Exercise 2 solution

COMP = str.maketrans("ACGT", "TGCA")

def revcomp(s):
    return s.translate(COMP)[::-1]

seq = "TTGAATTCAAGGATCCGCGCAATTAGAATTCC"
sites = []
for i in range(len(seq) - 5):
    piece = seq[i:i + 6]
    if piece == revcomp(piece):
        sites.append((i, piece))
print(sites)
print(len(sites))
print(revcomp("GAATTC") == "GAATTC", revcomp("GGATCC"), revcomp("ATGGCC"))

Output:

[(2, 'GAATTC'), (10, 'GGATCC'), (25, 'GAATTC')]
3
True GGATCC GGCCAT

Reading it: a six-letter site reads the same as its own reverse complement, so it is a palindrome in the DNA sense: GAATTC and GGATCC pass, ATGGCC does not. Three sites turn up, at positions 2, 10 and 25 (counting from 0). Sites like these are where many restriction enzymes cut (check).