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Nucleus in biology can be very well defined as a dense organelle present in most eukaryotic cells, typically a single rounded structure bounded by a double membrane, containing the genetic material.

Nucleus can also be defined as a specialized, usually spherical mass of protoplasm encased in a double membrane, and found in most living eukaryotic cells, directing their growth, metabolism, and reproduction, and functioning in the transmission of genic characters.

What is a Nucleus ?

  • The nucleus is the controlling center of eukaryotic cell. It contains most of the genetic materials of the cell.
  • It was first observed by Antonie von Leeuvenhoek in the lumen of RBC’s and was named in the year 1833 by Robert brown who discovered it in plant cells.

  • Most eukaryotic cells have one nucleus, but some have many nuclei and certain cells like matured RBC’s do not have it.
  • Nuclei differ in size depending upon the cell type. Most nuclei are spherical, but multi-lobed nuclei are also common, such as those found in polymorph nuclear leukocytes or mammalian epididymal cells.
  • A nucleus has four components: Nuclear envelope, nucleolus, nucleoplasm and chromosomes.

Anatomy of Nucleus:

 anatomy of nuclueus

Structure of Nucleus:

Nuclear Envelope:

  • The nuclear envelope consists of two concentric membranes called the inner and outer membranes. The outer membrane is continuous with ER. In contrast the inner membrane carries unique proteins that are specific to the nucleus.
  • A network of intermediate filaments present on the nuclear side of the inner membrane is known as nuclear lamina which is made up of lamin proteins providing mechanical support to the nucleus.
  • Nuclear pores: The nuclear envelope contains nuclear pores for transport of macromolecules between cytoplasm and nucleus.
  • Each nuclear pore is formed from an elaborate structure called nuclear pore complex.
  • The proteins that make up the nuclear pore complex are known as nucleoporins.


  • It is a non-membrane bound dynamic body which disappears in the late prophase and reappears in the telophase stage of cell division.
  • Each nucleolus is produced is produced by a Nucleolar- Organizing Region (NOR) of a chromosome which is termed as nucleolar organizing chromosome.
  • It is a site of transcription of ribosomal RNA and assembly of ribosome.
  • It is responsible for synthesizing a large nascent precursor ribosomal RNA that is 45S in mammals and the processing of this RNA to yield mature ribosomal RNA of 18S, 5.8S, and 28S.
  • It consists of three major regions:
  1. Fibrillar centers: Containing rRNA genes in the form of partly condensed chromatin.
  2. Fibrillar component: Surrounds the fibrillar centers, which contains RNA molecules in the process of transcription.
  3. Granular regions: Outermost regions having mature ribosomal precursor particles.

Nucleolar-organizing region (NORs):

The nucleolus organizing region is a region of chromosome around which the nucleolus forms. In the human, the five chromosomes contain NORs, which can be identified as secondary constrictions on metaphase chromosomes. The position of secondary constriction in a chromosome is specific and species specific. As compared to secondary constrictions, the primary constrictions also called as centromere, attaches sister chromatids together.

Chromosomes and chromatin:

  • A chromatin is an organized structure of DNA and protein that is found in the nucleus of eukaryotic cells.
  • It contains a single dsDNA in coiled and condensed form.
  • The basic difference between chromatin and chromosomes is that chromatin is less condensed and extended DNA while chromosomes are highly condensed DNA.
  • The extent of chromatin condensation varies during the life cycle of the cell.
  • In non-dividing as well as interphase stages of cell, most of the chromatin remain relatively de-condensed.
  • The light staining, less condensed portions of chromatin are termed as euchromatin. This region is transcriptionally active and contains most of the transcribing genes.
  • The darkly stained and highly condensed region of chromatin is termed as heterochromatin. This region is transcriptionally inactive.
chromosomes and chromatin


The constricted region of linear chromosomes is known as the centromere. Although this constriction is called the centromere, it is usually not located exactly in the center of the chromosome and, in some cases, is located almost at the chromosome’s end.


Telomeres are specialized structures, which cap the ends of eukaryotic chromosomes. It consists of a long array of short, tandemly repeated sequences.

Origin of Replication:

The origin of replication is a particular sequence in a chromosome at which replication is initiated. One chromosome contains multiple origins of replication.

Chromosome number:

All eukaryotic cells have multiple linear chromosomes. Every cell maintains a characteristic number of chromosomes. Depending upon the eukaryotic organism, the number of chromosomes varies from 2 to several hundreds.

Chromosome number in different eukaryotic organisms:

Species                                                        Haploid number of chromosomes
Saccharomyces cerevisiae                      16
Arabidopsis thaliana                                06
Drosophila thaliana                                  04 
Mus musculus                                           19 +X and Y
Homo sapiens                                           22 + X and Y

Functions of Nucleus:             

  • It is the prime control center of cellular activities. It controls the enzymatic activities.
  • It controls the process of synthesizing proteins.
  • It regulates the hereditary characteristics of an individual
  • It participates in the process of cell division, differentiation and growth of cell.
  • It controls the reliability of genes and its expression.
  • Helps in the transport of RNA and DNA across the cell.
  • Nucleolus is called ribosomal factory of the cell.
  • Transcription takes place in nucleus.

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