I don't know what I did - it's wrong . Can someone help me?

def insert_sequence(dna1, dna2, number):

    '''(str, str, int) -> str
    Return the DNA sequence obtained by inserting the second DNA sequence
    at the given index. (You can assume that the index is valid.)  

    >>> insert_sequence('CCGG', 'AT', 2)
    'CCATGG'
    >>> insert_sequence('TTGC', 'GG', 2)
    'TTGGGC'
    '''
    index = 0
    result = '';
    for string in dna1:
        if index == number:
            result = result + dna2

            result = result + string
            index += 1

            print(result)