5' and 3'
For all Tricks, It is necessary that you actually see these pictures in your mind.
You will be wonderstruck to know that you need not to read it again because now you know it and you will remember it forever.
A DNA have 2 ends one is called 5' and another 3'. It is the position in the pentose sugar. 5' end contains PO4 and 3' end contains OH¯ group.
It means 3' end contains OH¯