The basic foundation of any organism, a cell, consists of outer membranes (cell wall or cell membrane), an inner gel-like structure (cytoplasm or protoplasm), and cell organelles (mitochondria, lysosome, nucleus, endoplasmic reticulum, vacuole, etc.,) in the cytoplasm.
The nucleus is a cell organelle that carries the genetic material (DNA-deoxyribonucleic acid) of a cell. The nucleus of eukaryotes is membrane-bound, whereas the prokaryotes lack the outer membrane. The commonly known functions of the nucleus are to control and regulate cellular activities like growth and metabolism and carry genes (structure with genetic information).
The nucleus is a sphere-shaped cell organelle consisting of a thin layer of covering called karyotheca or nuclear envelope. On the inside, the nucleus consists of a fluid-like substance called nucleoplasm or karyolymph that houses fine filaments called chromatin.
A Dutch microscopist, Antonie van Leeuwenhoek, 1710, observed the nucleus in the blood cells of amphibians and birds and reported it as a centrally placed clear area. Likewise, Fontana reported it as an ovoid structure in the isolated epidermal cells of eel’s skin in 1781.
However, Robert Brown was the first to use the term “nucleus” to report a prominent body in an orchid cell. According to him, “the nucleus was one of the regular features of a cell,” starting the concept of nucleated cells. Here nucleus is a diminutive form of the Latin word “nux,” meaning nut, whereas the meaning of nucleus is kernel or center part.
Structure of Nucleus
The nucleus is spherical and consists of the following structural components; nuclear envelope, nucleoplasm, nuclear matrix, chromatin, and nucleolus.
The nucleus is bounded by a thin layer called a nuclear envelope or karyotheca. Inside the envelope is a fluid called nucleoplasm. The nuclear matrix (protein with fibrils), nucleoprotein (chromatin), and more spherical body called nucleolus are suspended in the nucleoplasm.
- Chemical composition: The chemical composition of the nucleus is 9-12% DNA, 5% RNA, 3% lipids, 15% simple basic proteins like histone or protamines, and about 65% complex acid or neutral proteins, including enzymes like polymerase for the DNA and RNA synthesis, organic phosphates, and salts and ions like Mg++, Ca++, and Fe++.
The nuclear envelope is the outermost part of a nucleus and separates it from the cytoplasm. It has two membranes: inner and outer. The thickness of each membrane is 75 Å thick and composed of trilaminar lipoprotein similar to the plasma membrane. An intermembrane or perinuclear space of 250Å width separates the two membranes. In some places, the outer membrane is covered with ribosomes that conduct protein synthesis and is continuous with the rough endoplasmic reticulum (RER).
The inner membrane has a dense layer called nuclear lamina that is closely associated with the nucleoplasm. The lamina comprises the 30-100 nm thick network of filaments made up of proteins called lamina A, B, and C. Its function is to provide shape and support to the inner membrane. It also connects the inner membrane with the chromatin for keeping the chromosomes in the periphery of the nucleus. In addition, the lamina plays a role in the breakdown and reformation of the nuclear envelope during mitosis.
The envelope has pores that control the passage of some molecules and particles. The fusion of the inner and outer membrane makes the pores in the envelope. There may exist 1000 to 10,000 pores per nucleus. Each nuclear pore has a pore complex, which is approximately cylindrical and projects into the nucleoplasm and cytoplasm. The complex has two rings, called annuli. One ring is located at the cytoplasmic rim of the pore and another at the nucleoplasmic edge. Each ring has eight symmetrically arranged subunits that enclose a channel of about 100 to 200 Å wide. Monosaccharides, disaccharides, and amino acids can pass freely through the pores, whereas it does not control the passage of RNA and proteins. It also acts as a barrier to some molecules like the DNA of chromosomes.
Functions of the nuclear envelope
- It maintains the shape.
- The envelope acts as the barrier between cytoplasm and nucleoplasm.
- It controls the flow of components in and out of the nucleus.
The nucleoplasm is a fluid present in the nucleus which is transparent. The nucleoplasm houses nucleolus and chromatin. It consists of nucleotides, polymerases, and metal ions like Mn++ and Mg++ for synthesizing DNA and RNA. In addition, it contains proteins like basic histones, acidic or neutral non-histones, etc., required for DNA and lipids. There are other proteins needed for forming ribosomal subunits as well. Thus, formed ribosomal units and RNAs pass out of the nucleoplasm with the help of nuclear pores.
Functions of nucleoplasm
- The site for the synthesis of RNAs and ribosomal subunits.
- It houses the nuclear matrix, chromatin, and nucleolus.
- It maintains the turgidity of the nucleus.
A network of thin, crisscrossed, protein-containing fibrils connected end to end with the nuclear envelope is called a nuclear matrix. It acts as a nuclear skeleton and remains intact when DNA and chromatin get removed.
Functions of nuclear matrix
- It maintains the shape of the nucleus.
- The nuclear matrix acts as an anchor for chromatin.
- It plays a role as a structural component for transcription and replication.
- The nuclear matrix also implicates the processing of newly formed RNA and helps transport it out of the nucleus.
Flemming termed chromatin in 1879. During interphase (non-dividing stage of cell division) occurs as a fine filament, so it is also called chromatin fibers. The fibers occur in criss-cross that gives the appearance of a diffuse network often referred to as nuclear or chromatin reticulum. It occupies most of the nucleus. The chromatin fiber is extended chromosomes and usually has approximately 250 Å diameter.
The chromatin fibers form short, thick, rod-like bodies during cell division called chromosomes. The chromosomes change back to the chromatin fibers after division. Normally, chromatin fibers are uncoiled, extended, and scattered in the nucleoplasm, called euchromatin. These represent the true chromatin during interphase. Likewise, another chromatin fiber called heterochromatin remains compacted and coiled even during the non-division state. It represents an inactive part of chromosomes and has less DNA and more RNA. It is of two types; constitutive and facultative.
According to the theory proposed by Kornerg and Thomas (1974), nucleosomes are a chain of similar subunits in chromatin fibers. It contains a core particle wrapped by a DNA strand. The core particle is eight histone proteins. The wrapping DNA is 140 nucleotides long. A short DNA linker (of 60 nucleotides) connects each nucleosome. So, a nucleosome and a linker have approximately 200 total nucleotides known as chromatosome. The chromatin fibers are similar to the chain of beads here, and the bead is a nucleosome. Prokaryotes lack nucleosomes.
Function of chromatin
- The most important and only function of chromatin is the formation of chromosomes during cell division.
F. Fontana discovered nucleolus in 1781 in the slime of the eel skin. Most cells have nucleolus except muscle cells and male reproductive cells. Its plural form is nucleoli. It is generally spherical, but other shapes also exist. Different organisms have different numbers of nucleoli. It disappears during cell division and reforms at the end of cell division and at specific sites called nucleolar organizers or nucleolar organizer regions (NORs) of specific chromosomes (nucleolar chromosomes). The nucleolus is positioned at the center of the nucleus but occupies a specific position in its chromosome. It lacks a membrane, and calcium ions are responsible for keeping it intact. It has four regions; the Fribrillar region, granular region, amorphous region, and nucleolar chromatin.
- Fibrillar region or nucleolonema: It consists of indistinct fibrils of about 50-100 Å in diameter. The fibril represents the long rRNA precursor molecule before enzymes cut off some segments from it.
- Granular region: It has spherical, electron-dense particles (granules) of about 150-200 Å in diameter. The granules are ribosomal units almost ready for transport in the cytoplasm.
- Amorphous region or par amorpha: It is a structure-less proteinaceous matrix where the granular or fibrillar region suspends.
- Nuclear chromatin: It has 100 Å thick chromatin fibers. It has a part of nucleolar chromosomes that follows a tortuous path from the granular and fibrillar components of the nucleolus. It has many copies of DNA, which direct rRNA synthesis (ribosomal RNA).
Functions of Nucleolus
- It synthesizes and stores rRNA.
- Nucleolus also stores ribosomal proteins.
- Likewise, it forms ribosomal subunits by wrapping the rRNA with ribosomal proteins.
- It also plays a role in cell division.
Function of Nucleus
The functions of the nucleus are as follows:
- It directs the synthesis of structural proteins to maintain the integrity of cells.
- The nucleus regulates the cell’s metabolism with the help of synthesized enzymatic proteins.
- It contains genetic information required for the organism’s reproduction, development, and behavior.
- The nucleus is necessary for replication.
- Ribosome subunits form in the nucleus.
- It develops genetic variations resulting in evolution.
- It regulates the activation of certain genes helping in bringing cell differentiation.
- Nucleus. Genome.gov. (2022). Retrieved 4 August 2022, from https://www.genome.gov/genetics-glossary/Nucleus.
- Bachs, O., Agell, N. (1995). The Cell Nucleus: Structure and Main Functions. In: Calcium and Calmodulin Function in the Cell Nucleus. Molecular Biology Intelligence Unit. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-21686-6_2