I use these handouts by giving each student the lowest level (in this case "DNA Level"). They work at their own pace, and when finished, check their work against answer keys posted in multiple places around the room. They need to re-do or at least be certain they understand any they got wrong; once confident in the first level, they pick up the second level and repeat. This allows students a lot of practice, makes it easy to see how far students get before they get stuck, provides movement breaks, and differentiates for students who are ready for a challenge. I circulate and focus on students who I know need help, or see need help from their progress on the practice sheets.