DNASeq = "ATGTCTCATTCAAAGCA"	
SeqLength = float(len(DNASeq))	

BaseList = list(set(DNASeq))	
for Base in BaseList:	
	Percent = 100 * DNASeq.count(Base) / SeqLength	
	print "%s: %4.1f" % (Base,Percent)
 

%whos

def calc (MySeq):
  SeqLength = float(len(MySeq))
  ComponentList = list(set(MySeq))
  for Component in ComponentList:
    Percent = 100 * MySeq.count(Component) / SeqLength
    print "%s: %4.1f%%" % (Component,Percent)

MySeq="TTGGGGGGCGAAAA"

calc(MySeq)

calc("TGTTNNTCTCCCCAAAA")