III. Watson-Crick Base Pairs

Opening image: Structure of an AT base pair from a DNA molecule. The drawing represents 10 superimposed structures consistent with the NMR spectra of the molecule in solution.

GC base pair. All four base pairs have the same overall dimensions. The distance separating the backbone C1' atoms, shown in green, is very important. In the "ideal" W-C model this distance is 10.85 Å for all four base pairs. However, the values range from 10.4 to 10.9 Å.

  GC base pairs are joined by three hydrogen bonds.

  Toggle alternate views of an AT base pair. An "ideal" AT base pair is planar and joined by two hydrogen bonds. Is that true for this AT base pair?

  We now return to the AT base pair from the opening image. NMR models represent "snapshots" of the thermal motion of the molecule in solution. Note the range of motion for different atoms (the lines coalesce if motion is limited).
 

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