Cytoplasmic components, diffusion

The study of diploids of the types i z IFi z or i a IFi a demonstrated that the inducible allele is dominant to the constitutive allele and that it is active in the trans-position with respect to both a and z. This shows that i gene mutations belong to an independent cistron, governing the expression of z, y, and a through production of a diffusible cytoplasmic component. The dominance of the inducible to the constitutive allele suggests that the former corresponds to the active form of the i gene. 

Although a nuclear membrane undoubtedly exists during interphase, the relative permeability of that membrane is not known. Is the nuclear membrane a perforated envelope that prevents the extrusion of chromosomes from the nuclei into the cytoplasm, but otherwise allows at least some cytoplasmic components to flow freely from cytoplasm to nucleus and vice versa The porous structure of the membrane seems to favor such a concept however, the pores are not necessarily true openings and, therefore, the problem of relative permeability remains crucial. Permeability has been investigated in various ways. Small molecules or ions pass freely from cytoplasm to nucleus. In fact, the exchange of sodium is so rapid that up to the present time it has been impossible to follow its transfer. Little is known of the transfer of other molecules such as glucose, amino acid, etc. Whether they are transferred by passive diffusion or by active transport is not known. Deoxyribonucleosides have been reported to accumulate within the nucleus in the course of liver regeneration. 

Fig. 4.1 Mechanism of action of cyclosporine. Cyclosporine readily diffuses into the cytoplasm of the target cells where it binds to cyclophilins. The cyclosporine-cyclophilin complex stably associates with calcineurin and inhibits calcineurin activity. Calcineurin is a Ca2+-dependent enzyme— serine/threonine phosphatase— which after activation by Ca2+, dephosphorylates a cytosolic component of NFAT (NFATc, cytosolic factor of activated T cells). After dephosphorylation, NFATc migrates from the cytoplasm to the nucleus where it associates with NFATn and induces transcription of several cytokine genes including IL-2. Cyclosporine inhibits calcineurin activity after associating with cyclophilins, resulting in the inhibition of IL-2 production and other cytokines (see Color Insert)...

Fig. 7.2. The structure of the translocation pathway in mycelial cords. (A) Hyphae fanning out at the distal end of a cord of Phanerochaete velutina (scanning electron microscopy by A. Yarwood) (B) Internal structure of a cord of Serpula lacrymans, showing vessels and cytoplasm-filled hyphae and extracellular matrix material. (C) Diagram of the components of the translocation pathway (adapted from Cairney, 1992) V, vessel hypha f, foraging front a, anastomosis (D) A cord system in beech woodland showing both corded mycelium and diffuse growth in contact with the wood substrate.

Hgb consists of a protein component with two a and two p chains each chain is linked to a heme group consisting of a porphyrin ring structure with an iron atom chelated at its center, which is capable of binding oxygen. The initial step in the synthesis of heme from the substrate succinyl CoA and glycine requires the presence of pyridox-ine phosphate (vitamin Be) as a catalyst. Following its synthesis in the cytoplasmic mitochondria of the RBC, heme diffuses into the extra-mitochondrial space, combines with the completed a and p chains, and forms Hgb. 

The tumors are either partially encapsulated or nonencapsulated. On low-power magnification, EMC tends to grow in a diffuse or multinodular pattern. The tumors are composed of small ducts composed of cuboi-dal cells with pink cytoplasm and centrally or basally located round nuclei. This epithelial cell component... 

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