Final answer:
The center of the DNA molecule, the rungs of the ladder, are composed of nitrogenous bases - adenine, thymine, cytosine, and guanine.
Step-by-step explanation:
The center of the DNA molecule, the rungs of the ladder, are composed of nitrogenous bases. These bases are adenine (A), thymine (T), cytosine (C), and guanine (G). The two strands of DNA are bonded together across pairs of bases, with adenine always bonding with thymine, and cytosine always bonding with guanine.