Polynucleotide helices, left-handed

The heart of the Watson-Crick model is the postulate that a molecule of DNA is a complementary double helix. It consists of two antiparallel polynucleotide strands coiled in a right-handed manner about the same axis to form a double helix. As illustrated in the ribbon models in Figure 28.6, chirality is associated with a double helix left-handed and right-handed double helices are related by reflection just as enantiomers are related by reflection. 

Z form. A duplex DNA structure in which the usual type of hydrogen bonding occurs between the base pairs but in which the helix formed by the two polynucleotide chains is left-handed rather than right-handed. 

Until recently, only right-handed double helices were known. A left-handed helix structure was observed for the first time in the synthetic hexameric DNA containing alternating cytosine and guanine bases [d(CGCGCG)]. Both polynucleotide strands are wound around themselves in a left-handed sense. The phosphate groups show a zick-zack-type pattern along the screw this structure therefore was named Z-DNA (Fig. 2). 

There is a large variability possible in the structures of double stranded DNA due to the fact that (compared to polypeptides) many more bonds can be rotated in the backbone of each monomer (Scheme 14). The most common and physiologically most important structure is the B-DNA helix. It consists of two polynucleotide chains running in opposite direction which coil around a common axis to form a right-handed double helix. In the helix, the phosphate and deoxyribose units of each strand are on the outside, and the purine and pyrimidine bases on the inside. The purine and pyrimidine bases are paired by selective hydrogen bonds adenine is paired with thymine, and guanine with cytosine (Scheme 15). The structure is very flexible and can form a supercoil with itself, or around proteins. It can form a left-handed supercoil around histones to form nucleosomes which assemble in yet another helical structure to form chromatin. ... 

Z form Z-DNA is a left-handed helix found in polynucleotides with alternating purines and pyrimidines in each strand, as in the following ... 

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