Cell Biology

Nucleus Definition, Structure, Diagram, and Functions

Cell biology describes the nucleus is the big organelle, with a membrane-bounded structure that holds the genetic material form of numerous linear...

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This article writter by MN Editors on November 27, 2021

Microbiology Notes is an educational niche blog related to microbiology (bacteriology, virology, parasitology, mycology, immunology, molecular biology, biochemistry, etc.) and different branches of biology.

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Nucleus Definition, Structure, Diagram, and Functions
Nucleus Definition, Structure, Diagram, and Functions

Cell biology describes the nucleus is the big organelle, with a membrane-bounded structure that holds the genetic material form of numerous linear DNA molecules arranged into chromosomes, which are the structures that make up the nucleus. In the field of cell biology, the nucleus’s function is to serve as the central point of control for the cells. This is due to the fact that it holds the genetic material that code for the essential functions of cells. 

The nucleus is the organelle that is responsible for maintaining DNA’s integrity and controlling the activities of cells like metabolism, growth and reproduction through the regulation of the expression of genes. The nucleus is by far the largest cell’s cytoplasm. In mammalian cells the average size is about 6 inches. However, there are cells which lack nuclei, namely human red blood cells. There are other cells that have a higher proportion of cells, e.g. osteoclasts.

Nucleus Definition

When it comes to biology,”nucleus” generally refers to the cell’s nucleus. It is described as the organelle within the cell that houses the chromosomes. Some cells do not contain nuclei. If a cell does not have nucleus, it is known as anucleated. In addition to this the term “nucleus” can also be employed in different biological fields. For example, in botany, the term “nucleus” could also be a reference to the central kernel in the nut, or seed, or the central point of the starch granule. The neuroanatomy Nucleus is a nerve cell bodies in the brain and the spinal cord.

In other fields of science the term “nucleus” could refer to the central part or the central area within which the other components are grouped or collected. For example in Physics the term “nucleus” is used to refer to the positively charged center of an atom which usually has neutrons and protons. In Chemistry Nucleus refers to the fundamental arrangement of atoms that occurs in chemical compounds by the substitution of atoms with changing their the structure. In Astronomy the nucleus is the central point of a comet or center or brightest area of a galaxy or nebula. In Meteorology the term “nucleus” refers to a particle in where water vapor molecules gather in the air and create water droplets or crystals of ice.

A nucleus is an enormous double-membraned organelle, which is often called”the “central cell” within the cell due to the fact that it houses the chromosomes that carry all the genes. It is not only found in eukaryotic cells but also on prokaryotic cell lines. In addition to the chromosomes there are various other structures within the nucleus. They are collectively known as nuclear bodies. The fluid part that is found in nucleus cells are known as the nucleoplasm.

Structure of Nucleus/Parts of Nucleus


  • The nucleic acid (e.g. DNA, the RNA) as well as proteins (e.g. histones) is referred to as chromatin.
  • In the course of cell division, cell’s chromatin is compressed to create an it’s a chromosome.
  • The fundamental structural unit of chromatin is the nucleosome.
  • Each nucleosome consists of a DNA segment wrapped around the histone protein’s cores.
  • The primary role of chromatin is pack DNA into a smaller size that fits into the cell.
  • There are two main forms of chromatin, the heterochromatin and the euchromatin.
  • The structure of euchromatin is loose, allowing replication and transcription, while the heterochromatin is less condensed and, consequently, less active.

Nuclear DNA

  • The nuclear DNA is significant proportions of the genome of a cell (the tiny portion comes from extranuclear DNA that is found in mitochondria or the chloroplasts).
  • The DNA outside of the nucleus is known as extranuclear DNA. This extranuclear DNA like cpDNA found in mitochondria, and chloroplasts in is present in multiple copies as there are multiple chloroplasts as well as mitochondria, while there is generally only one nucleus in cells.
  • Cells, then have multiple copies of mtDNA as well as cpDNA typically in the thousands.
  • Nuclear DNAs are compressed into chromatin-like structures via histones, whereas cpDNA and mtDNA aren’t.

Nuclear bodies

  • A nuclear body is described as a non-membraned mostly proteinsaceous body in the nucleus. As mentioned earlier the nucleolus is considered to be one of nuclear body, and is the most popular.
  • It is distinguished by its granular round appearance.
  • It is used for the production of ribosomes which are in turn one of the main components of protein synthesis.
  • Other nuclear body types are Cajal gems and cajal (Gemini in Cajal bodies) Polymorphic interphase karyosomal connection (PIKA) domains promyelocytic leukemia protein (PML) bodies paraspeckles, splicing speckles as well as perichromatin fibrils and Clastosomes.
  • Nuclear bodies are classified as straightforward (type I as well as type II) and more complex (type III Type IVa, type IVa as well as type V).
Nuclear bodies
Source: https://nitrocdn.com/BzukxzxIDWSkBjOuXIuFVkjjEriFmqlw/assets/static/optimized/rev-5165cad/wp-content/uploads/2019/11/liver-cell-with-nucleus-and-nucleolus-768×432.jpg

Nuclear matrix

  • Nuclear matrix functions comparable to the cytoskeleton found in the cell’s cytoplasm.
  • The fibrillary structure which provides structural support for the shape and size of the nucleus.
  • It is much more dynamic even when compared to the cytoskeleton.
  • It is comprised of it’s nuclear layer. The latter is the fibrous and dense network that encircles to the nucleus envelope.


  • The term “nucleoplasm” refers to the nucleus’ protoplasm like the cytoplasm found is found in the other parts of the cell.
  • The nucleoplasm is made up of different components (e.g. the nuclear body, chromosomes the nuclear matrix) that are contained within an envelope called the nuclear.
  • The liquid component of the nucleoplasm is referred to as the nucleosol (just as the cytosol is to it’s cytoplasm).
  • Nucleoplasm is the substance that forms a gelatinous layer inside the nuclear envelope.
  • Also known as karyoplasm, this semi-aqueous substance is similar to the cytoplasm . It is made up of mainly water, dissolved enzymes, salts, and organic molecules suspended in.
  • The nucleolus as well as chromosomes are protected by nucleoplasm which serves to cushion and protect content of nucleus.
  • Nucleoplasm can also support the nucleus, helping it keep its shape. Furthermore, it provides the medium through which various substances like enzymes and nucleotides (DNA and subunits of RNA) are transported through the nucleus. Substances exchange between nucleoplasm and the cytoplasm through nucleoplasmic pores.

Nuclear envelope

  • The nucleus’s envelope (also known as the nucleus membrane) is a type of biological membrane that surrounds the nucleus.
  • As with cell membranes, the nuclear membrane is a bilipid layer. So, the nuclear membrane functions in a similar way to a cell membrane for regulating the flow and exit of substances.
  • The nuclear envelope is a cellular structure with pores that control the flow of molecules between the nucleoplasm as well as the celluplasm. It is inaccessible to large molecules. This is why it is able to separate the contents of the nucleus from the cell tissue and allows for the passage of specific molecules.
  • Nuclear transport for large molecules (e.g. proteins or RNAs) takes place via active transport system that contains carrier proteins, whereas the transport of smaller molecules and ions is done by passively passing through these pores in the nucleus.


  • The nucleus could be described as bilobed multi-lobed or tri-lobed based on the amount of lobes.
  • White blood cells are an illustration one with lobed nucleus.


  • In the nucleus, there is a thick, non-membrane structure that is made up of RNA as well as proteins referred to as the nucleolus.
  • A few eukaryotic organisms have nuclei that contain as many as four nucleoli.
  • The nucleolus is home to organizers for nucleolar RNA that are part of chromosomes that have genes for ribosome synthesizing on them. The nucleolus assists in the production of Ribosomes by transcribing and joining subunits of ribosomal DNA. These subunits join and form a “ribosome” the process of protein synthesis.
  • The nucleolus vanishes after cells divide and is rebuilt following the cell division has ended.

Functions of Nucleus

The nucleus serves as a location for gene transcription, which is separated from the place of transcription in the cytoplasm. This allows levels of gene control that are not accessible to prokaryotes. The primary purpose of the cell’s nucleus is to regulate gene expression as well as to facilitate reproduction of DNA in the cycle of cell growth.

  • It regulates the genetic characteristics of an animal.
  • Organelles are also involved in the synthesis of proteins as well as development, cell division and differentiation.
  • Storage of hereditary material the genes that are made up of thin and long DNA (deoxyribonucleic acid) strings, known as Chromin.
  • Protein storage and the storage of RNA (ribonucleic acid) within the nucleolus.
  • The nucleus is the site for transcription, where messenger (mRNA) (mRNA) are made to enable protein production.
  • In the process of cell division the chromatins are placed in chromosomes within the nucleus.
  • Production of Ribosomes (protein factories) within the nucleolus.
  • Transport of regulatory factors or energy molecules by nuclear pores.

Biological Importance of Nucleus

The nucleus is the biggest cell’s cytoplasmic organelle in mammals. In mammalian cells the average size is about 6um. There are some cells deficient in nuclei, for instance human red blood cell, there are certain cells with more nuclei e.g. osteoclasts. This implies that the osteoclasts have a greater degree of activity in the sense of regulation of genes than Red blood cells. After maturation, red blood cells shed their nucleus and provide more affinity to gasses, e.g. oxygen.

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Microbiology Notes is an educational niche blog related to microbiology (bacteriology, virology, parasitology, mycology, immunology, molecular biology, biochemistry, etc.) and different branches of biology.

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