Nucleus pores

The formation of a liquid phase from the vapour at any pressure below saturation cannot occur in the absence of a solid surface which serves to nucleate the process. Within a pore, the adsorbed film acts as a nucleus upon which condensation can take place when the relative pressure reaches the figure given by the Kelvin equation. In the converse process of evaporation, the problem of nucleation does not arise the liquid phase is already present and evaporation can occur spontaneously from the meniscus as soon as the pressure is low enough. It is because the processes of condensation and evaporation do not necessarily take place as exact reverses of each other that hysteresis can arise. 

The smaU nucleus of the yeast ceU is surrounded by a membrane or tonoplast, which has many pores with an average diameter of about 0.085 p.m. 

Expoitins are transport receptors at the nuclear pore complex needed for the selective export of proteins from the nucleus into the cytoplasm. They recognize nuclear export signal sequences of cargo proteins. 

From the conceptual diagram in Fig. 15, it is obvious that if the radius of the nucleus exceeds the critical radius r the nucleus will grow into a macroscopically ruptured small pore. The passive film is more or less defective and the size of the defect will fluctuate from moment to moment. It is therefore reasonable to assume a certain probability that pore nuclei larger than the critical radius are formed in the film. 

According to Eq. (23), the critical pore radius r greatly decreases with increasing electrode potential. It is seen that above a certain critical potential AE b the active barrier as well as the critical pore radius decreases steeply with anodic potential. This critical potential AE is the lowest potential of pore formation and below this potential the passive film is stable against electrocapillary breakdown because of an extremely high activation barrier and the large size of pore nucleus required. 

After synthesis, the mRNA exits the nucleus through a nuclear pore and proceeds to the ribosome for translation into protein. Competing with export and translation is the process of message degradation by cellular ribonucleases. The competition between degradation and translation provides another mechanism to regulate the levels of individual messages. 

The nucleus (37-44) is the most prominent structure within the cytoplasm. It is bounded by a nuclear envelope containing circular pores that are 30-100 nm in diameter. The outer nuclear envelope may be continuous with the er. 

The nucleus is surrounded by the nuclear envelope, which takes on a lumenal structure connected to the endoplasmic reticulum. The transport of proteins into (and out of) the nucleus occurs through the nuclear pore complex (NPC), a large complex composed of more than 100 different proteins (Talcott and Moore, 1999). Because NPC forms an aqueous pore across the two membranes, small proteins less than 9 nm in diameter can pass through it simply by diffusion. However, most of the transports of both proteins and RNAs are mediated by an active transport mechanism. It is now clear that there is heavy traffic through the NPC in both directions. Proteins are not only imported into the nucleus but also actively exported from it as well. There are many reasons for nuclear export. One reason is to send some shuttle proteins back after their import another is for some viral proteins to export their replicated genomes outside the nucleus. 

Nuclear Envelope The membrane system of the cell nucleus that surrounds the nucleoplasm. It consists of two concentric membranes separated by the perinuclear space. The structures of the envelope where it opens to the cytoplasm are called the nuclear pores (nudear pore). [NIH]... 

Nuclear Pore An opening through the nudear envelope formed by the nudear pore complex which transports nuclear proteins or RNA into or out of the cell nucleus and which, under some conditions, acts as an ion channel. [NIH]... 

Ultrastructural examination of nuclei in situ showed they were not surrounded by a continuous double-layered membrane, but that the membrane was interrupted by pores (Callan and Tomlin, 1950). These were not holes but were highly organized structures involved in transport between the nucleus and the cytosol. 

The neuron contains severai speciaiized components, caiied organeiies, which are anaiogous to the organs of a body. The nucleus is approximateiy 3-18 pm in diameter and contains deoxyribonucleic acid (DNA). The nuciear membrane has pores to aiiow passage of substances in and out of the nucieus. The nucieus aiso has a body within caiied the nucieoius, which manufactures ribosomes and contains ribonucieic acid (RNA), a coating of DNA, and severai enzymes. 

Figure 4 Transfer of DNA from cytoplasm into the nucleoplasm. The DNA-containing complex can enter the nucleus by (1) crossing the membrane during mitotic nuclear membrane breakdown (2) diffusion through nuclear pore for small particles and (3) targeted uptake through the nuclear pore, facilitated by a nuclear localization sequence. Abbreviations NLS, nuclear localization sequence IMP, importin.

Rough endoplasmic reticulum Nuclear pore Nucleus... 

The exchange of substances between the nucleus and the cytoplasm is mediated by pore complexes with complicated structures, which traverse the nuclear membrane. The nuclear pores consist of numerous proteins that form several connected rings of varying diameter. Low-molecular structures and small... 

Regarding the localization of the majority of proteasomes to the nuclear rim and nuclear envelope, it seems likely, that proteasomal degradation requires nuclear export or translocation processes, which directs nuclear proteins at least to the nuclear pore or to the inner surface of the nucleus. This implies that all components of the ubiquitination machinery have to be active inside the nucleus, v ich seems to be a prerequisite for the specific export process. It is also possible that nucleus-specific kinases or E2/E3 enzymes promote the triggering of nuclear proteins for rapid degradation. 

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