Computing GC Content

[uwl]

I have been trying to find the solution to a programming problem which asks me to find the GC content. The example online displays:

Sample Dataset
>Rosalind_6404
CCTGCGGAAGATCGGCACTAGAATAGCCAGAACCGTTTCTCTGAGGCTTCCGGCCTTCCC
TCCCACTAATAATTCTGAGG
>Rosalind_5959
CCATCGGTAGCGCATCCTTAGTCCAATTAAGTCCCTATCCAGGCGCTCCGCCGAAGGTCT
ATATCCATTTGTCAGCAGACACGC
>Rosalind_0808
CCACCCTCGTGGTATGGCTAGGCATTCAGGAACCGGAGAACGCTTCAGACCAGCCCGGAC
TGGGAACCTGCGGGCAGTAGGTGGAAT

Sample Output
Rosalind_0808
60.919540

My code runs properly and I already tried entering the solutions I find with the correct amount of decimal place values.

from collections import Counter
myDna = ''
with open('rosalind_gc.txt', 'r') as data:
     for line in data:
          if '>' in line:
               continue
          myDna += line.strip()
myNucleotideCounts = Counter(myDna)
myGC = (myNucleotideCounts['G'] + myNucleotideCounts['C']) / float(len(myDna))
print('Dna GC Content = {0}'.format(myGC)) 

Would someone be able to provide suggestions as to what might be the problem and how to fix this in the above code?