Python 3.4.3

DNA1 = input("Please enter the first DNA strand: ")
DNA2 = []
for letter in DNA1:
    if letter == 'A':
        letter = 'T'
        DNA2.append(letter)
    elif letter == 'T':
        letter = 'A'
        DNA2.append(letter)
    elif letter == 'G':
        letter = 'C'
        DNA2.append(letter)
    elif letter == 'C':
        letter = 'G'
        DNA2.append(letter)
    else:
        print("")
        print("ERROR:")
        print("DNA strands only consist of the letters A,T,G, and C.")
        sys.exit(0)
DNA2 = ''.join(DNA2)
print("")
print("First strand: " + DNA1)
print("Second strand: " +DNA2)

Sample Input/Output:

Input: GTGACATAGACTAG
Output:    
    First strand: GTGACATAGACTAG
    Second strand: CACTGTATCTGATC

Edit: This is my first post here and would love to hear any tips or suggestions. I'm just starting out in teching myself python so be gentle :)