Nucleus nuclear assembly

Molecular communication between the nucleus and the cytosol requires the movement of macromolecules through nuclear pores. RNA molecules synthesized in the nucleus are exported to the cytosol. Ribosomal proteins synthesized on cytosolic ribosomes are imported into the nucleus and assembled into 60S and 40S ribosomal subunits in the nucleolus completed subunits are then exported back to the cytosol. A variety of nuclear proteins (RNA and DNA polymerases, histones, topo-isomerases, proteins that regulate gene expression, and so forth) are synthesized in the cytosol and imported into the nucleus. This traffic is modulated by a complex system of molecular signals and transport proteins that is gradually being elucidated. 

Separation energy This is defined as the energy to remove one nucleon from a nucleus. Consider an example of a neutron removed from a nucleus, taking that nucleus to be 29Si just for purposes of illustration. The 29Si nucleus is assembled from 14 protons and 15 neutrons. Removing one neutron can be represented by the nuclear-identity equation... 

Nucleus The nucleus is separated from the cytosol by a double membrane, the nuclear envelope. The DNA is complexed with basic proteins (histones) to form chromatin fibers, the material from which chromosomes are made. A distinct RNA-rich region, the nucleolus, is the site of ribosome assembly. The nucleus is the repository of genetic information encoded in DNA and organized into chromosomes. During mitosis, the chromosomes are replicated and transmitted to the daughter cells. The genetic information of DNA is transcribed into RNA in the nucleus and passes into the cytosol where it is translated into protein by ribosomes. 

The alpha particle is a helium nucleus produced from the radioactive decay of heavy metals and some nuclear reactions. Alpha decay often occurs among nuclei that have a favorable neutron/proton ratio, but contain too many nucleons for stability. The alpha particle is a massive particle consisting of an assembly of two protons and two neutrons and a resultant charge of +2. 

Euchromatin generally corresponds to looped 30-nm fibers. Heterochromatin is more highly condensed. Figure 1-1-14 shows an electron micrograph of an interphase nucleus containing euchromatin, heterochromatin, and a nucleolus. The nudeolus is a nuclear region spedalized for ribosome assembly (discussed in Chapter 3). 

Each cell nucleus contains one or more dense nucleoli, regions that are rich in RNA and may contain 10-20% of the total RNA of cells. Nucleoli are sites of synthesis and of temporary storage of ribosomal RNA, which is needed for assembly of ribosomes. The nuclear envelope is a pair of membranes, usually a few tens of nanometers apart, that surround the nucleus. The two membranes of the pair separate off a thin perinuclear space (Fig. 1-7). The membranes contain "pores" -130 ran in diameter with a complex structure (see Fig. 27-8).38/39 There is a central channel -42 ran in diameter, which provides a route for controlled passage of RNA and other large molecules from the nucleus into the cytoplasm and also from the cytoplasm to the nucleus. Smaller -10 nm channels allow passive diffusion of ions and small molecules. 

Assembly and release. The assembly of the capsid and its association with nucleic acid is then followed by release of the virus front the cell. This may occur in different ways, depending upon the nature of the virus. Naked viruses may be released slowly and extruded without cell lysis, or released rapidly by disruption of the cell membrane. DNA viruses, which mature in the nucleus, tend to accumulate within infected cells over a long period. Enveloped viruses generally acquire their envelope and leave the cell by budding through the nuclear or cytoplasmic membrane at a point where virus-specified proteins have been inserted. The budding process is compatible with cell survival. 

The difference between the actual nuclear mass and the mass of all the individual nucleons, which must be assembled to make the nucleus, is called the total binding energy, Blol(A, Z). It represents the work necessary to dissociate the nucleus into separate nucleons or the energy that would be released if all the nucleons came together to form the nucleus. We write... 

Figure 2.4 Schematic model of how the nucleus AZ is assembled from an IV = Z nucleus from W. E. Meyerhof, Elements of Nuclear Physics. Copyright 1967 by McGraw-Hill Book Company, Inc. Reprinted by permission of McGraw-Hill Book Company, Inc.

Figure 12.2 Model for importin-mediated nuclear import and export. NLS-containing proteins in the cytoplasm bind to importin family members in the cytoplasm and are translocated through the NPC. Once inside the nucleus, Ran-GTP interacts with the importin-cargo complex to facilitate complex dissociation. Similarly, proteins to be exported from the nucleus interact with one of the exportins, but in contrast to the case with importins, assembly of this complex is promoted by Ran-GTP. As the complex, presumably with Ran-GTP bound, enters the cytoplasm, the GTP is hydrolyzed to GDP, which induces the release of the cargo from exportin.

The third strategy is used by viruses whose nucleocapsids are produced in the nucleus of the cell, with assembly and maturation involving the nuclear membrane. The enveloped viruses accumulate in the endoplasmic reticulum and are carried to the cell surface via vacuoles. Some viruses that use this process, such as herpesvirus, are cytolytic (cause cell lysis). 

---