Zip it Up: Base pairing in DNA

Okay, now that we know which end is which, let’s take two short strands of DNA and fix ‘em up. One strand has the sequence 5’-ACCGTAG-3’. The other just happens to have the sequence 5’-CTACGGT-3’. Let’s get these two kids together. The first strand we leave in the 5’ to 3’ orientation, and the other we flip around to the 3’ to 5’ orientation. Bring them close together and—hey! They stick.

Why? Because the sequence of bases in the two strands is complementary. And what does that mean?