucleate respectively. There are also a type of cells, that lose their nucleus when they reach maturity, in a process known as enucleation. Such cells are called enucleate cells, and serve certain specialized . The Red Blood Cells (RBC) in mammals and the sieve tube cells in plants are enucleate.
Functions of a Nucleus
The following is a list of the functions of the nucleus:
- Nucleus stores the genetic entropy necessary for reproduction, growth and metabolism of not only the cell that it controls, but also of the organism as a whole. It controls the transfer and replication of hereditary molecules (DNA and RNA) between the parent cell and the child cell. Nucleus ensures equal distribution and exact copying of the genetic content during the process of cell replication. This is the main function of nucleus in animal cells.
- The nucleus sustains and controls the cell growth by orchestrating the synthesis of structural proteins in the cell.
- Nucleus is the place for DNA transcription in which messenger RNA (mRNA) that synthesize protein, are produced. The nucleus contains various types of proteins which can either directly control transcription or are indirectly involved in regulating the process.
- The process of energy and nutrient metabolism in the cell is regulated by the nucleus by directing the synthesis and functioning of enzymes, which are a type of protein.
- The selective diffusion of cell's regulatory and energy molecules through the pores in the nuclear membrane is presided over by the nucleus.
- Nucleus is responsible for the secretion of ribosomes. The ribosomes, in turn are responsible for synthesizing all types of proteins.
Thus, the cell nucleus stores all the chromosomal DNA of an organism, upon which the entire behavior and appearance of the organism depends. It is also responsible for the growth patterns of the body, and controls correct replication of cells, which leads to the growth of that species. Looking at how vital the functions of a nucleus are, it is clear why it is referred to as the control center of the cell.