A linear DNA duplex has two 5' ends, and a circle has none
- The idea
- Predict
- See it
- Why
- Check
The idea
“A double-stranded DNA molecule has one 5' end, whatever its shape.”
It's easy to picture a DNA molecule as one rope with one front end. It's really two strands that run in opposite directions.
The short version
The mistake: A double-stranded DNA molecule has one 5' end, whatever its shape.
What's actually true: Each strand of a linear molecule has one 5' end, so a linear duplex has two. A circular one has none.
- Every DNA strand has a 5' end, with a phosphate on the sugar's 5' carbon, and a 3' end, with an OH on the 3' carbon.
- The two strands of a double helix are antiparallel, which means they run in opposite directions. So each end of a linear molecule has one 5' end and one 3' end.
- Count strands, not molecules. A linear duplex gives two 5' ends. A circular one, like a bacterial chromosome or plasmid, gives none, because each strand is a closed loop.
On the exam: Count strands, not molecules. Each linear strand has one 5' end. A circle has none.