def getNucleotideCodon(sequence, x1) :
	"""Returns the entire codon of the nucleotide at pos x1 in sequence, 
	and the position of that nocleotide in the codon in a tuple"""

	if x1 < 0 or x1 >= len(sequence) :
		return None

	p = x1%3
	if p == 0 :
		return (sequence[x1: x1+3], 0)
	elif p ==1 :
		return (sequence[x1-1: x1+2], 1)
	elif p == 2 :
		return (sequence[x1-2: x1+1], 2)