input: Write a python function to calculate the GC content of a DNA sequence.
  assert calculate_gc_content('ACTGCTAGCTACTAGC') == 50.0
  assert calculate_gc_content('GCGCGC') == 100.0
  assert calculate_gc_content('ATATAT') == 0.0
output: def calculate_gc_content(dna_sequence):
      gc_count = dna_sequence.count('G') + dna_sequence.count('C')
      gc_content = (gc_count / len(dna_sequence)) * 100
      return gc_content