Protein import, nucleus materials

The eytoplasm is a viscous fluid and contains within it systems of paramount importance. These are the nucleus, responsible for the genehc make-up of the cell, and the ribosomes, whieh are the site of protein synthesis, hi addihon are found granules of reserve material suehas polylydioxybutyric add, an energy reserve, and polyphosphate or volutin granules, the exact funchon of which has not yet been elucidated. The prokaiyohc nueleus or bacterial chromosome exists in the cytoplasm in the form of a loop and is not surrounded by a nuclear membrane. Bacteria cany other chromosomal elements episomes, which are portions of the main chromosome that have become isolated firm it, and plasmids, whieh may be called miniature chromosomes. These are small annular pieees of DNA whieh carry a limited amount of genetic information. 

In addition to their plasma membrane eukaryotic cells also contain internal membranes that define a variety of organelles (fig. 17.2). Each of these organelles is specialized for particular functions The nucleus synthesizes nucleic acids, mitochondria oxidize carbohydrates and lipids and make ATP, chloroplasts carry out photosynthesis, the endoplasmic reticulum and the Golgi apparatus synthesize and secrete proteins, and lysosomes digest proteins. Additional membranes divide mitochondria and chloroplasts into even finer, more specialized subcompartments. Like the plasma membrane, organellar membranes act as barriers to the leakage of proteins, metabolites, and ions they contain transport systems for import and export of materials, and they are the sites of enzymatic activities as diverse as cholesterol biosynthesis and oxidative phosphorylation. 

The cell body contains many structures of importance. The nucleus is usually located in the center of the cell body. It contains widely dispersed, fine chromatin material. The chromatin is composed of deoxyribonucleic acid (DNA) and its associated histone proteins. The nucleolus contains the specific portion of DNA encoding the ribonucleic acid (RNA) of future ribosomes. 

After years of work it has become possible to make the generalization that the nucleus of every cell invariably consists largely of nucleoproteins —compounds of nucleic acids and proteins. A few of the most important and interesting materials containing nucleoproteins are the chromosomes, the genes, and certain viruses, bacteriophages, and antigens. Yeast and... 

Synchrotron-based nuclear resonance methods have revealed the vibrational dynamics of the iron atom in numerous systems, including alloys, amorphous materials, nanomaterials, and materials under high pressure. The above-mentioned selectivity for the probe nucleus is particularly valuable for biological macromolecules, which may contain many thousands of atoms, but a localized active site is often the true center of interest. Since its availability, NRVS has been applied to study the vibrational dynamics of Fe in proteins, porphyrin model compounds, " and iron-sulfur clusters. It is shown that NRVS can provide frequencies, amplitudes, and directions for Fe vibrations in the samples. It helps to clarify mode assignments in vibrational spectra and reveals many important vibrational modes of Fe that cannot be seen by other methods. In particular, NRVS reveals low-frequency motions of the Fe down to below 100 cm that control biological reactions. The applications presented here use Fe as the probe nucleus, but the principle applies to other Mossbauer isotopes such as " Sn, Kr, Ni, and Zn if appropriate sources are available. 

The mechanical properties of the nucleus, the stiffest component of the cell, are important for the overall cellular response. It is, probably, even more significant that forces transmitted from the cell surface and acting on the nucleus can alter gene expression and protein synthesis. Kan et al. (1999a) have modeled the nucleus as a viscous fluid and analyzed the effect of the nucleus on the leukocyte recovery. Guilak et al. [2000] have estimated the linear viscoelastic properties of nuclei of chondrocytes. Caille et al. [2002] used a model of nonlinear elastic material to estimate Young s modulus of endothelial cell nuclei. Recently, Dahl et al. [2004], by using the micropipette technique, have estimated the mechanical properties of the cell s nuclear envelope. 

Accordingly, we shall concentrate our attention on the genetic material that is localized in the chromosomes of the nucleus. Chromosomes consist predominantly of proteins and nucleic acids. The proteins can be divided into histones, which are basic proteins, and nonhistones. In addition, there are enzyme proteins that have specific functions. The second, most important group of chromosome constituents, the nucleic acids, were discovered about 100 years ago by the Swiss scientist Miescher in Tubingen. Nucleic acids are the carriers of genetic information. For the moment, we shall not consider the evidence for this assertion, and instead concern ourselves with the chemical structure of the nucleic acids. 

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