def count_nucleotides(dna, nucleotide):
    """ (str, str) -> int
    
    Return the number of occurrences of nucleotide in the DNA sequence dna.
    
    >>> count_nucleotides('ATCGGC', 'G')
    2
    >>> count_nucleotides('ATCTA', 'G')
    0
    """
    return dna.count(nucleotide)