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When synthesizing DNA strands and beginning to "connect" them, how quickly does it become a helix?

In this answer, canadianer says

The helical structure is more stable than the "straight" form (because of base stacking interactions), and so it forms spontaneously.

This then makes me think of the following scenario: say one is constructing two strands of DNA out of deoxyribonucleotides. Then, one begins connecting them with hydrogen bonds. Before the hydrogen bonds are added, my understanding is the two strands would be straight and not helical (if this is wrong, please correct me on this) so at some point as the two strands are connected, it would "spontaneously" become helical.

The question, then, is at which point. After one "connection"? Two? All of the connections necessary to complete the piece of DNA?

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  • $\begingroup$ Yes, in general that's correct. $\endgroup$ – canadianer Apr 4 '17 at 0:28
  • $\begingroup$ I will do soon, glad it helped. $\endgroup$ – canadianer Apr 4 '17 at 0:40
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    $\begingroup$ @canadianer has answered your question, but you should be aware that it is of little interest to scientists because of its hypothetical nature and experimental untestability, and the fact that it does not address a problem in biological or physical sciences. DNA is polymerized by enzymes, and we understand something about what occurs in these reactions. We also know there is not the sort of folding problem that exists for proteins. We cannot add deoxynucleotides to a pre-existing helix (which involves making phosphodiester bonds) in the way you suggest without enzymes, template and primer. $\endgroup$ – David Apr 5 '17 at 7:59
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In general, a single stranded nucleic acid is helical in the absence of other secondary structure. The base stacking that drives helix formation does occur between adjacent bases in the same strand. Note, however, that such structures are dynamic and dependent on properties such as temperature and salt concentration. You can see an example of a single-stranded RNA helix in the following crystal structure (1MHK):

enter image description here

Note the blue helix at the bottom of the image is actually a duplex in the crystal.

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