"The DNA strand is missing the pairing element. Take each character, get its pair, and return the results as a 2d array."

dna-pairing.js

function pair(str) {
  var tmp = str.split(''), // Make a copy without altering str
      match = {    // An object to easily match pairs
        'C': 'G',
        'G': 'C',
        'A': 'T',
        'T': 'A'
      },
      result = [], // Initialize some variables
      currentPair,
      letter;
  
  while (tmp.length > 0) {
    letter = tmp.shift(); // Also acts as the iterator :thumbsup:
    currentPair = [letter, match[letter]]; // Match it up!
    result.push(currentPair); // Push it … push it real good
  }
  
  return result;
}

pair("GCG");