text: function pairElement (str) {
    var output = []
    var pairs = {
      G: 'C',
      C: 'G',
      A: 'T',
      T: 'A'
    }
    for (var i = 0, len = str.length; i < len; i++) {
      var arr = []
      arr.push(str[i], pairs[str[i]])
      output.push(arr)
    }
    return output
  }
  
  console.log(pairElement('GCG'))
  