Interactive · 2 questions
The two free ends of each DNA sugar-phosphate backbone are quite different from each other. On one end, a phosphate group is attached to a 5' carbon. This is known as the 5' end.
The other end of the sugar-phosphate backbone has a hydroxyl group attached to a 3' carbon. What do you think this end is known as?
The other end, because it has a hydroxyl group attached to a 3' carbon, is known as the 3' end. The sugar-phosphate backbone has directionality from 5' to 3', meaning that it runs from the 5' end to the 3' end.
Note how the 5' end is attached to a phosphate group and the 3' end is attached to a hydroxyl group. Image credit: http://www.forensicindia.com/fingerprint/dnapair.gif.
Which direction does DNA run in?
The two strands of DNA run in an antiparallel direction, which is just a fancy way of saying that they run 5' to 3', but in opposite directions so that one strand's 5' end is across from the other strand's 3' end and vice versa.
See how the 5' end of the left strand faces the 3' end of the right strand? That's antiparallel! Image credit: https://myndbook.com/view/956