Ribonucleic acids continued

The nucleus of the cell (Figure 1.2) is composed of a porous nuclear membrane, the nucleolus, and soluble materials. The nucleolus contains ribonucleic acids (RNA) and genetic materials also termed chromatin that code for the proteins synthesized upon the ribosomes in the cell cytoplasm. The nuclear membrane is continuous with the outer membrane of the endoplasmic reticulum. Messenger RNA synthesized in the nucleus is transported across the nuclear membrane and is involved in protein synthesis. It fits into the groove between the large and small rRNA subunits (Figure 1.2)... 

As shown in Sec. I, uracils have represented, for more than 90 years, a class of compounds that continually attract organic chemists, biochemists, medicinal chemists, and photobiologists. Uracils were first detected as constituents of ribonucleic acids, from which they were prepared by hydrolysis. Nucleosides derived from uracil are called uridine, pseudouridine, and uridine phosphate, respectively. Recently, uracil moieties were detected in the antibiotic Tunicamycin (85JA7761). 

Deoxyribonucleic acid (DNA) contains all the information required to build the cells and tissues of an organism. The exact replication of this information in any species assures its genetic continuity from generation to generation and is critical to the normal development of an individual. The information stored in DNA is arranged in hereditary units, now known as genes, that control identifiable traits of an organism. In the process of transcription, the information stored in DNA is copied into ribonucleic acid (RNA), which has three distinct roles in protein synthesis. 

Floxuridine is a pyrimidine antimetabolite with its primary effect to interfere with the synthesis of deoxyribonucleic acid (DNA) and to a lesser extent inhibit the formation of ribonucleic acid (RNA). It is indicated in palliative management of GI adenocarcinoma metastatic to the liver administered by continuous regional intra-arterial infusion as long as cancer does not extend beyond the area perfused by a single artery. Floxuridine, an antimetabolite and anti-neoplastic agent (0.1 to 0.6 mg/kg daily by intra-arterial infusion), is used to treat brain, breast, head, neck, liver, gallbladder, and bile duct cancer (see also Figure 15). 

Similarly, adenosine is a true inducer of purine nucleosidase. But at a level of 0.5% its repressing effect is great enough to prevent the appearance of enzyme. Use of ribonucleic acid (RNA) or of the phosphate ester of adenosine (AMP) by the fungus leads to a low but continuous supply of adenosine and a resultant high enzyme concentration (Reese, 1968). 

Among all particulate fractions of the cells, the nucleus is most likely to suffer leakage during tissue fractionation. As pointed out before, in contrast to mitochondria, the nucleus is not surrounded by a continuous membrane, but rather by a perforated envelope. As with any other particulate preparation, it is difficult to evaluate the amount of material that has been adsorbed onto the nucleus. Deoxyribonucleic acid and ribonucleic acid are both apt to adsorb proteins therefore, it would not be surprising if proteins that are sometimes considered genuine constituents of the nucleus remain associated with the nuclear preparation only because they are artifacts of the preparative procedure. 

Polynucleotide phosphorylase is the only known enzyme that forms high polymers of nucleotides, and therefore it is believed to have an important function in the synthesis of ribonucleic acids. Several points remain to be determined about RNA synthesis, however. The relatively poor reaction with the guanine nucleotide is hard to reconcile with the large amounts of guanine found in nucleic acids. Another question relates to the mechanism of control of the synthesis to form specific nucleic acids. While it is possible to devise hypothetical control mechanisms, such as the participation of protein as a template, continued investigation is required to answer the question. 

Kennell, D. Inhibition of host protein synthesis during infection of Escherichia coli by bacteriophage T4. I. Continued synthesis of host ribonucleic acid. J. Virol. 2, 1262-1271 (1968). 

---