The site finder

You will write three small functions that find a restriction site and cut a piece of DNA at it.

The idea. To test a palindrome, pair the bases and reverse the result. To cut, find every place where the site starts, and cut the top strand after the chosen base of the site.

Your tasks.

1. Finish pair_up. It gives the bottom strand of a site, read from its 5 prime end.
2. Finish is_palindrome. It is True when the site is the same as its paired and reversed form.
3. Finish fragments. It gives the sizes of the pieces of the top strand, from the left, when the DNA is cut after cut_after bases of every site.