Thymine, 1-methyl-, structure

Cytosine and thymine were first isolated by hydrolysis of calf thymus tissue by Albrecht Kossel (1853-1927) and A. Neumann during 1893-1894. Thymine s structure was published in 1900 and confirmed over the next several years when it was synthesized by several investigators. In 1903, cytosine was synthesized by Henry Lord Wheeler (1867-1914) and Treat B. Johnson, confirming its structure. Uracil was first isolated in 1900 from yeast nucleic acid found in bovine thymus and herring sperm. The methylation of uracil produces thymine thymine is also called 5-methyluracil because methylation takes place at the fifth carbon in uracil to produce thymine. 

The dimer has been shown by Blackburn and Davies40 to have a geometrical structure similar to that of thymine dimer (10). The proof consisted in observation of the 13C—H satellite NMR spectrum of 5,5 -dideutero-,3,3 -dimethyl uracil dimer (prepared by methylation of the dimer from photolysis of uracil-5-rf). The magnitude of the H6H6 coupling constant showed that these hydrogens were vicinal, and was... 

The pyrimidine base units cytosine, thymine, and uracil contain six-member nitrogenous ring structures with various points of unsaturation. Thymine and uracil are similar, containing the same double bond between carbons 5 and 6 and the same two ketone groups on C-2 and C-4 of the ring, but differ only in the presence of a methyl... 

The structure is identical to that of the T-A base pair shown in Sec. 18.8 of the text, except that the methyl group in the thymine unit is replaced by a hydrogen. 

The replacement of thymine by uracil has no significant effect on the hydrogen bonding, as RNA does not use base pairing to form complementary dimers it is of less importance than it would be for DNA, but the removal of the methyl group may have an influence on the tertiary structures that RNA can adopt. From this it is clear that DNA is a better method of storing information whereas RNA is more suited to turn that information into a protein sequence. This is done by the ribosome, composed of ribosomal RNA (rRNA), which translates the codons of the mRNA sequence into a protein by matching three base sequences to those of tRNA that have the appropriate amino acids attached. 

Mollegaard NE, Nielsen PE. Increased temperature and 2-methyl-2,4-pentanediol change the DNA structure of both curved and uncurved adenine/thymine-rich sequences. 

The similarly linked dimer (155 R = Me, R = H) has been isolated213 after irradiation of DNA in vitro and in vivo possibly by combination of cytosine and thymine (accompanied by deamination). The macrocyclic tetramer (157) has also been isolated on irradiation of DNA 214 it is thought to be the dimer of the above product (155 R = Me, R = H). The trans-syn structure of 157 has been confirmed by X-ray diffraction analysis of the hexa-A -methyl derivative.215 These are all biologically significant photoreactions as described in Section III, B, 2. Further irradiation of the trans-syn and cis-syn cyclobutane-type dimers (see Section III,E, 2) of 1,3-dimethyluracil with a sensitizer has been found to produce the 5,5 -linked dimer (158).216... 

The covalent structures of RNA and DNA differ in one other way. Whereas RNA contains uracil, DNA contains a methylated uracil derivative termed thymine. This modification also serves to protect the integrity of the genetic sequence, although it does so in a less direct manner. As we will see in Chapter 27. the methyl group present in thymine facilitates the repair of damaged DNA, providing an additional selective advantage. 

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