Question

I’m writing a program in which I can Reverse the sequence and Replace all As with Ts, all Cs with Gs, all Gs with Cs, and all Ts with As. the program is to read a sequence of bases and output the reverse complement sequence. I am having trouble to do it so can anyone please help me with this by having a look on my code:

word = raw_input("Enter sequence: ")
a = word.replace('A', 'T')
b = word.replace('C', 'G')
c = word.replace('G', 'C')
d = word.replace('T', 'A')
if a == word and b == word and c == word and d == word:
    print "Reverse complement sequence: ", word

And I want this sort of output:

Enter sequence: CGGTGATGCAAGG
Reverse complement sequence: CCTTGCATCACCG

Regards

Answer 1

I would probably do something like:

word = raw_input("Enter sequence:")

# build a dictionary to know what letter to switch to
swap_dict = {'A': 'T', 'T': 'A', 'C': 'G', 'G': 'C'}

# find out what each letter in the reversed word maps to and then join them
newword = ''.join(swap_dict[letter] for letter in reversed(word))

print "Reverse complement sequence:", newword

I don’t quite understand your if statement, but the above code avoids needing one by looping over each letter, deciding what it should become, and then combining the results. That way each letter only gets converted once.

Edit: oops, I didn’t notice that you wanted to reverse the string too. Fixed.