a program to change letters in DNA nucleotide

I'm making a program to change the sequence of DNA. When finding the letter "a", replace it with a "t", and opposite. But I've encountered a problem cause when the program runs, it replaces "a" with "t", then replaces "t" with "a" again.

How can I fix that?

The code:

def opposite_nucleotide(dna):
     dna = dna.replace("a", "t")
     dna = dna.replace("t", "a")
     dna = dna.replace("g", "c")
     dna = dna.replace("c", "g")
     return dna

dna3 = input("Enter the DNA to represent it's oppsite one: ")
dna4 = opposite_nucleotide(dna3)
print(dna4)

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