The nitrogenous bases point inward on the ladder and form pairs with bases on the other side, like rungs. Each base pair is formed from two complementary nucleotides (purine with pyrimidine) bound together by hydrogen bonds. The base pairs in DNA are adenine with thymine and cytosine with guanine.
The four nitrogenous bases present in DNA are adenine (A), guanine (G), cytosine (C) and thymine (T). In RNA, the only differing nitrogenous base is uracil (U) which replaces thymine in DNA and differs thymine only by the missing methyl group at carbon 5 of the pyrimidine ring. These bases are grouped as either a Purine (A and G) or a Pyrimidine (C and T in DNA and U in RNA).
The nitrogenous base is one of the three components of a nucleotide which in turn join up to form DNA. The three components of a nucleotide are a phosphate group, a pentose sugar and a nitrogenous base.
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