A partner strand runs the opposite way
- The idea
- Predict
- See it
- Why
- Check
The idea
“To write the partner, I pair each base and keep the same order, 5' to 3', left to right.”
Every pair you wrote is probably right. The partner strand just runs the other way, so its 5' end is on the opposite side.
The short version
The mistake: To write the partner, I pair each base and keep the same order, 5' to 3', left to right.
What's actually true: The strands run in opposite directions, so the partner, written 5' to 3', has its bases in reverse order.
- Each strand has a 5' end, with a phosphate on the sugar's 5' carbon, and a 3' end, where the 3' carbon carries a free OH. That gives the strand a direction.
- The two strands of a double helix are antiparallel. One strand's 5' end lies across from the other's 3' end.
- DNA polymerase can only add nucleotides to a free 3' OH. So a new strand is always built 5' to 3', toward the template's 5' end.
- To write a partner, pair each base and reverse the order so it reads from its own 5' end. For RNA, use U in place of T.
On the exam: Partner strands run opposite ways. Pair the bases, then reverse them to read from the new strand's 5' end.