DNA Structure Activity

Problem 7: Are the two strands of DNA parallel or anti-parallel?

Are the two strands of DNA parallel or anti-parallel? Switch to the view labeled "Antiparallel DNA strands". Only the sugar rings of each base pair are shown. The sugar carbon atoms are colored blue and yellow. Notice that directly opposite each blue sugar on one strand is a yellow sugar on the other strand. Notice that the different colored sugar rings make two helical spirals that wrap around a common axis. The "+" and "-" zoom tools from the Tool Bar can be used for zooming in and out. Using the red oxygen atoms as a reference, determine if the two strands of DNA spiral in the:

A. same direction. The strands are "parallel".
B. opposite direction. The strands are "antiparallel".
In the illustration for this problem, note that the oxygen atom is pointing up on one strand and down on the other, indicating that the sugar-phosphate backbones are moving in opposite directions. The strands of a DNA double helix are said to be "antiparallel" because the have the same chemical structure, but are opposite in direction.

The Biology Project
University of Arizona
Modified: May 27, 1997
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