Polynucleotide chain, mononucleotides

The Polynucleotide Chain Contains Mononucleotides Linked by Phosphodiester Bonds... 

Watson-Crick model for DNA replication. The double helix unwinds at one end. New strand synthesis begins by absorption of mononucleotides to complementary bases on the old strands. These ordered nucleotides are then covalently linked into a polynucleotide chain, a process resulting ultimately in two daughter DNA duplexes. 

Pseudo-polynucleotide chains can be formed by replacing 3 —> 5 linking with 2 —> 5 linking between the mononucleotide units [78]. Such chains are known to result from the action of interferons (Chapter 11) and have been identified in bacteria. Double helices can be formed by 2 5 linked chains and between 2 —> 5 and 3 —> 5 linked chains can be synthesised. These compounds are effective inhibitors of protein synthesis and will block cell growth (Chapter 12.13). Triply linked ribose rings involving both 2 —> 5 and 3 —> 5 connections (99) are believed to occur as lariat intermediates in the biosynthesis of m-RNA [79]. 

The enzyme mechanism for DNA synthesis was clarified by Kornberg and his associates [103-108] in bacterial systems. A single DNA strand is a polynucleotide chain in which the individual nucleotides are linked by ester bonds between the carbon 3 of one deoxyri-bose and the carbon 5 of another deoxyribose belonging to an immediately adjacent nucleotide. There are two distinct aspects in the synthesis of a single strand formation of the ester bond and the alignment of the nucleotide in the proper sequence. On the basis of observations made on the biosynthesis of coenzymes, Kornberg postulated that the precursor used for the formation of the polynucleotide chain was an activated 5 -mononucleotide, namely, a triphosphate. 

The specificity of snake venom phosphodiesterase, unlike that of RNase, is not concerned with the nature of the bases. Therefore, it can release one by one all nucleotides found in the polynucleotide chain. However, in contrast to pancreatic RNase, the snake venom phosphodiesterase acts at the other end of the phosphodiester bond, and 5 -mononucleotides are produced. 

Exonuclease. Exonuclease removes successive mononucleotide units from the polynucleotide chain in stepwise fashion, starting from the 3-end. 

Although most polynucleotides are naturally occurring prodncts (for which the term nncleic acid is best reserved), many synthetic polynucleotides, generally of lower molecular weight, have now been prepared and smdied. Very short chains, obtained either synthetically or by breakdown of natnral polynucleotides, which contain just a few mononucleotide units, are often called ohgonncleotides. 

Nucleic acids (DNA and RNA) are polynucleotides consisting of four different mononucleotides. The base sequence of the DNA chain determines the sequence of amino acids in an organism s proteins. Hydrogen bonding between specific base pairs results in DNA s double helical structure and is the key to the process of protein synthesis. 

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