Cellular nucleus

Nucleic acid Nucleon Nucleus, cellular Numbat Number theory Numeration systems Nut... 

The specific role of vitamin A in tissue differentiation has been an active area of research. The current thinking, developed in 1979, involves initial dehvery of retinol by holo-B >V (retinol-binding protein) to the cell cytosol (66). Retinol is then ultimately oxidized to retinoic acid and binds to a specific cellular retinoid-binding protein and is transported to the nucleus. Retinoic acid is then transferred to a nuclear retinoic acid receptor (RAR), which enhances the expression of a specific region of the genome. Transcription occurs and new proteins appear during the retinoic acid-induced differentiation of cells (56). 

Prokaryotic cells have only a single membrane, the plasma membrane or cell membrane. Because they have no other membranes, prokaryotic cells contain no nucleus or organelles. Nevertheless, they possess a distinct nuclear area where a single circular chromosome is localized, and some have an internal membranous structure called a mesosome that is derived from and continuous with the cell membrane. Reactions of cellular respiration are localized on these membranes. In photosynthetic prokaryotes such as the cyanobacteria,... 

Zell-kera, n. cell nucleus, -kemteilung, /. Biol.) nudear division, -kdrper, m. cellular body or substance, -masse, /. cellular substance. 

Proteasomal degradation also plays an essential role in the activation of cellular signaling pathways. A prototype for this is the control of NF-kB signaling, which has a pivotal role in inflammatory responses. Upon stimulation the inhibitory IicBa protein is phos-phorylated and thereby becomes a target substrate for K48-polyubiquitination. Proteasomal degradation of IkBu releases the transcription factor NF-kB, which subsequently translocates to the nucleus and activates specific target genes. 

Alternatively, one interesting drug delivery technique exploits the active transport of certain naturally-occurring and relatively small biomacromolecules across the cellular membrane. For instance, the nuclear transcription activator protein (Tat) from HIV type 1 (HlV-1) is a 101-amino acid protein that must interact with a 59-base RNA stem-loop structure, called the traus-activation region (Tar) at the 5 end of all nascent HlV-1 mRNA molecules, in order for the vims to replicate. HIV-Tat is actively transported across the cell membrane, and localizes to the nucleus [28]. It has been found that the arginine-rich Tar-binding region of the Tat protein, residues 49-57 (Tat+9 57), is primarily responsible for this translocation activity [29]. 

Much of the RNA synthesized from DNA templates in eukaryotic cells, including mammalian cells, is degraded within the nucleus, and it never serves as either a strucmral or an informational entity within the cellular cytoplasm. 

The size of plasmids used for transfection can vary considerably, but most plasmids are 4,000 to 10,000 base pairs in size. Despite their differences all plasmids face the same barriers when transfected. Transfected DNA has to cross the cell membrane or the endosomal membrane, it has to be transported into the nucleus, and it has to be protected against cellular nucleases and degradation... 

All enveloped human vimses acquire their phospholipid coating by budding through cellular membranes. The maturation and release of enveloped influenza particles is illustrated in Fig. 3.8. The capsid protein subunits are transported flom the ribosomes to the nucleus, where they combine with new viral RNA molecules and are assembled into the helical capsids. The haemagglutinin and neuraminidase proteins that project fiom the envelope of the normal particles migrate to the cytoplasmic membrane where they displace the normal cell membrane proteins. The assembled nucleocapsids finally pass out from the nucleus, and as they impinge on the altered cytoplasmic membrane they cause it to bulge and bud off completed enveloped particles flxm the cell. Vims particles are released in this way over a period of hours before the cell eventually dies. 

Fig. 2.2 Simplified scheme of oxidant/antioxidant regulation ofNF-KB activation. Different stimuli, leading to an increase of ROS generation inside the ceU, activate the phosphorylation of IkB inhibitory protein and the subsequent proteolysis. Thioredoxin (Trx) may reduce activated NF-kB proteins facilitating nuclear translocation.Qnce released from IkB, the NF-kB complex translocates into the nucleus and the binding to DNA domain in the promoters and enhancers of genes such as TNF-a, IL-1, proliferation and chemotactic factors, adhesion molecule. Some of these genes, in turn, may further induce NF-kB activation, leading to a vicious circle if the regulatory cellular system escapes from...

Because the carotenoids favour hydrophobic domains they are generally localised in the membranes and lipoproteins of animal cells. In this location they can influence the oxidation of membrane lipids and prevent the passage of free radicals from one cellular compartment to another. Thus, DNA in the nucleus is protected from intracellularly generated ROS by (at least) the nuclear membrane and from extracellular ROS by a number of membranes. Should ROS reach the nucleus, base oxidation can occur. The base most susceptible to oxidation is guanine, although all other bases can also be affected. The cell has the ability to detect damaged bases, excise them. 

---