If we think of the dna double helix as a twisted ladder, what makes up the rungs or steps of the ladder?

DNA refers to deoxyribo nucleic acid. It is the genetic material of most of the living organisms except for viruses which have RNA (ribo nucleic acid) as their genetic material.

The structure of DNA can best explained by the double helical model proposed by James Watson and Francis Crick in 1953. According to this model, the structure of the DNA is compared to a ladder. It is made up of two polynucleotide strands. A nucleotide refers to a sugar, phosphate and a nitrogenous base. The sugar-phosphate backbone make the sides of the ladder and the nitrogenous bases form the rungs or the steps of the ladder.

A nitrogenous base is defined a a nitrogen containing molecule exhibiting the same chemical properties as of a base. Nitrogenous bases of DNA can be classified into purines and pyrimidines. Adenine(A) and guanine(G) have two rings and they are the purines, cytosine(C) and thymine(T) are the pyrimidines with one ring.

The rungs of the ladder show the complementary base pairing between adenine and thymine, guanine and cytosine showing two and three hydrogen bonds respectively.

Hence one of the nitrogenous base along with a sugar and a phosphate make up the nucleotide of a DNA molecule. Many such nucleotides form a polynucleotide chain with nitrogenous bases resembling the rungs of the ladder and sugar and phosphate making the two sides of the ladder.

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the fossil record provides evidence of a common ancestor to many species.