Nucleus– Class 11 | Chapter – 8 | Short Notes Series PDF

Nucleus: 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. Antonie van Leeuwenhoek observes a ‘lumen’  , in the nucleus, in the red blood cells of salmon. Unlike mammalian red blood cells, those of other vertebrates still contain nuclei.

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 consists 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 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.

Structure 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.

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. The nuclear envelope is connected with the endoplasmic reticulum in such a way that the internal compartment of the nuclear envelope continuous with the lumen of 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 the nucleus and cytoplasm. The nuclear membrane is made up of lipoproteins, perinuclear space, pores, annuli material, an inner dense lamella.


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. 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. 

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.


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. The nucleolus disappears when a cell undergoes division and reforms after the completion of the cell division.

Characteristics of Nucleus

Some of the important features are summarised below.

  • One or more nucleoli may be present within a nucleus. Four nucleoli are found in each nucleus in an onion.
  • The nucleolus disappears in the late prophase stage.
  • Reappears in the telophase stage
  • It is a storehouse of RNA.

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By Team Learning Mantras