python replace multiple elements in string with str methods

Hello I am learning to program with python. I am trying to write a function that takes a string of DNA and returns the compliment, I have been trying to solve this for a while now and looked through the python documentation but couldn't work it out. I have written the doc string for the function so you can see what the answer should look like. I have seen a similar question asked on this forum but I could not understand the answers. I would be grateful if someone can explain this using only str formatting and loops / if statements, as I have not yet studied dictionaries/lists in detail. thank you in advance

I tried str.replace but could not get it to work for multiple elements, tried nested if statements and this didn't work either. I then tried writing 4 separate for loops, but to no avail.

def get_complementary_sequence(dna):

    """ (str) -> str

    Return the DNA sequence that is complementary 
    to the given DNA sequence.

    >>> get_complementary_sequence('AT')
    TA
    >>> get_complementary_sequence('GCTTAA')
    CGAATT

    """

    for char in dna:
        if char == A:
            dna = dna.replace('A', 'T')
        elif  char == T:
            dna = dna.replace('T', 'A')
        # ...and so on