######################################################################
##### READ IN FASTA FILE AND STORE CONTENTS IN *SEQUENCE* ############
######################################################################

file = open("yeastGenome.txt")

# Read in first line of file and check for FASTA format
headerLine = file.readline()
if (headerLine[0] == '>'):  # First character is '>'
    print "The file is in FASTA format."
else:
    print "The file is not in FASTA format."

# Read in the rest of the file (i.e., the sequence)
sequence = file.read()

# Remove all carriage returns from the sequence
sequence = sequence.replace("\n", "")



######################################################################
##### DETERMINE GC CONTENT OF *SEQUENCE* #############################
######################################################################

# Calculate GC content of sequence
numberOfGuanines = float(sequence.count('G'))
numberOfCytosines = float(sequence.count('C'))
totalNucleotides = float(len(sequence))
GC_content = (numberOfGuanines + numberOfCytosines) / totalNucleotides
print GC_content