code: def _extract_total_number_of_chars(self):
          
          self._get_gene_codes_and_seq_lengths()
  
          sum = 0
          for seq_length in self._gene_codes_and_lengths.values():
              sum += sorted(seq_length, reverse=True)[0]
          self.number_chars = str(sum)
docs: sets `self.number_chars` to the number of characters as string.