The reasoning behind the alternation of the sugar and phosphate molecules on a DNA ladder lies in the combination of bases to form the rungs of the ladder. The rungs are only between the sugar molecules. Two bases line up between these molecules and bond together, creating the rung appearance. There are no bases in between the phosphate molecules, thus there are no rungs between these molecules. There is open space that gives the DNA ladder the step appearance.
The ladders of DNA that makeup the genes can be twisted or not twisted. The ones that are not twisted have a lower amount of impact in accordance with the way that the DNA affects the appearance and other decisions. The DNA models that are twisted, or look like a spiraled staircase, are the ones that contain the most pertinent information. The alternation of the sugars and phosphates within the twisted DNA models combine with the nitrogen that makes up the bases of the pairs in the DNA strand. They move together in a way that causes them to become twisted.Learn more about Molecular Biology & DNA