# get the fraction of → GC content of a DNA sequence in percent
from Bio.SeqUtils import GC
GC('TGCAA')
40.0 # Sequence includes 40% of the bases G or C
http://biopython.org/DIST/docs/api/Bio.SeqUtils-pysrc.html#GC
# without using Biopython
http://saml.rilspace.org/calculating-gc-content-in-python