Ok I can see the 5 end is negative, is the 3 end positive due to OH. So in other strand this goes other way round. Is this what they mean when they say there is reverse polarity in a DNA molecule. Thanks!!
Yes. If you think of each strand of DNA as an arrow pointing from the 5' end to the 3' end, when the two strands of DNA pair together, the arrows would be pointing in opposite directions as opposed to the same direction.