Thymine, in DNA

Oxygen-free reactions of psoralens, when in close proximity to the target, proceed via the first excited states in which the 3,4-and the 4, 5 7r-bonds of the pyrone and furan moieties, respectively, can undergo C4-cyclization reactions with, e.g., unsaturated bonds of lipids, or the C5=C6 double bonds of thymine in DNA. In reactions with DNA the psoralen is believed to intercalate with DNA in the dark. Subsequent irradiation at 400 nm usually leads to furan-side 4, 5 -monoadduct formation, whereas irradiation at 350 nm increases the formation of crosslinks in which the furan and pyrone rings form C4 cycloadducts to thymines on opposite strands [95], Subsequent irradiation of the 4, 5 -monoadducts at 350 nm leads to formation of crosslinks and conversion into pyrone-side 3,4-monoadducts. Shorter wave-... 

DNA incorporates the monosaccharide deoxyribose, while RNA has ribose. One of the four bases is different RNA has uracil in place of thymine in DNA. Finally, RNA has no double helix. 

A. Biotin Incorporate in base thymine in DNA probe sequence or uracil in RNA probe sequence... 

Ultraviolet light induces the formation of dimers between adjacent thymines in DNA (also occasionally between other adjacent pyrimidines). The formation of thymine dimers interferes with DNA rephcation and normal gene expression. Thymine dimers are eliminated from DNA by a nucleotide excision-repair mechanism (Figure 1-2-4). 

Figure 20.4 The bases present in RNA and DNA and the Watson-Crick base pairing relationships. Uracil is present in RNA but is replaced by thymine in DNA that is, the pairs C-G and T-A are found in DNA the pairs C-G but U-A are found in RNA. The pairing is brought about by hydrogen bonding, indicated by a broken line.

Figure 8.3 5-Methylcytosine deamination to thymine in DNA. R Phosphodesoxyribose of the DNA backbone.

In order to understand the effect of temperature we consider the energetics involved for the stepwise formation of T(C6)H in DNA. The proton-coupled electron transfer from cytosine to thymine in DNA is depicted in reaction 1 and protonation of the thymine anion radical in reaction 2. 

The two strands of DNA are held together by the molecular attractions that occur between nucleotides. Because of their molecular structures, however, the nucleotides are particular in the attractions they have for each other. Guanine and cytosine, for example, are best attracted to each other, while adenine and thymine are best attracted to each other. Accordingly, within the DNA double helix for each adenine on one strand there is a thymine on the opposing strand to which it is attracted. The number of adenines and thymines in DNA, therefore, is always the same. 

Another key difference between RNA and DNA is the presence of thymine in DNA instead of the uracil in RNA. Thymine is simply uracil with an additional methyl group. The four common bases of DNA are cytosine, thymine, adenine, and guanine. 

The sugar present in RNA is ribose, while DNA contains deoxyribose. Also, uracil is present in RNA this is replaced by thymine in DNA. 

The three pyrimidine bases are the simpler and they are uracil (U), thymine (T), and cytosine (C). Cytosine is found in DNA and RNA, uracil in RNA only, and thymine in DNA only. 

These purines and pyrimidines join to the sugar-phosphate backbones of nucleic acids through repeating /3-linked AT-glycosidic bonds involving the N9 position of purines and the N1 position of pyrimidines. There are two classes of nucleic acids ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). DNA and RNA differ in one of their nitrogenous base components (uracil in RNA, thymine in DNA) and in their sugar (ribose) moiety, as indicated in Fig. V-2. 

TT-slackcd cytosine (and presumably also for thymine) in DNA and provides the basic understanding of potential phototogenotoxicity via triplet-triplet sensitization. 

The latter reactions have also been studied at the level of polynucleotides. In particular, replacement of thymine in DNA by 5-iodouracil or 5-bromouracil (both of which are thymine analogues) enhances the sensitivity to UV irradiation Since both... 

Classic Watson-Crick base pairs are formed by unique hydrogen-bonding interactions between the nitrogenous bases of DNA and RNA. The purine adenine associates specifically with the pyrimidine thymine in DNA (or the related unmethylated analog, macil, in RNA), and the pmine guanine interacts with the pyrimidine cytosine. These complementarity rules. 

A heterocyclic base, guanine, adenine, cytosine, uracil (in RNA), or thymine (in DNA). The sugar is substituted at Cl by the base, which is attached by a / glycosyl Cl -N link. 

Photosensitizing dyes work through different mechanisms in the cell. For example, psoralens (Chapter 4.2.2) react under long-wavelength UV-irradiation with the thymine in DNA by cyclobutane addition forming mono- and diadducts. In this way the synthesis of nucleic acids is blocked. Pathological cell growth, as in the skin disease psoriasis, is thus prevented. 

Base pairing The complementary hydrogen bonding of cytosine with guanine and of adenine with thymine (in DNA) or with... 

Howard-Flanders P, Boyce RP.Simson E, Theriot L (1962) A genetic locus in E.coli K12 that controls the reactivation of UV-photoproducts associated with thymine in DNA. Proc Natl Acad Sci USA 48 2109-15... 

Chargaff rule the amount of adenine and thymine in DNA is equal the amount of cytosine and guanine are equal. (A = T, C = G). The amount of purines equals the amount of pyrimidines. 

The five-carbon sugar in RNA is ribose, and the sugar in DNA is 2 -deox)Tribose. The only difference between these two sugars is the absence of an hydroxyl group on the 2 carbon of 2 -deoxyribose. The purines in both DNA and RNA are adenine and guanine. Both DNA and RNA contain the pyrimidine cytosine however, the fourth base is thymine in DNA and uracil in RNA. The chemical compositions of DNA and RNA are summarized in Table 24.1. 

Purines are bases found in the nucleosides and nucleotides that make up nucleic acids. In nucleic acids, the purines match up with specific pyrimidine bases. The matching between purines and pyrimidines forms "base-pairs" in which adenine pairs with thymine (in DNA) or uracil (in RNA). Guanine pairs with the base cytosine in either nucleic acid. 

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