Answer: Nitrogenous bases guanine(G), adenine(A), cytosine(C) and thymine(T) forms the rungs of the ladder in twisted ladder structure of DNA.
Explanation:
Rungs of the ladder are normally horizontal small logs placed one after the other with small gaps between them on the two vertically standing logs of the ladder.
Since, DNA resembles the twisted ladder structure the rungs of the ladder structure of DNA are the organic nitrogenous basic molecules which are associated to their respective base with the help of hydrogen bonding.
In DNA, there are four bases guanine(G), adenine(A), cytosine(C) and thymine(T). These bases through hydrogen bonding are associated with there respective bases :adenine is bonded to thymine and guanine is bonded cytosine.
(A=T): Two hydrogen bonds between the bases
(C≡G): Three hydrogen bonds between the bases.
