# complement_1.py
# Author: Sami Khuri
# Last updated: January 15, 2016
# Purpose: A function to construct the complement of a given DNA sequence 
# Program uses a function, a dictionary, for loop, string concatination
#         and a docstring: first line of function enclosed in """.

def complement(s): 
    """Return the complementary sequence string of s.""" 
    basecomp = {'A': 'T', 'C': 'G', 'G': 'C', 'T': 'A'}
     
    seqcomp = ""
    for base in s:
        seqcomp = seqcomp + basecomp[base]
    return seqcomp
       
dna_seq = 'CCGGAAGAGCTTACTTAG' 
print "DNA sequence:\t", dna_seq        
print "Its complement:\t", complement(dna_seq)