What is a Nucleus? A Comprehensive Guide with Worksheet

  • The Nucleus is an important cell organelle that is present in the eukaryotic cell.
  • It is derived from the Latin word, ‘nucleus’ or ‘ nuculeus’. It means kernel or seed.
  • It is first described by Robert Brown as the cell organelle.
  • It was named chromatin by the Flemming.
  • It is double-membraned.
  • It consists of genetic material. 
  • The presence of a nucleus is one of the distinguishing features of the eukaryotic cell. 
  • All the cellular activities are directed and coordinated by the nucleus.
  • In the prokaryotic cell, the true nucleus is absent.
  • In the case of a eukaryotic cell, the nucleus is present in all cells except the Red Blood Cell (RBC) and sieve tube cell (phloem). 
  • In the different types of cells, there is a different number of the nucleus. 
  • Most of the cells consist of only one type of nucleus i.e uninucleated while some of them consist of two nuclei i.e binucleate. 
  • The multinucleated cell consists of two or more nuclei. Example: slime mold.
  • The size of the nucleus is 5-25 µm. So, it is considered to be the largest organelles. 
  • It occupies about 8% of the total cell volume.
  • There is a variation in the shape as well as the position too. 
  • The nucleus is of different shapes like round, oval, elliptical, or lobed.
  • The Nucleus is present in a different position.
  • In the animal cell, it is present in the center.
  • In the plant cell, it is present in the periphery. It is because in the center there is the presence of a large water-filled vacuole.

Structure of Nucleus

The structure of the nucleus consists of the following parts. They are:  

Nuclear envelope

  • It is of the double- membrane and surrounds the nucleus.
  • Outer and inner membrane are present in it. The outer membrane of the nucleus is continuous with the ER ( Endoplasmic reticulum ). On its outer surface, there is the presence of many ribosomes. 
  • Perinuclear space is present between the membranes.
  • It is impermeable to large molecules like proteins and RNA.
  • Small molecules and ions can move freely.
  • Nuclear pores are present in the nuclear envelope. These pores are the small gaps present in the envelope. 
  • Chemical composition: lipo-proteinous

The function of the nuclear envelope

  • It gives the shape to the nucleus.
  • It protects the internal constituents of the nucleus.
  • It controls and regulates the movement of the substances which enter and exits the nucleus.


  • During the cell division, it disappears in the late prophase. 
  • In the Telophase stage, it reappears. 
  • The structure of the nucleolus consists of three main regions. They are :
  • Fibrillar centers: In the form of partly condensed chromatin, ribosomal RiboNucleic Acids (rRNA) genes are present.
  • Fibrillar component:  It surrounds the fibrillar centers where RNA molecules are present.
  • Granular regions: It consists of the mature ribosomal precursor particles. These are the outermost regions.
  • In the nucleus, the nucleolus may be present 1 to 4 in number.
  • It is rounded in structure and naked.
  • It is dense and is stained dark in color.
  • Chemical composition: RNA and protein

Functions of the Nucleolus

  • RNA is synthesized and stored in it.
  • Sub-units of ribosomes are formed.
  • During the cell division, it forms the spindle.
Nucleus Structure

Other nuclear bodies

Different types of nuclear bodies are present in the nucleus. They are:

  • Cajal bodies
  • Gemini of Cajal bodies
  • PIKA ( Polymorphic interphase karyosomal association)
  • PML ( Promyelocytic leukemia) bodies
  • Paraspeckles
  • Splicing speckles

Chromatin reticulum

  • Inside the nucleus, there is the presence of chromatins.
  • During the cell division,  it is condensed in the chromosome. 
  • It is dense in structure and is thread-like.
  • It consists of proteins and DNA.
  • Chromosome remains in the form of chromatin fibers at the interphase stage of cell division. 
  • They are differentiated into two distinct regions as heterochromatin and euchromatin.

Comparison between heterochromatin and euchromatin can be done based on the following properties:

  • Stain: Heterochromatin is dark stained and euchromatin is a lightly stained region of chromatin reticulum.
  • Condensation: Heterochromatin is highly condensed and euchromatin is a less condensed region.
  • Proportion: Heterochromatin forms the small part whereas euchromatin forms the major part of the chromatin reticulum.
  • Activeness: Heterochromatin is genetically inactive and euchromatin is genetically active.


  • In 1888 A.D Waldaye gave the term chromosome. 
  • Chromosome consists of the gene and genes consist of the DNA.
  • All the genetic information, heredity characters are present in it. 
  • A Chromosome consists of the following parts which are revealed by the electron microscope. They are:
  • Chromonemata: It is known as the subchromatid. There are two subunits. Chromatids are the two subunits of the metaphasic chromosome.
  • Centromere: In the different chromosomes, there is the presence of the constricted regions in different places. Based on the positions they are categorized as metacentric, submetacentric, acrocentric, telocentric chromosomes. 

Metacentric chromosome: Centromere is present in the middle and it forms two equal arms of a chromosome.

Sub-metacentric chromosome: Centromere is present nearer to one end of the chromosome due to which resulting arms are unequal. One is a long arm while another is a shorter arm.

Acrocentric chromosome: Centromere is present close to the end. It also results in unequal arms of a chromosome. One will be very short whereas one will be a very long arm.

Telocentric chromosome: It has got a terminal centromere.

Nuclear organizer (secondary constriction I)

  • A Nuclear organizer is a constriction that is present near one end of the chromosome. It is necessary for the formation of the nucleolus.
  • One or more secondary constrictions may be present.

Satellite: It is very short like the sphere. It is present beyond the nucleolar organizer. It is the non-staining secondary constriction and is the small fragment. It is present only in a few chromosomes.

Telomeres: It is the tip of the chromosome. It prevents the ends of the chromosome from sticking together.

Nuclear matrix

  • It is also called a nuclear scaffold.
  • With the use of non-ionic detergents, nucleases, and high salt buffers, extraction of the nuclear matrix from the cell’s nucleus is possible.
  • It consists of the nuclear lamina which is a network of intermediate filaments.
  • It consists of lamin proteins.
  • It consists of the nuclear matrix which provides t mechanical support to the nucleus. It acts like the cytoskeleton. 
  • A Nuclear matrix is a network of fibers and filaments.


  • Nucleoplasm is a clear and transparent, gelatinous substance.
  • It is also known as the karyoplasm.
  • It surrounds the nucleolus and chromosomes.
  • It consists of water, minerals, sugar, protein, nucleotides, enzymes, and RNA.
  • Nucleoplasm is a semifluid substance.

Function of nucleoplasm

  • It forms the spindle proteins which aids in cell division.
  • It protects the contents of the nucleus.
  • It provides the medium by which the enzymes and nucleotides get transported throughout the nucleus.
  • RNA and DNA are synthesized in it.
  • Nucleolus and chromatin reticulum are held by nucleoplasm.
  • It provides support by acting as the nuclear skeleton.

Nucleus Structure Free Worksheet

Nucleus Structure Worksheet


Functions of the Nucleus

Storing genetic material

  • Genetic material like DNA is stored in the nucleus. 
  • It is the cellular hereditary material. 
  • The information which is encoded in the DNA is passed to the offsprings from the parents.

DNA replication

  • Replication is the process of copying the parental DNA.
  • It occurs in the cell nucleus. 
  • It takes place in the S phase of the interphase of the cell cycle.


  • It provides the site for genetic transcription. 
  • It allows the level of gene regulation which are not available to prokaryotes. It consists of a variety of proteins. 
  • It either mediates the transcription process directly or is they may be involved in regulating the process. 
  • Different proteins involved in it are helicases, RNA polymerases, topoisomerases, etc.

Other functions

  • The nucleus controls the gene expression and helps in the replication of DNA during the cell cycle.
  • Coordinates and regulates cellular activities like cell division, protein synthesis, and growth.
  • The formation of ribosomes occurs in the nucleolus.
  • Through the nuclear pores only selective transportation is allowed. 
  • Organic evolution: It involves variation and can induce genetic change.
  • Nucleolus stores the proteins and RNA.
  • In the nucleus, during the cell division, chromatins are arranged into chromosomes.

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About Author

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Sushmita Baniya

Sushmita Baniya is pursuing her Master’s degree in Medical Microbiology from the National College of Science and Technology (NIST), Kathmandu, Nepal. She did her Bachelor’s degree in Microbiology from Birendra Multiple Campus, Chitwan, Nepal. She is interested in Genetics and Molecular Biology.

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