def contains_sequence(dna1, dna2):
    """ (str, str) -> bool
    
    Return True if and only if DNA sequence dna2 occurs in the DNA sequence
    dna1.
    
    >>> contains_sequence('ATCGGC', 'GG')
    True
    >>> contains_sequence('ATCGGC', 'GT')
    False
    
    """
    if dna2 in dna1:
        return True
    else:
        return False