Ideas about cell structure have changed considerably over the years. Early biologists saw cells as simple membranous sacs containing fluid and a few floating particles. Today’s biologists know that cells are infinitely more complex than this.
There are many different types, sizes, and shapes of cells in the body. For descriptive purposes, the concept of a “generalized cell” is introduced. It includes features from all cell types. A cell consists of three parts: the cell membrane, the nucleus, and, between the two, the cytoplasm.
Within the cytoplasm lie intricate arrangements of fine fibers and hundreds or even thousands of miniscule but distinct structures called organelles.
A. Cell wall.
The outermost structure of most plant cells is a dead and rigid layer called cell wall. It is mainly composed of carbohydrates such as cellulose, pectin, hemicellulose and lignin and certain fatty substances like waxes.
Ultra-structurally cell wall is found to consist of a microfibrillar network lying in a gel-like matrix. The microfibrils are mostly made up of cellulose. There is a pectin-rich cementing substance between the walls of adjacent cells which is called middle lamella.
The cell wall which is formed immediately after the division of cell, constitutes the primary cell wall. Many kinds of plant cells have only primary cell wall around them. Primary cell wall is composed of pectin, hemicellulose and loose network of cellulose microfibrils.
In certain types of cells such as phloem and xylem, an additional layer is added to the inner surface of the primary cell wall at a later stage. This layer is called secondary cell wall and it consists mainly of cellulose, hemicellulose and lignin.
In many plant cells, there are tunnels running through the cell wall called plasmodesmata which allow communication with the other cells in a tissue. The cell wall constitutes a kind of exoskeleton that provides protection and mechanical support to the plant cell. It determines the shape of plant cell and prevents it from desiccation
B. Plasma membrane.
Every kind of animal cell is bounded by a living, extremely thin, and delicate membrane called plasmalemma, cell membrane, or plasma membrane.
In-plant cells, the plasma membrane occurs just inner to the cell wall, bounding the cytoplasm. The plasma membrane exhibits a tri-laminar (i.e., three-layered) structure with a translucent layer sandwiched between two dark layers.
At the molecular level, it consists of a continuous bilayer of lipid molecule (i.e., phospholipids and cholesterol) with protein molecules embedded in it or adherent to its both surfaces. Some carbohydrate molecules may also be attached to the external surface of the plasma membrane, they remain attached either to protein molecules to form glycoproteins or to lipids to form glycolipids.
The plasma membrane is a selectively permeable membrane; its main function is to control selectively the entrance and exit of materials. This allows the cell to maintain a constant internal environment (homeostasis).
Transport of small molecules such as water, oxygen, carbon dioxide, ethanol, ions, glucose, etc., across the plasma membrane takes place by various means such as osmosis, diffusion and active transport.
The process of active transport is performed by special type of protein molecules of plasma membrane called transport proteins or pumps, consuming energy in the form of ATP molecules. For bulk transport of large-sized molecules, plasma membrane performs endocytosis (i.e., endocytosis, pinocytosis, receptor-mediated endocytosis and phagocytosis) and exocytosis both of these processes also utilize energy in the form of ATP molecules.
Various cell organelles such as chloroplasts, mitochondria, endoplasmic reticulum and lysosomes are also bounded by membranes similar to the plasma membrane. All the cellular membranes have a basic trilaminar unit membrane construction.
However, their structure and extent of activity are mainly depended on the relative proportion of their constituent protein and lipid molecules. Thus, membranes that are metabolically highly active, e.g., those of mitochondria and chloroplasts have a greater proportion of proteins and more granular appearance than those membranes which are relatively less active, e.g., myelin sheath of certain nerve fibers.
The plasma membrane is followed by the cytoplasm which is distinguished into following structures:
The plasma membrane is followed by the colloidal organic fluid called matrix or cytosol. The cytosol is the aqueous portion of the cytoplasm (the extranuclear protoplasm) and of the nucleoplasm (the nuclear protoplasm).
It fills all the spaces of the cell and constitutes its true internal milieu. The cytosol is particularly rich in differentiating cells and many fundamental properties of the cell are because of this part of the cytoplasm.
The cytosol serves to dissolve or suspend the great variety of small molecules concerned with cellular metabolism, e.g., glucose, amino acids, nucleotides, vitamins, minerals, oxygen, and ions.
In all types of cells, the cytosol contains the soluble proteins and enzymes which form 20 to 25 percent of the total protein content of the cell. Among the important soluble enzymes present in the matrix are those involved in glycolysis and in the activation of amino acids for protein synthesis. In many types of cells, the cytosol is differentiated into the following two parts:
- The ectoplasm or cell cortex is the peripheral layer of cytosol which is relatively non-granular, viscous, clear, and rigid.
- Endoplasm is the inner portion of cytosol which is granular and less viscous.
a). Cytoskeleton and micro trabecular lattice.
The cytosol of cells also contains fibers that help to maintain cell shape and mobility and that probably provides anchoring points for the other cellular structures. Collectively, these fibers are termed as the cytoskeleton. At least three general classes of such fibers have been identified.
- The thickest are the microtubules (20 nm in diameter) which consists primarily of the tubulin protein. The function of microtubules is the transportation of water, ions or small molecules, cytoplasmic streaming (cyclosis), and the formation of fibers or asters of the mitotic or meiotic spindle during cell division. Moreover, they form the structural units of the centrioles, basal granules, cilia and flagella.
- The thinnest are the microfilaments (7 nm in diameter) which are solid and are principally formed of actin protein. They maintain the shape of cell and form contractile component of cells, mainly of the muscle cells.
- The fibers of middle order are called the intermediate filaments (IFs) having a diameter of 10 nm. They having been classified according to their constituent protein such as desmin filaments, keratin filaments, neurofilaments, vimentin and glial filaments.
Recently, cytoplasm has been found to be filled with a three-dimensional network of interlinked filaments of cytoskeletal fibers, called microtra-becular lattice. Various cellular organelles such as ribosomes, lysosomes, etc., are found anchored to this lattice. The microtrabecular lattice being flexible, changes its shape and results in the change of cell shape during cell movement.
2). Cytoplasmic structures.
In the cytoplasmic matrix certain non-living and living structures remain suspended. The non-living structures are called paraplasm or inclusions, while the living structures are membrane bounded and are called organoids or organelles. Both kinds of cytoplasmic structures can be studied under the following headings:
I) Cytoplasmic inclusions.
The stored food and secretory substances of the cell remain suspended in the cytoplasmic matirx in the form of refractile granules forming the cytoplasmic inclusions.
The cytoplasmic inclusions include oil drops, triacylglycerols (e.g., fat cells of adipose tissue), yolk granules (or deutoplasm, e.g., egg cells), secretory granules, glycogen granules (e.g., muscle cells and hepatocytes of liver) and starch grains (in plant cells).
II) Cytoplasmic organelles.
Besides the separate fibrous systems, cytoplasm is coursed by a multitude of internal membranous structures, the organelles (literally the word organelle means a tiny organ). Membranes close off at specific regions of the eukaryotic cells performing specialized tasks:
oxidative phosphorylation and generation of energy in the form of ATP molecules in mitochondria;
- formation and storage of carbohydrates in plastids;
- protein synthesis in rough endoplasmic reticulum;
- lipid (and hormone) synthesis in smooth endoplasmic reticulum;
- secretion by Golgi apparatus;
- degradation of macromolecules in the lysosomes;
- regulation of all cellular activities by nucleus;
- organization of spindle apparatus by centrosomes and so forth.
Membrane-bound enzymes catalyze reactions that would have occurred with difficulty in an aqueous environment. The structure and function of some important organelles are as follows:
1). Endoplasmic reticulum (ER).
Within the cytoplasm of most animal cells is an extensive network (reticulum) of membrane-limited channels, collectively called endoplasmic reticulum (or ER). Some portion of ER membranes remains continuous with the plasma membrane and the nuclear envelope.
The outer surface of rough ER has attached ribosomes, whereas smooth ER do not have attached ribosomes. Functions of smooth ER include lipid metabolism (both catabolism and anabolism; they synthesize a variety of phospholipids, cholesterol and steroids); glycogenolysis (degradation of glycogen; glycogen being polymerized in the cytosol) and drug detoxification (by the help of the cytochrome P-450).
On their membranes, rough ER (RER) contain certain ribosome specific, transmembrane glycoproteins, called ribophorins I and II, to which are attached the ribosomes while engaged in polypeptide synthesis. As a growing secretory polypeptide emerges from ribosome, it passes through the RER membrane and gets accumulated in the lumen of RER.
Here, these polypeptide chains undergo tailoring, maturation, and molecular folding to form functional secondary or tertiary protein molecules. RER pinches off certain tiny protein filled vesicles which ultimately get fused to cis Golgi.
RER also synthesize membrane proteins and glycoproteins which are cotranslationally inserted into the rough ER membranes. Thus, endoplasmic reticulum is the site of biogenesis of cellular membranes.
2). Golgi apparatus.
It is a cup-shaped organelle which is located near the nucleus in many types of cells. Golgi apparatus consists of a set of smooth cisternae (i.e., closed fluid-filled flattened membranous sacs or vesicles) which often are stacked together in parallel rows.
It is surrounded by spherical membrane bound vesicles which appear to transport proteins to and from it. Golgi apparatus consists of at least three distinct classes of cisternae: cis Golgi, median Golgi and trans Golgi, each of which has distinct enzymatic activities.
Synthesized proteins appear to move in the following direction: rough ER→ cis Golgi→ median Golgi → trans Golgi→ secretory vesicles/cortical granules of egg/ lysosomes or peroxisomes.
Thus, the size and number of Golgi apparatus in a cell indicate the active metabolic, mainly synthetic, state of that cell. Plant cells contain many freely distributed sub-units of Golgi apparatus, called dictyosomes, secreting cellulose and pectin for cell wall formation during the cell division.
Generally, Golgi apparatus performs the following important functions:
The packaging of secretory materials (e.g., enzymes, mucin, lactoprotein of milk, melanin pigment, etc.) that are to be discharged from the cell.
- The processing of proteins, i.e., glycosylation, phosphorylation, sulphation, and selective proteolysis.
- The synthesis of certain polysaccharides and glycolipids.
- The sorting of proteins destined for various locations (e.g., lysosomes, peroxisomes, etc.) in the cell.
- The proliferation of membranous element for the plasma membrane.
- Formation of the acrosome of the spermatozoa.
The cytoplasmof animal cells contains manytiny, spheroid or irregular-shaped,membrane-bounded vesiclesknown as lysosomes. The lysosomesare originated from Golgiapparatus and contain numerous(about 50) hydrolytic enzymes(e.g., acid phosphatase that iscytochemically identified) for intracellularand extracellular digestion.
They digest the material takenin by endocytosis (such as phagocytosis,endocytosis and pinocytosis),parts of the cell (by autophagy)and extracellular substances.Lysosomes have a high acidic medium(pH 5) and this acidificationdepends on ATP- dependent protonpumps which are present inthe membrane of lysosomes andwhich accumulate protons (H+) insidethe lysosomes.
Lysosomes exhibitgreat polymorphism, i.e.,there are following four types oflysosomes:
- primary lysosomes(storage granules),
- secondary lysosomes(digestive vacuoles),
- residualbodies and
- autophagic vacuoles.
The lysosomes of plant cellsare membrane-bounded storage granules containing hydrolytic digestive enzymes, e.g., large vacuolesof parenchymatous cells of corn seedlings, protein or aleurone bodies and starch granules ofcereal and other seeds.
4). Cytoplasmic vacuoles.
The cytoplasm of many plants and some animal cells (i.e., ciliate protozoans) contains numerous small or large-sized, hollow, liquid-filled structures, the vacuoles. These vacuoles are supposed to be greatly expanded endoplasmic reticulum or Golgi apparatus.
The vacuoles of animal cells are bounded by a lipoproteinous membrane and their function is the storage, transmission of the materials, and the maintenance of internal pressure of the cell. The vacuoles of the plant cells are bounded by a single, semipermeable membrane known as tonoplast.
These vacuoles contain water, phenol, flavonols, anthocyanins (blue and red pigment), alkaloids and storage products, such as sugars and proteins.
These are tiny circular membrane-bound organelles containing a crystal-core of enzymes (such as urate oxidase, peroxidase, D-amino oxidase, and catalase, e.g., liver cells and kidney cells). These enzymes are required by peroxisomes in detoxification activity, i.e., in the metabolism or production and decomposition, of hydrogen peroxide or H2O2 molecules which are produced during neutralization of certain superoxide—the end products of mitochondrial or cytosolic reactions.
Peroxisomes are also related with β-oxidation of fatty acids and thermogenesis like the mitochondria and also in degradation of the amino acids. In green leaves of plants, peroxisomes carry out the process of photorespiration.
These organelles develop in a germinating plant seed (e.g., castor bean or Ricinus) to utilize stored fat of the seed (i.e., to metabolize the triglycerides). Glyoxysomes consist of an amorphous protein matrix surrounded by a limiting membrane.
The membrane of glyoxysomes originates from the ER and their enzymes are synthesized in the free ribosomes in the cytosol. Enzymes of glyoxysomes are used to transform the fat stores of the seed into carbohydrates by way of the glyoxylate cycle.
Mitochondria are oxygen-consuming ribbon-shaped cellular organelles ofimmense importance. Each mitochondrion is bounded by two unit membranes.
The outer mitochondrialmembrane resembles more with the plasma membrane in structure and chemical composition.It contains porins, proteins that render the membrane permeable to molecules having molecularweight as high as 10,000.
Inner mitochondrial membrane is rich in many enzymes, coenzymes andother components of electron transport chain. It also contains proton pumps and many permease proteins for the transport of various molecules such as citrates, ADP, phosphate and ATP. Innermitochondrial membrane gives out finger-like outgrowths (cristae) towards the lumen of mitochondrionand contains tennis-racket shaped F1 particles which contain ATP-ase enzyme for ATPsynthesis.
Mitochondrial matrix which is the liquid (colloidal) area encircled by the inner membrane, contains the soluble enzymes of Krebs cycle which completely oxidize the acetyl-CoA (an end product of cytosolic glycolysis and mitochondrial oxidative decarboxylation) to produce CO2, H2O and hydrogen ions.
Hydrogen ions reduce the molecules of NAD and FAD, both of which pass on hydrogen ions to respiratory or electron transport chain where oxidative phosphorylation takes place to generate energy- rich ATP molecules.
Since mitochondria act as the ‘power-houses’ of cells, they are abundantly found on those sites where energy is earnestly required such as sperm tail, muscle cell, liver cell (up to 1600 mitochondria), microvilli, oocyte (more than 300,000 mitochondria), etc.
Mitochondria also contain in their matrix single or double circular and double stranded DNA molecules, called mt DNA and also the 55S ribosomes, called mitoribosomes. Since mitochondria can synthesize 10 per cent of their proteins in their own protein-synthetic machinery, they are considered as semi-autonomous organelles.
Mitochondria may also produce heat (brown fat), and accumulate iron-containing pigments (Heme ferritin), ions of Ca2+ and HPO4 2– (or phosphate; e.g., osteoblasts of bones or yolk proteins).
Plastids occur only in the plant cells. They contain pigments and may synthesize and accumulate various substances. Plastids are of the following types:
- Leucoplasts are colorless plastids of embryonic and germ cells lacking thylakoids and ribosomes.
- Amyloplasts produce starch.
- Proteinoplasts accumulate protein.
- Oleosomes or elaioplasts store fats and essential oils.
- Chromoplasts contain pigment molecules and are colored organelles.
Chromoplasts impart a variety of colors to plant cells, such as red color in tomatoes, red chilies and carrots, various colors to petals of flowers and green color to many plant cells. The green colored chromoplasts are called chloroplasts.
They have chlorophyll pigment and are involved in the photosynthesis of food and so act like the kitchens of the cell. Chloroplasts have diverse shapes in green algae but are round, oval or discoid in shape in higher plants.
Like mitochondria, each chloroplast is bounded by two membranous envelopes, both of which have no chlorophyll pigment. However, unlike mitochondria there occurs third system of membranes within the boundary of inner membrane, called grana.
The grana form the main functional units of chloroplast and are bathed in the homogeneous matrix, called the stroma. Stroma contains a variety of photosynthetic enzymes and starch grains. Grana are stacks of membrane-bounded, flattened discoid sacs, arranged like neat piles of coins.
A chloroplast contains many such interconnected grana on which are located various photosynthetic enzymes and the molecules of green pigment chlorophyll and other photosynthetic pigments to trap the light energy. They contain DNA, ribosomes and complete protein synthetic machinery.
Ribosomes are tiny spheroidal dense particles (of 150 to 200 A0 diameter) that contain approximately equal amounts of RNA and proteins. They are primarily found in all cells and serve as a scaffold for the ordered interaction of the numerous molecules involved in protein synthesis.
Ribosome granules may exist either in the free state in the cytosol (e.g., basal epidermal cells) or attached to RER (e.g., pancreatic acinar cells, plasma cells or antibodies-secreting lymphocytes, osteoblasts, etc.).
Ribosomes have a sedimentation coefficient of about 80S and are composed of two subunits namely 40S and 60S. The smaller 40S ribosomal subunit is prolate ellipsoid in shape and consists of one molecule of 18S ribosomal RNA (or rRNA) and 30 proteins (named as S1, S2, S3, and so on).
The larger 60S ribosomal subunit is round in shape and contains a channel through which growing polypeptide chain makes its exit. It consists of three types of rRNA molecules, i.e., 28S rRNA, 5.8 rRNA and 5S rRNA, and 40 proteins (named as L1, L2, L3 and so on).
10). Microtubules and microtubular organelles.
With rare exceptions, such as human erythrocyte, microtubules are found in the cytoplasm of all types of eukaryotic cells. They are long fibers (of indefinite length) about 24 nm in diameter. In cross section each microtubule appears to have a dense wall of 6 nm thickness and a light or hollow centre. In cross section, the wall of a microtubule is made up of 13 globular subunits, called protofilaments, about 4 to 5 nm in diameter.
Chemically, microtubules are composed of two kinds of protein subunits: α-tubulin (tubulin A) and β-tubulin (tubulin B), each of M.W. 55,000 daltons. The wall of a microtubule is made up of a helical array of repeating α and β tubulin subunits.
Assembly studies have indicated that the structural unit is an αβ dimer of 8 nm length. Thus, in each microtubule, there are 13 protofilaments, each composed of αβ dimers that run parallel to the long axis of the tubule.
The repeating unit is an αβ heterodimer which is arranged ‘head to tail’ within the microtubule, that is αβ→ αβ→αβ. Thus, all microtubules have a defined polarity: their two ends are not structurally equivalent.
Microtubules undergo reversible assembly- disassembly (i.e., polymerization– depolymerization), depending on the need of the cell or organelles. Their polymerization is regulated by certain MAPs or microtubule- associated proteins (e.g., Tau protein).
The assembly of microtubules involves preferential addition of subunits (αβ dimers) to one end of tubule, called A end (or net assembly end); the other end of the tubule is called D end (or net disassembly end).
Such an assembly involves the hydrolysis of GTP to GDP. Thus, assembly of tubulin in the formation of microtubules is a specifically oriented and programmed process. Centrioles, basal bodies and centromeres of chromosomes are the sites of orientation for this assembly.
Calcium and calmodulin (an acidic protein having four Ca2+ binding sites) are some other regulating factors in the in vivo polymerization of tubulin. Certain drugs such as colchicine and vinblastin, are found to block the polymerization of tubulin.
The following cell organelles are derived from special assemblies of microtubules:
10.1). Cilia and flagella.
Ciliary and flagellar cell motility is adapted to liquid media and is executed by a minute, specially differentiated appendices, called cilia and flagella. Both of these organelles have very similar structures; they differ mainly in size and number (i.e., flagella are longer and fewer in number, while cilia are short and numerous).
Cilia are used for locomotion in isolated cells, such as certain protozoans (e.g., Paramecium). or to move particles in the medium, as in air passages and oviduct. Flagella are generally used for locomotion of cells, such as the spermatozoon and Euglena (protozoan).
All cilia and flagella are built on a common fundamental plan: a bundle of microtubules called the axoneme (1 to 2 nm in length and 0.2 μm in diameter) is surrounded by a membrane that is part of the plasma membrane.
The axoneme is connected with the basal body which is an intracellular granule lying in the cell cortex and which originates from the centrioles. Each axoneme is filled with ciliary matrix, in which are embedded two central singlet microtubules, each with the 13 protofilaments and nine outer pairs of microtubules, called doublets.
This recurring motif is known as the 9 + 2 array. Each doublet contains one complete microtubule, called the A sub fiber, containing all the 13 protofilaments. Attached to each A sub fiber is a B sub fiber with 10 protofilaments.
Sub fiber A has two dynein arms which are oriented in a clockwise direction. Doublets are linked together by nexin links. Each sub fiber A is also connected to the central microtubules by radial spokes terminating in fork-like structures, called spoke knobs or heads.
Propulsion by both cilia and flagella is caused by bending at their base. Cilia move by a whiplike
power stroke fueled by hydrolysis of ATP, followed by a recovery stroke.
Flagellar movement is also powered by ATP hydrolysis. In contrast to cilia, they generally move by waves that emanate from the base and spread outward toward the tip.
10.2) Basal bodies and centrioles.
Basal bodies and centrioles are similar in structure and function; both act as nucleating centres from which microtubules grow. Centrioles are cylinders that measure 0.2 μm × 0.5 μm. This cylinder is open on both ends, unless it carries a cilium or flagellum (then it is called basal body or kinetosome).
The wall of a centriole has nine groups of microtubules arranged in a circle. Each group, called blade is a triplet formed of three tubules — A, B, and C that are skewed toward the centre. Tubule A has 13 protofilaments, while tubules B and C have only 10 protofilaments each.
There are no central microtubules in the centrioles and no dynein arms like the cilia; however, triplets are linked by connectives. The procentriole (or daughter centriole) is formed at right angles to the centriole and is located near the proximal end of the centriole.
Both centrioles are found in a specially differentiated region the centrosome, cell centre or centrosphere. The centrosome is juxtanuclear (L., juxta = near) and firmly attached to the nuclear envelope.
At the time of cell division two pairs of centrioles are formed and form the spindle of microtubules which help in the separation and movement of chromosomes during concluding stages of cell divisions.
The nucleus is a centrally located and spherical cellular component which controls all the vital activities of the cytoplasm and carries the hereditary material the DNA in it. The nucleus consists of the following three structures:
Nucleus being the heart of every type of eukaryotic cell, contains the genes, the hereditary units. Genes are located on the chromosomes which exist as chromatin network in the non- dividing cell, i.e., during interphase.
The chromatin has two forms:
- Euchromatin is the well-dispersed form of chromatin that takes lighter DNA-stain and is genetically active, i.e., it is involved in gene duplication, gene transcription (DNA- dependent RNA synthesis), and phenogenesis or phenotypic expression of a gene through some type of protein synthesis.
- Heterochromatin is the highly condensed form of chromatin which takes dark DNA-stain and is genetically inert. Such type of chromatin exists both in the region of the centromere (called constitutive heterochromatin) and in the sex chromatin (called facultative heterochromatin) and is late replicating one.
Chemically, the chromatin contains a single DNA molecule, equal amount of five basic types of histone proteins, some RNA molecules and variable amount of different types of acidic proteins. In fact, the chromatin has its unit structures in the form of nucleosomes.
The chromatin binds strongly to the inner part of nuclear lamina, a 50 to 80 nm thick fibrous lamina lining the inner side of the nuclear envelope. Nuclear lamina is made up of three types of proteins, namely lamin A, B and C.
Lamin proteins are homologous in structure to IF proteins and serve the following functions:
- They anchor parts of interphase chromatin to the nuclear membrane. They tend to interfere with chromatin condensation during interphase of cell cycle.
- Lamins may play a crucial role in the assembly of interphase nuclei after each mitosis.
2. Nuclear envelope and nucleoplasm.
The nuclear envelope comprises two nuclear membranes— an inner nuclear membrane that is lined by nuclear lamina and an outer nuclear membrane that is continuous with rough ER. At certain points, the nuclear envelope is interrupted by structures called pores or nucleopores.
Nuclear pores contain octagonal pore complexes that regulate exchange between the nucleus and cytoplasm. The number of nucleopores is found to be correlated with the transcriptional activity of the cell.
For example, in the frog Xenopus laevis oocytes (which are very active in transcription) have 60 pores/ μm 2 (and up to 30 million pore complexes per nucleus), whereas frog’s mature erythrocytes (inactive in transcription) have only about 3 pores/μm2 (and a total of only 150 to 300 pores per nucleus)
The nuclear envelope binds the nucleoplasm which is rich in those molecules which are needed for DNA replication, transcription, regulation of gene actions and processing of various types of newly transcribed RNA molecules (i.e., tRNA, mRNA and other types of RNA).
Nucleus contains in its nucleoplasm a conspicuous, darkly stained, circular sub organelle, called nucleolus. Nucleolus lacks any limiting membrane and is formed during interphase by the ribosomal DNA (rDNA) of nucleolar organizer (NO).
Nucleolus is the site where ribosomes are manufactured. It is here where ribosomal DNA transcribes most of rRNA molecules and these molecules undergo processing before their step-wise addition to 70 types of ribosomal proteins to form the ribosomal sub-units.