Polydeoxyribonucleotides, single-strande

DNA ligase (NAD+) [EC 6.5.1.2] (also referred to as polydeoxyribonucleotide synthase (NAD+), polynucleotide ligase (NAD+), DNA repair enzyme, and DNA join-ase) catalyzes the reaction of NAD+ with (deoxyribo-nucleotide) and (deoxyribonucleotide) to produce AMP, nicotinamide nucleotide, and (deoxyribonucleo-tide)( +m). This forms a phosphodiester at the site of a single-strand break in duplex DNA. RNA can also act as substrate to some extent. 

DNA is a polydeoxyribonucleotide that contains many monodeoxy-ribonucleotides covalently linked by 3 ->5 -phosphodiester bonds. With the exception of a few viruses that contain single-stranded DNA, DNA exists as a double-stranded molecule, in which the two strands wind around each other, forming a double helix. In eukaryotic cells, DNA is found associated with various types of proteins (known collectively as nucleoprotein) present in the nucleus, whereas in prokaryotes, the protein-DNA complex is present in the nucleoid. 

Single-strand break a break in a double-stranded DNA molecule which involves only one of the two strands, so that the molecule remains together. One S.s.b. is required to initiate unwinding of the double helix during replication. S.s.b. are caused by endonucleases, physical conditions or chemicals. Such breaks are repaired by the polydeoxyribonucleotide synthetases 6.5.1.1 and 6.5.1.2). 

Single-stranded polydeoxyribonucleotides are, as a rule, less stacked and structured than their ribo counterparts. They frequently show different titration behavior [e.g., poly(dC) forms the double helical acid form two pH units higher than poly(C) under the same experimental conditions] and optical properties. 

To define the specificity of SLE antibodies reacting with denatured DNA Koffler et al., (1971) used polydeoxyribonucleotides as test antigens, or as inhibitors of the reaction. The specificities were multiple antibodies directed against a determinant common to one or more polydeox5nibonucleotides and to single-stranded DNA, and antibodies with single base specificities were observed. Whatever the specificity of these antibodies, no significant difference appears to exist in the reactivity with DNA from different sources. 

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