Thymine 2 - -nucleotide

Deoxyrihonucleotides are generally formed by reduction of ribonucleoside diphosphates. This involves a series of redox reactions in which NADP+ and FAD play a role (see Section 15.1.1), with a subsequent electron transport chain. DNA contains thymine rather than uracil, so thymidine triphosphate (dTTP) is a requirement. Methylation of dUMP to dTMP is a major route to thymine nucleotides, and is dependent upon N, A °-methylenetetrahydrofolate as the source of the methyl group (see Box 11.13). 

Nucleoside monophosphates are converted to their triphosphates by enzymatic phosphorylation reactions. Ribonucleotides are converted to deoxyribonucleotides by ribonucleotide reductase, an enzyme with novel mechanistic and regulatory characteristics. The thymine nucleotides are derived from dCDP and dUMP. 

Calculation of As was carried out for hole transfer in finite DNA duplexes in water, in which one strand includes a G3(T) G3 sequence, n = 0-6, where G and T are, respectively, guanine and thymine nucleotide bases (in the complementary strand, of course, G and T are paired respectively, with cytosine and adenine bases) [23], The D and A sites (the solute) were taken as the middle unit of each G3 triad, or as alternative models for the n = 0 case, using one or both of the inner members of the G3 units). These structures lead to D/A sites in contact or separated by intervening bases ranging in number from 1 to 8. Using a base stacking separation of 3.4 A yields rDA = (m+ 1) (3.4 A), m = 0-8 (an estimate closely supported by detailed molecular force field calculations). 

Fig. 9.6 Photochemical formation of the cyclobutane-type dimer by irradiation of thymine nucleotides.

Mutagenesis can occur as the result of exposure to ionizing radiation and certain chemicals. Ionizing radiations include cosmic rays, x rays, and ultraviolet light. It is of interest to note that melanoma, caused almost exclusively by exposure to the ultraviolet radiation from the snn, is the most rapidly increasing lethal cancer in the United States. In melanoma, ultra violet radiation induces what is called thymine dimers. This means that two thymine nucleotides next to each other in a DNA sequence form an abnormal bond. Thymine dimers in a... 

Chloro-9-cyclopentyl-8-azapurine inhibited synthesis of DNA, RNA, and protein in E. coli. Blockage of thymine-nucleotide formation was the first effect seen. Alkylation of enzymes by the 6-chloro substituent was suggested as a mechanism. This azapurine inhibited the RNA polymerase from E. coli, but not that from M. lysodeikticus. Formyltetrahydrofolate synthetases, of both mammalian and bacterial origins, were strongly inhibited. The same azapurine, at 0.3 mM, markedly inhibited the steroid-induced synthesis of A -3-ketosteroid isomerase in Pseudomonas testoster-oni ... 

The treatment of cancer frequently involves drugs that interfere with DNA synthesis. For example, 5-fluorouracil (5-FU) prevents the conversion of dUMP to dTMP, reducing the level of thymine nucleotides required for DNA synthesis. Methotrexate prevents formation of tetrahy-drofolate from its more oxidized precursors. As a result, the formation of both thymine for DNA synthesis and the purines for DNA and RNA synthesis is inhibited. 

D) contain long stretches of thymine nucleotides that produce the poly(A) tail of mRNA... 

B. 5-Fluorouracil prevents conversion of dUMP to dTMP. DNA synthesis is inhibited because of a lack of thymine nucleotides, so this compound is used to treat cancer. 

The conformationally-locked 3 -amino thymidine derivative (47) was prepared and incorporated into DNA as a dimer unit, attaching the amino group via a carbamate or phosphoramidate linkage.Both modifications were found to be destabilising towards both RNA and DNA targets. The hybridisation properties of an amino-modified bicyclic thymine nucleotide (48) have been exam-ined. Using alternate T and (48) residues there is an increase in duplex stability with d(A)io, with no pH dependence. When the amino group is acetylated, the duplex stability is decreased. The locked C-nucleoside oxazole derivative (49) has been incorporated into TFOs where it was found to selectively enhance the stability with a C.G base pair. ... 

Figure 22.17 Salvage and de novo synthetic pathways to thymine nucleotides. 

Figure 22.17 outlines the de novo and salvage synthetic pathways to thymine nucleotides. dUTP, an intermediate in the de novo pathways that begins with UDP, is readily recognized by DNA polymerases and can be incorporated into DNA in place of dXTP. The uracil from a dUMP residue in a DNA strand pairs with adenine (like thymine from a dXMP residue would), so there is no loss of or change in information in the DNA. However, dUMP residues can also arise from spontaneous deamination of dCMP. When this DNA is replicated, a mutation at the site will result because cytosine is meant to pair with guanine, not adenine. 

Colin Tuma is being treated with 5-fluorouracil (5-FU), a pyrimidine base similar to uracil and thymine. 5-FU inhibits the synthesis of the thymine nucleotides required for DNA replication. Thymine is normally produced by a reaction catalyzed by thymidylate synthase, an enzyme that converts deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP). 5-FU is converted in the body to F-dUMP, which binds tightly to thymidylate synthase in a transition state complex and inhibits the reaction (recall that thymine is 5-methyl uracil). Thus, thymine nucleotides cannot be generated for DNA synthesis, and the rate of cell proliferation decreases. 

Fluorouracil (5-FU) undergoes metabolism to form 5-fluoro-2 -deoxyuridine 5 -phosphate (5-dUMP). This metabolite forms a covalently bound ternary complex with thymidylate synthase and its coenzyme A-methylenetetrahydrofolate. The synthesis of thymine nucleotides is blocked, and a thymineless death of cells results. The answer is (E). 

How do we target DNA synthesis Two typical drugs (Fig. 40.1) are 5-fluorouradl and methotrexate, and both of them target the supply of thymine nucleotides. As we saw in Topics 33 and 34, thymine is one of the four DNA bases. RNA has uracil instead of thymine, and the difference between them is that thymine has an extra CH3 group (Fig. 40.2). Uracil thus supplies building blocks for RNA, but it is also a precursor in the pathway to make the thymine building blocks for DNA. Uracil is converted into deoxyuridine monophosphate, and in the next step the CH3 group is added to make deoxythymidine monophosphate (Fig. 40.3). The... 

Uridine nucleotides are also the precursors for de novo synthesis of the thymine nucleotides, which can also be obtained through the salvage pathways from deoxyuri-dine or deoxythymidine. 

Biosynthesis of thymine nucleotides. This is shown in Fig. 2. Since thymine is a constituent of DNA, the corresponding nucleotides contain 2-deoxyribose. Thymidylic acid (TMP) is therefore more correctly dTMP (deoxythymidine 5 -monophosphate). The reaction sequence is CMP - CDP - dCDP -v dCMP ->dUMP- TMP (dTMP)- TDP (dTDP)- TTP (dITP). Methylation of dUMP to TMP is catalysed by th idylate synthase (EC 2.1.1.45). The cofiictor, AP,A/ -methylenetetrahydrofolic acid, transfers the active Cl unit to C5 of dUMP, and it also functions as a reducing agent in the formation of the methyl group from the active Cl unit. 

The ribonucleotide reductases form deoxycytidine phosphates which are destined for incorporation into DNA these nucleotides are diverted to some extent into the deoxyuridylate pool, and thence into the thymine nucleotides, through the action of deoxycytidylate deaminase. This diversionary flow into the thymine pathway is regulated by the demand for the terminal products, dTTP and dCTP the valve controlling this flow is deoxycytidylate deaminase, the activity of which is subject to allosteric regulation by dTTP and dCTP. 

UMP, the first complete pyrimidine nucleotide synthesized de novo, may be converted to the functional cytosine and thymine nucleotides by the variety of reactions diagrammed in Fig. 2. 

In such strategies, two comonomers are intentionally defined as 0 and 1 bits. The monomer T is a thymine nucleotide, which is used to start the sequence and facilitates the pol rmer characterization, (b) Extended-ASCII encoded text that was implemented in the primary structure of a polymer using the monomer-based digital code, (c) Schematic representation of the primary structure of the corresponding sequence-coded polymer and its characterization by negative mode electrospray mass spectrometry (ESI-MS). 

UV results in degradation of genetic material. The most efficacious wavelengths are about 260 nm. The most commonly used lamp design is low-pressure mercury vapor lamps, which have a monochromatic output of 254 nm. At this wavelength the light forms dimers between adjacent thymine nucleotides in DNA chains. The resulting thymine dimers inhibition the correct replication and transcription of the DNA (Friefelder, 1987). The role of pH in UV action has not been demonstrated (Haas, 1999). 

If a sample of DNA isolated from a microorganism culture were analyzed and found to contain 1.5 mol of cytosine nucleotides and 0.5 mol of adenosine nucleotides, what would be the amounts of guanine and thymine nucleotides in the sample ... 

The double-stranded helix also provides a basis for a replicative mechanism (834). If each strand of the double helix serves as a template on which a daughter strand is synthesized, the arrangement of bases on the strand being formed will be complementary to tiie parent strand. For example, an adenine nucleotide in the parent compels the incorporation of a thymine nucleotide in the dau ter strand the same also holds for guanine and c3rtosine. Thus a dau ter strand is formed from each parent strand. Each daughter is the same as the other parent strand and the net result is the formation of two double helices similar to that of the parent (Rg. 6). 

When the fake thymine nucleotide is added to the replicating DNA, the chain cannot continue to form, because the —N=N =NH group on the sugar prevents future phosphate linkages. 

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