DNASeq = "ATGTCTCATTCAAAGCA"
SeqLength = float(len(DNASeq))
 
BaseList = "ACGT"
for Base in BaseList:
  Percent = 100 * DNASeq.count(Base) / SeqLength
  print "%s: %4.1f" % (Base,Percent)
 

%whos

def calc(MyDNASeq):
  SeqLength = float(len(MyDNASeq))
  BaseList = "ACGT"
  for Base in BaseList:
    Percent = 100 * MyDNASeq.count(Base) / SeqLength
    print "%s: %4.1f%%" % (Base,Percent)

MyDNASeq="TTGGGGGGCGAAAA"

calc(MyDNASeq)

calc("TGTTTTTCTTTTTCCCCCCCAAAA")