Nucleotides in nucleic acid

The macromolecules of cells are built of units—amino acids in proteins, nucleotides in nucleic acids, and carbohydrates in polysaccharides—that have structural polarity. That is, these molecules are not symmetrical, and so they can be thought of as having a head and a tail. Polymerization of these units to form macromolecules occurs by head-to-tail linear connections. Because of this, the polymer also has a head and a tail, and hence, the macromolecule has a sense or direction to its structure (Figure 1.9). 

Much research has centered on identifying the source of the purine ring in caffeine. Two possible sources are likely methylated nucleotides in the nucleotide pool and methylated nucleotides in nucleic acids. Extensive experimental work by Suzuki and Takahashi27-30 proposes a scheme whereby caffeine is synthesized from methylated purines in the nucleotide pool via 7-methylxanthosine and theobromine. Information relating to the formation of 7-methylxanthine from nucleotides in the nucleotide pool is sparse. They also provide data that demonstrate that theophylline is synthesized from 1-methyladenylic acid through 1-methylxanthine as postulated by Ogutuga and Northcote.31... 

In RNA, the base T found in DNA is replaced by uracil, which is similar in structure to T, but lacks the methyl group. The nucleotides in nucleic acids are linked by phosphodiester bonds between the 3 -hydroxyl of one nucleoside and the 5 -hydroxyl of the sugar of its neighbour in the sequence, as was first shown by Alexander Todd3 in 1952 (Figure 4.13). 

Now that we have the monomers, which are pretty complex in this case, it remains to define how they are joined together to create the polymer. The amino acids in proteins are linked by peptide bonds. The nucleotides in nucleic acids are linked by phosphodiester bonds, as is shown in figure 12.2. These DNA phosphodiester bonds are very stable. Indeed, samples of largely intact DNA can be recovered from organisms that have been extinct for thousands of years. This remarkable stability should not come as a surprise the central importance of DNA in all forms of life requires that it be stable to various sorts of insults. 

Phosphodiester Bonds Link Successive Nucleotides in Nucleic Acids... 

In 1950, when the study of ribosomes began, no methods for determining the sequences of amino acids in proteins or of nucleotides in nucleic acids existed.11 Sanger published the sequences of the two short chains of insulin in 1953, and the first transfer RNA sequence was published by Holley in 1965.21 Never-... 

FIGURE 12.39. Nucleosides and nucleotides in nucleic acids, (a) Designation in DN.4. and (b) pyrimidines and purines. 

A base linked to a sugar is called a nucleoside a base linked to a sugar linked to a phosphate is called a nucleotide or a nucleoside phosphate. The nucleotides in nucleic acids are joined to one another by a second phos-phodiester bond that joins the 5 -phosphate of one nucleotide to the 3 -OH group of the adjacent nucleotide. Such a doubly esterified phosphate is called aphosphodi-ester group (Figure 23-2). 

Adenine and guanine are purines, which contain a pair of fused rings cytosine, thymine, and uracil are pyrimidines, which contain a single ring (Figure 2-15). The bases are often abbreviated A, G, C, T, and U, respectively these same singleletter abbreviations are also commonly used to denote the entire nucleotides in nucleic acid polymers. In nucleotides. 

Sequences of nucleotides in nucleic acids are written widi one-letter symbols starting with die 5 -terminus at the left toward 3 -tea minus at die right. Deoxyribonucleotides in DNA is prefixed with d. If it is not known whether a residue is A or G, die appropriate abbreviation for purine is R Y is used for pyrimidine, C or T. 

Paper chromatography was in use over 50 years ago for fractionation and quantitative analysis of nucleotides in nucleic acid hydrolysates. Techniques for paper and thin-layer chromatography were developed over 30 years ago for the analysis of phosphosaccharide mixtures. [40] Thin-layer chromatography (TLC) has now generally replaced paper chromatography for the analysis of organo-phosphorus mixtures. It is usually faster and more sensitive than the latter. 

The nucleotides in nucleic acids are held together by phospho-diester bonds between the 3 -hydroxyl group of a sugar (ribose or deoxyribose) and a phosphate group on the 5 -carbon of another sugar. 

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