The nucleolus (also called nucleole) is a non-membrane bound structure composed of proteins and nucleic acids found within the nucleus. Ribosomal RNA is transcribed and assembled within the nucleolus. The nucleolus ultrastructure can be visualized through an electron microscope, while the organization and dynamics can be studied through fluorescent protein tagging and fluorescent recovery after photobleaching (FRAP). Malfunction of nucleoli can be the cause for several human diseases.


Structure

Three major components of the nucleolus are recognized: the fibrillar centers (FC), the dense fibrillar component (DFC) and granular components (GC). However, it has been proposed that this particular organization is only observed in higher eukaryotes and that it evolved from a bipartite organization with the transition from anamniotes to amniotes. Reflecting the substantial increase in the rDNA intergenic region, an original fibrillar component would have separated into the FC and the DFC. Another structure identified within many nucleoli, (particularly in plants) is a clear area in the center of the structure referred to as a nucleolar vacuole.

Function and ribosome assembly

Nucleoli are formed around specific genetic loci called Nucleolar Organizing Regions (NORs), first described by Barbara McClintock. Because of this non-random organization, the nucleolus is defined as a "genetically determined element." A NOR is composed of tandem repeats of rRNA genes, which can be found in several different chromosomes. The human genome for example, contains more than 200 clustered copies of the rRNA genes on five different chromosomes(13,14,15,21,22). In a typical eukaryote, a rRNA gene consists of a promoter, internal and external transcribed spacers (ITS/ETS), rRNA coding sequences (18S, 5.8S, 28S) and an external non-transcribed spacer.

In ribosome biogenesis, three eukaryotic RNA polymerases (pol I, II, III) are required which function in a coordinated manner. In an initial stage, the RNA genes are transcribed as a single unit within the nucleolus by RNA pol I. In order for this transcription to occur, several pol I-associated factors and rDNA-specific transacting factors are required. In yeast, the most important are: UAF (upstream activating factor), TBP (tata-box binding protein) and CF (core factor), which bind promoter elements and form the pre-initiation complex (PIC), which is in turn recognized by RNA pol I. In humans, a similar PIC is assembled with SLI, the promoter selectivity factor (composed of TBP and TBP-associated factors, or TAFs), IF (the transcription initiation factor) and UBF (upstream binding factor).

Transcription of the ribosomal gene yields a long precursor molecule (45S pre-rRNA) which still contains the ITS and ETS. Further processing, which involves methylation and endo/exonuclease activity is therefore needed to generate the 18S RNA, 5.8S and 28S RNA molecules. In eukaryotes, the RNA modifying enzymes are brought to their respective recognition sites through interaction with guide RNA's which bind these specific sequences. These guide RNA's belong to the class of small nucleolar RNA's (snoRNA's) which are complexed with proteins and exist as small-nucr-ribonucleoprotein (RNP) particles (snoRNP's). Once RNA is processed, the RNA molecules are ready to be assembled into ribosomes. However, an additional RNA molecule, the 5S RNA, is necessary for this biogenesis. In yeast, the 5S rDNA sequence is localized in the external non-transcribed spacer and is transcribed in the nucleolus by RNA pol III. In higher eukaryotes and plants, the situation is more complex, for the 5S DNA sequence lies outside the NOR and is transcribed in the nucleoplasm after which it finds its way into the nucleolus to participate in the ribosome assembly. This assembly not only involves the rRNA, but ribosomal proteins as well. The genes encoding these r-proteins are transcribed by pol II in the nucleoplasm by a "conventional" pathway of protein synthesis (transcription, pre-mRNA processing, nuclear export of mature mRNA and translation on cytoplasmic ribosomes). The mature r-proteins are then "imported" into the nucleolus. Association and maturation of rRNA's and r-proteins result in the formation of the 40S and 60S subunits of the ribosome. These are exported through the nuclear pore complexes to the cytoplasm where they remain free or will become associated with the endoplasmic reticulum.

A continuous chain between the nucleoplasm and the inner parts of the nucleolus exists through a network of nucleolar channels. In this way, macromolecules with a molecular weight up to 2000 kDa are easily distributed throughout the nucleolus

In cell biology, a mitochondrion (plural mitochondria) is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 10 micrometers (μm) in diameter. Mitochondria are sometimes described as "cellular power plants" because they generate most of the cell's supply of adenosine triphosphate (ATP), used as a source of the chemical energy. In addition to supplying cellular energy, mitochondria are involved in a range of other processes, such as signaling, cellular differentiation, cell death, as well as the control of the cell cycle and cell growth. Mitochondria have been implicated in several human diseases, including mitochondrial disorders and cardiac dysfunction, and may play a role in the aging process. The word mitochondrion comes from the Greek μίτος or mitos, thread or chondrion, granule.

Several characteristics make mitochondria unique. The number of mitochondria in a cell varies widely by organism and tissue type. Many cells have only a single mitochondrion, whereas others can contain several thousand mitochondria. The organelle is composed of compartments that carry out specialized functions. These compartments or regions include the outer membrane, the intermembrane space, the inner membrane, and the cristae and matrix. Mitochondrial proteins vary depending on the tissue and the species. In humans, 615 distinct types of proteins have been identified from cardiac mitochondria; whereas in Murinae (rats), 940 proteins encoded by distinct genes have been reported. The mitochondrial proteome is thought to be dynamically regulated. Although most of a cell's DNA is contained in the cell nucleus, the mitochondrion has its own independent genome. Further, its DNA shows substantial similarity to bacterial genome.

PLASMA MEMBRANE

All living cells, prokaryotic and eukaryotic, have a plasma membrane that encloses their contents and serves as a semi-porous barrier to the outside environment. The membrane acts as a boundary, holding the cell constituents together and keeping other substances from entering. The plasma membrane is permeable to specific molecules, however, and allows nutrients and other essential elements to enter the cell and waste materials to leave the cell. Small molecules, such as oxygen, carbon dioxide, and water, are able to pass freely across the membrane, but the passage of larger molecules, such as amino acids and sugars, is carefully regulated.

According to the accepted current theory, known as the fluid mosaic model, the plasma membrane is composed of a double layer (bilayer) of lipids, oily substances found in all cells (see Figure 1). Most of the lipids in the bilayer can be more precisely described as phospholipids, that is, lipids that feature a phosphate group at one end of each molecule. Phospholipids are characteristically hydrophilic ("water-loving") at their phosphate ends and hydrophobic ("water-fearing") along their lipid tail regions. In each layer of a plasma membrane, the hydrophobic lipid tails are oriented inwards and the hydrophilic phosphate groups are aligned so they face outwards, either toward the aqueous cytosol of the cell or the outside environment. Phospholipids tend to spontaneously aggregate by this mechanism whenever they are exposed to water.

Within the phospholipid bilayer of the plasma membrane, many diverse proteins are embedded, while other proteins simply adhere to the surfaces of the bilayer. Some of these proteins, primarily those that are at least partially exposed on the external side of the membrane, have carbohydrates attached to their outer surfaces and are, therefore, referred to as glycoproteins. The positioning of proteins along the plasma membrane is related in part to the organization of the filaments that comprise the cytoskeleton, which help anchor them in place. The arrangement of proteins also involves the hydrophobic and hydrophilic regions found on the surfaces of the proteins: hydrophobic regions associate with the hydrophobic interior of the plasma membrane and hydrophilic regions extend past the surface of the membrane into either the inside of the cell or the outer environment.

Plasma membrane proteins function in several different ways. Many of the proteins play a role in the selective transport of certain substances across the phospholipid bilayer, either acting as channels or active transport molecules. Others function as receptors, which bind information-providing molecules, such as hormones, and transmit corresponding signals based on the obtained information to the interior of the cell. Membrane proteins may also exhibit enzymatic activity, catalyzing various reactions related to the plasma membrane.

Since the 1970s, the plasma membrane has been frequently described as a fluid mosaic, which is reflective of the discovery that oftentimes the lipid molecules in the bilayer can move about in the plane of the membrane. However, depending upon a number of factors, including the exact composition of the bilayer and temperature, plasma membranes can undergo phase transitions which render their molecules less dynamic and produce a more gel-like or nearly solid state. Cells are able to regulate the fluidity of their plasma membranes to meet their particular needs by synthesizing more of certain types of molecules, such as those with specific kinds of bonds that keep them fluid at lower temperatures. The presence of cholesterol and glycolipids, which are found in most cell membranes, can also affect molecular dynamics and inhibit phase transitions.

Eukaryotic animal cells are generally thought to have descended from prokaryotes that lost their cell walls. With only the flexible plasma membrane left to enclose them, these primordial creatures would have been able to expand in size and complexity. Eukaryotic cells are generally ten times larger than prokaryotic cells and have membranes enclosing interior components, the organelles. Like the exterior plasma membrane, these membranes also regulate the flow of materials, allowing the cell to segregate its chemical functions into discrete internal compartments.

ABOUT ME

Hai my फ्रिएंड्स
I am PAVAN। I am born & brought up in a small villege naming Semali Kalyan which is located in jhalawar distt॥My schooling spend in govt ups semli kalyan ,After that I went to JNV Pachpahar for my plus two education but i leave it in 9th। then i score 82% in secondary exam from govt। sss raipur.After I went to kota for PMT , but could not get pass out.After that I just came to jhalawar for my Gratuation. Now I am working as an MANAGER of S.B.I.But the strugle cant get complete.
I am unmarried till and there is no taget to being merried early.i think She shall loving&caring and does all the need full things for me . I believe, I get her through My God only,
Ohh॥!!I am sorry.!! Dear ones,I would not like to tell you anything more...So this is all For now ॥!! Thks for reading out my testimony ..