# Cell Nucleus: Definition, Structure, Components, and Functions > Learn what the cell nucleus is, how the nuclear envelope, pores, chromatin, and nucleolus are organized, and how the nucleus stores DNA and regulates... Canonical URL: https://biologynotesonline.com/nucleus-structure-and-functions/ Author: Sourav Pan Last updated: September 12, 2026 ![Cell Nucleus: Definition, Structure, Components, and Functions](https://biologynotesonline.com/wp-content/uploads/2024/04/Structure-of-the-Cell-Nucleus-and-Its-Components.webp) The nucleus is a membrane-bound cell organelle present as a characteristic feature of eukaryotic cells, which contains most of the genetic material (DNA) of the cell. It is surrounded by a nuclear envelope and remains separated from the cytoplasm. This is one of the major features that differentiate a eukaryotic cell from [prokaryotic cell](https://biologynotesonline.com/prokaryotic-vs-eukaryotic-cells/). However, all the cellular DNA is not present inside the nucleus. A small amount of DNA is also found in [mitochondria](https://biologynotesonline.com/mitochondria/), and in plant and algal cells, plastids such as chloroplasts also contain DNA. The nucleus is not same as the nucleolus. Nucleolus is a dense non-membranous region present inside the nucleus and is mainly associated with rRNA formation and early ribosome assembly. It is also different from the nucleoid found in prokaryotic cells. Bacteria and Archaea do not possess a true membrane-bound nucleus. Their major DNA is present in a region called “nucleoid”, which is not enclosed by a nuclear envelope. Although the nucleus is a characteristic feature of eukaryotic cells, every mature eukaryotic cell does not necessarily retain one. Mature mammalian erythrocytes (red blood cells) are one such example. During their formation, the nucleus is expelled from the developing erythroblast. This process is referred to as “enucleation”. ## Structure of the Nucleus The nucleus is made up of different structural components. The following are the major structures- ![Cutaway diagram of a eukaryotic nucleus showing the double nuclear envelope, nuclear pores, lamina, nucleoplasm, chromatin, nucleolus, nucleosomes, and continuity with the endoplasmic reticulum.](https://biologynotesonline.com/wp-content/uploads/2024/04/Structure-of-the-Cell-Nucleus-and-Its-Components-1024x768.webp)Cutaway diagram of a eukaryotic nucleus showing the double nuclear envelope, nuclear pores, lamina, nucleoplasm, chromatin, nucleolus, nucleosomes, and continuity with the endoplasmic reticulum. - Nuclear envelope- It is a double membrane covering which surrounds the nucleus and separates its contents from cell cytoplasm. The two membranes are called outer nuclear membrane and inner nuclear membrane. A narrow perinuclear space occurs between them. The outer membrane continues with the [endoplasmic reticulum (ER)](https://biologynotesonline.com/endoplasmic-reticulum/), and ribosomes may also remain attached on its cytoplasmic surface. - Nuclear pore complexes- Nuclear pores are present throughout the nuclear envelope. These pores contain large protein structures known as nuclear pore complexes (NPCs). They regulate movement of materials into and out of the nucleus. Proteins generally enter into nucleus, whereas different RNA molecules and ribosomal subunits pass out through these pores. - Nuclear lamina- It is a fibrous supporting network found just below the inner nuclear membrane. In animal cells, the major proteins forming this layer are called lamins. The nuclear lamina gives mechanical support to the nucleus. It also remains associated with chromatin. - Nucleoplasm- The internal space of nucleus contains a semifluid material called nucleoplasm. Chromatin, nucleolus and different nuclear molecules are found within it. - Chromatin- This is the DNA-protein material present inside the nucleus. Nuclear DNA is mainly associated with histone proteins, forming repeating bead-like units known as nucleosomes. During interphase, chromatin occurs in different degree of condensation. When the cell enters into division, it becomes more highly condensed. Visible [chromosomes](https://biologynotesonline.com/chromosome/) are then formed. - Nucleolus- It is a dense region of the nucleus which lacks a surrounding membrane. One or more nucleoli can be present in a nucleus. Its major role is associated with formation and processing of rRNA. The early assembly of ribosomal subunits also takes place in this region. ## Nuclear Variation Across Cells The nucleus shows variation in its number, shape and position in different eukaryotic cells. It mainly depends upon the type and specialization of cell. ![Comparison of a uninucleate cell, multinucleated skeletal muscle fiber, mature plant cell with a peripheral nucleus, and mammalian erythroblast losing its nucleus during enucleation.](https://biologynotesonline.com/wp-content/uploads/2024/04/Variation-in-Nucleus-Number-Position-and-Enucleation-1024x768.webp)Comparison of a uninucleate cell, multinucleated skeletal muscle fiber, mature plant cell with a peripheral nucleus, and mammalian erythroblast losing its nucleus during enucleation. ### Number of Nuclei Most of the eukaryotic cells contain a single nucleus and are called uninucleate cells. Some specialized cells contain many nuclei. Skeletal muscle fibres are multinucleated cells which are formed by fusion of many myoblasts. Osteoclasts also contain several nuclei, formed by fusion of mononuclear precursor cells. ### Shape and Position The shape and position of nucleus is not same in all cells. It may be spherical or ovoid, and changes according to the cell type and internal organization. In mature plant cells, a large central vacuole occupies most of the cell space. Due to this, cytoplasm and other organelles are pushed towards the periphery and the nucleus is generally present towards the side of cell. ### Anucleate Specialized Cells Some eukaryotic cells lose their nucleus during maturation. Mature mammalian erythrocytes ([red blood cells](https://biologynotesonline.com/red-blood-cell/)) are the common example. During their development, the nucleus becomes condensed and finally expelled from the erythroblast. This process is referred to as “enucleation”. The absence of nucleus in mature mammalian red blood cells is therefore a specialized condition. These cells are not prokaryotic, but are formed from nucleated eukaryotic precursor cells. ## Functions of the Nucleus The nucleus performs different functions related to genetic material and cellular activities. Some of the important functions are- ![Diagram showing DNA replication, transcription and RNA processing inside the nucleus, nucleolar ribosomal-subunit formation, RNA export, and protein import through nuclear pore complexes.](https://biologynotesonline.com/wp-content/uploads/2024/04/Functions-of-the-Nucleus-and-Nuclear-Pore-Transport-1024x768.webp)The nucleus combines genome storage and processing with regulated communication across nuclear pores: DNA replication, transcription and RNA processing occur within the nuclear compartment, while RNAs, ribosomal precursors and nuclear proteins move selectively between nucleus and cytoplasm. - Storage of genetic material- The nucleus stores most of the genetic information of cell in the form of DNA. It remains associated with proteins as chromatin. - [DNA replication](https://biologynotesonline.com/dna-replication-steps/)- Replication of nuclear DNA takes place inside the nucleus. In this process, DNA forms its copies before the cell division. - RNA formation- The nucleus is the major site of [transcription](https://biologynotesonline.com/transcription/), where information of DNA is used for formation of RNA. Processing of newly formed RNA also takes place here. - Formation of ribosomal subunits- The nucleolus is involved in rRNA formation and its processing. Early assembly of [ribosomal subunits](https://biologynotesonline.com/ribosomes-structure-and-functions/) also occurs in this region. - Regulation of gene expression- The nucleus helps in controlling gene expression by regulating transcription, RNA processing and movement of molecules through nuclear pores. - Nuclear transport- Nuclear pore complexes (NPCs) regulate movement between nucleus and cytoplasm. Proteins move into nucleus, whereas RNA molecules and ribosomal subunits are transported outside. ## Key Facts about Nucleus FeatureKey factsDefinitionThe nucleus is a membrane-bound organelle containing most of the cellular DNA.OccurrenceIt is a characteristic feature of eukaryotic cells. Prokaryotes do not possess a true nucleus.DNAMost of the DNA is present in nucleus, while some DNA also occurs in mitochondria and plastids.Nuclear envelopeIt is a double membrane covering around the nucleus. Inner and outer nuclear membranes are separated by perinuclear space.Nuclear poresNuclear pore complexes (NPCs) regulate movement of proteins, RNA and other molecules between nucleus and cytoplasm.Nuclear laminaA fibrous supporting layer present below the inner nuclear membrane. It is mainly formed of lamins in animal cells.NucleoplasmSemifluid internal material of nucleus in which chromatin and nucleolus remain present.ChromatinIt is the DNA-protein material of nucleus. DNA remains associated mainly with histone proteins and forms nucleosomes.NucleolusA dense, non-membranous region involved in rRNA formation, processing and early ribosomal subunit assembly.Main functionsStorage of genetic material, DNA replication, transcription, RNA processing, regulation of gene expression and nuclear transport.Number of nucleiMost cells are uninucleate. Skeletal muscle fibres and osteoclasts are examples of multinucleated cells.Shape and positionNuclear shape and position vary according to cell type and organization. In mature plant cells, nucleus is generally pushed towards the side by the large central vacuole.Anucleate cellsMature mammalian erythrocytes (RBCs) lose the nucleus during maturation by enucleation.Nucleus vs nucleolusNucleus is the membrane-bound organelle. Nucleolus is a region present inside the nucleus.Nucleus vs nucleoidA nucleus is membrane-bound, whereas the prokaryotic nucleoid is not enclosed by a nuclear membrane. ## References - Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular biology of the cell (4th ed.). Garland Science. [https://www.ncbi.nlm.nih.gov/books/NBK21054/](https://www.ncbi.nlm.nih.gov/books/NBK21054/) - Cooper, G. M. (2000). The cell: A molecular approach (2nd ed.). Sinauer Associates. [https://www.ncbi.nlm.nih.gov/books/NBK9839/](https://www.ncbi.nlm.nih.gov/books/NBK9839/) - Foisner, R. (n.d.). Dynamic connections of nuclear envelope proteins to chromatin and the nuclear matrix. In Madame Curie Bioscience Database. Landes Bioscience. [https://www.ncbi.nlm.nih.gov/books/NBK6125/](https://www.ncbi.nlm.nih.gov/books/NBK6125/) - Ji, P., Murata-Hori, M., & Lodish, H. F. (2011). Formation of mammalian erythrocytes: Chromatin condensation and enucleation. Trends in Cell Biology, 21(7), 409–415. [https://doi.org/10.1016/j.tcb.2011.04.003](https://doi.org/10.1016/j.tcb.2011.04.003) - Rye, C., Wise, R., Jurukovski, V., DeSaix, J., Choi, J., & Avissar, Y. (2016). Biology. OpenStax. [https://openstax.org/books/biology/pages/1-introduction](https://openstax.org/books/biology/pages/1-introduction) - Walters, A. D., Bommakanti, A., & Cohen-Fix, O. (2012). Shaping the nucleus: Factors and forces. Journal of Cellular Biochemistry, 113(9), 2813–2821. [https://doi.org/10.1002/jcb.24178](https://doi.org/10.1002/jcb.24178)