在dna中有四中碱基，分别为ATCG，其中符号A与T是互补，符号C与G是互补的，请编写函数DNA_strand(dna)

回答：DNA_strand(dna)函数的实现如下：def DNA_strand(dna):
complementary = ""
for letter in dna:
if letter == "A":
complementary += "T"
elif letter == "T":
complementary += "A"
elif letter == "C":
complementary += "G"
elif letter == "G":
complementary += "C"
return complementary