Nucleus – What is Nucleus

Nucleus – Structure and Functions of Nucleus


The cell is the basic and structural, functional, and biological unit of all known living organisms. A cell is the smallest unit of life. Cells are called the building blocks of life. The study of cells is called cell biology.
The human body consists of trillions of cells, all with their own particular function. Cells are the fundamental or key structure of all living organisms. The Cell provides structure for the body, take in nutrients from food and carry out important functions.
There are two types of cell; one is prokaryotic (bacteria) and other eukaryotic (plant, animal, fungi). The prokaryotes have no nucleolus- the DNA is in the cytoplasm, it can form small circular strands of DNA called plasmids. Likewise, Eukaryotic cells all have their DNA enclosed in the nucleus.
The cell is made up of two-thirds water, which means that two-thirds of the whole body is water. The rest is a mixture of molecules, mainly lipids, proteins, and carbohydrates. The cells turn the raw materials in the food that we eat into the molecules the body needs, using thousands of different chemical reactions.
The nucleus is an organelle that consists of the genetic information for that organism. In an animal cell, the nucleus is located in the central place of the cell. Likewise, in the plant cell, the nucleus is located more on the periphery due to the large water-filled vacuole in the centre of the cell.
The nucleus is a spherical-shaped organelle that is present in every eukaryotic cell. The Nucleus is the control centre of eukaryotic cells. It is also responsible for the coordination of genes and gene expression. The structure of the nucleus includes nuclear membrane, chromosomes, nucleoplasm, and nucleolus.
The nucleus is the most prominent organelle as compared to other cell organelles, which account for about 10 percent of the volume of the cell. In general, a eukaryotic cell has the only nucleus. However, some eukaryotic cells enucleate cells (without a nucleus) for example red blood cells. Some are multinucleate; it means it consists of two or more nuclei, for example, slime mould.
The Nucleus is detached from the rest of the cell or the cytoplasm by a nuclear membrane.
The nucleus was the first organelle to be discovered or detected. The microscopist Antonie van Leeuwenhoek observes a lumen the nucleus, in the red blood cells of salmon. Unlike mammalian red blood cells, those of other vertebrates still contain nuclei.
Not all the cells have a nucleus. Biology breaks cell types into eukaryotic (have a nucleus) and prokaryotic (no nucleus). The nucleus is not needed to have DNA.

Structure of the Nucleus:

The cell nucleus consists of a nuclear membrane, called the nuclear envelope, nucleoplasm, nucleolus, and chromosomes. Nucleoplasm, also called karyoplasm, is the matrix present inside the nucleus.
The nuclear membrane separates the constituents of the nucleus from the cytoplasm. Like the cell membrane, the nuclear envelope subsists of phospholipids that form a lipid bilayer. The envelope helps to maintain the shape of the nucleus and assists in coordinating the flow of the molecules into and out of the nucleus through nuclear pores. The nucleus of the cell contains the DNA. The DNA controls the form, function, and growth of the cell. The nucleus is similar to the brain in its functions of coordinating all the cell activities
The nuclear envelope is connected with the endoplasmic reticulum in such a way that the internal compartment of the nuclear envelope continues with the lumen of the Endoplasmic reticulum.
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Nuclear Membrane:

The nuclear membrane is a double layered system that encloses the elements of the nucleus. The outer layer of the membrane is combined with the endoplasmic reticulum. A liquid-filled space or perinuclear space is present between the two layers of a nuclear membrane.
The nucleus gets through the remaining of the cell or the cytoplasm through several openings called nuclear pores. Such nuclear pores are the sites for the exchange of large molecules between nucleus and cytoplasm.


Chromosomes are present in the form of strings of DNA and protein molecules called chromatin. The chromatin is classified further into heterochromatin and euchromatin based on the functions. The heterochromatin is a highly condensed, transcriptionally inactive form; mostly present adjoining to the nuclear membrane. On the other hand, euchromatin is a mild, less condensed organization of chromatin, which is found amply in a transcribing cell.
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The nucleolus is a solid, spherical-shaped structure found inside the nucleus. Some of the eukaryotic organisms have a nucleus that consists of up to four nucleoli. The nucleolus plays an implied/indirect role in protein synthesis by producing ribosomes.
These ribosomes are cell organelles made up of RNA and proteins; they are transported to the cytoplasm, which is then attached to the endoplasmic reticulum.
The Ribosome is the protein-producing organelles of a cell. Nucleolus disappears when a cell undergoes division and reforms after the completion of the cell division.

Functions of the Cell Nucleus:

The cell nucleus controls the hereditary characteristics of an organism. This organelle is also responsible for protein synthesis, growth, cell division, and differentiation. The important function is carried out by a cell nucleus.

  • Chromatin is referred to as the storage of hereditary material, the genes in the form of long and thin DNA strands.

  • The Nucleolus is referred to as storage of proteins and RNA in the nucleolus.

  • The Nucleus is a site for transcription in which messenger RNA is produced for protein synthesis.

  • The nucleus functions as the exchange of hereditary molecules that is RNA and DNA between the nucleus and the rest of the cell.

  • During the cell division, chromatins are arranged into chromosomes in the nucleus.

  • It functions the production of ribosomes in the nucleolus.

  • The nucleus functions the selective transportation of regulatory factors and energy molecules through nuclear pores.

The nucleus is the control center of an organism as it regulates the integrity of genes and the gene expression. The nucleus contains all the genetic material of an organism like DNA, genes, chromosomes etc.

The Nucleus of the Plant Cell:

Plant cells are eukaryotic cells that are found in the organism within the plant kingdom. Eukaryotic cells contain nucleus Plant cells differ from other eukaryotic cells because the organelles existing are different. Organelles are a major part of the cell.
In the plant cell there are different types of the nucleus:

  • Uninucleate cell: It is also referred to as monokaryotic cells, mostly plant cells which contain a single nucleus.

  • Bi-nucleate cell: It is also called a dikaryotic cell. It contains two nuclei at a time. The examples are one paramecium (have mega and micronucleus), balantidium, and liver cells and cartilage cells.

  • Multinucleate cells: It is also known as the polynucleated cell which contains more than 2 nuclei at a time. For example, plants latex cells and latex vessels. In animals, striated muscle cells and bone marrow cells.

  • Enucleate cells: Cells without a nucleus are called enucleate cells. However, some living cells like mature sieve tubes of phloem and RBC’s of mature mammals lack nuclei.

The Plant Cell has Four Parts of the Nucleus:

The parts of the nucleus are as follows:

  • Nuclear membrane or envelope or karyotheca

  • Chromatin threads or nuclear reticulum

  • Nuclear sap or nucleoplasm or karyolymph

  • Nucleolus.

  1. Nuclear Membrane: The nuclear membrane is made up of the outer and inner membrane, made up of lipoproteins, perinuclear space, pores, annuli material, and inner dense lamella. The outer membrane is continuous with the endoplasmic reticulum. The exchange of different substances between nucleus and cytoplasm takes place through minute pores already present in the nuclear membrane.

  1. Chromatin Threads: The term chromatin thread was proposed by W. Flemming. Chromatin threads are associated with one another and form a network called chromatin reticulum. At the time of cell division, the chromatin threads isolated from one another become thicker or massive and smaller and are now termed as chromosomes.

It is primarily nucleoprotein, made up of nucleic acid and basic protein histone. Nucleic acid contains sugar, nitrogenous bases, phosphate, and is a very complex organic acid.

Nucleic Acids are of Two Types:

DNA (Deoxyribonucleic acid) especially found in the cytoplasm in soluble form and is called soluble RNA. It is also present in some amounts in the ribosomes of nucleus, chromatin, and nucleolus. It is synthesized from DNA and is piled up in the nucleolus. It travels to the cytoplasm and gets attached to the ribosome.
Chromatin is basophilic in type and most of the chromatin material is transferred into the specific number of chromosomes during cell division. The chromatin material may be heterochromatin, sex chromatin, and euchromatin.

  1. Nuclear Sap:

The nuclear membrane encloses the clear, homogeneous, transparent, colloidal liquid of variable consistency. It is chiefly organized of nucleoproteins, a small amount of inorganic and organic substances like nucleic acids, proteins dissolved phosphorus, ribose sugars, minerals, enzymes, and nucleotides.

  1. Nucleolus:

It was observed by Wagner and the term was proposed by Browman, subsequently described by Fontana.

Characteristics of Nucleolus:

  • The one or more nucleoli may be present within a nucleus. Four nucleoli are found in each nucleus in an onion.

  • Nucleolus disappears in the late prophase stage.

  • Reappears in the telophase stage

  • It is storehouse of RNA.

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