BIOMOLECULES AND ENZYMES

Detailed explanation-1: -Four different types of nitrogenous bases are found in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). In RNA, the thymine is replaced by uracil (U). The chemical structures of A, G, C, T, and U are shown in (Fig.

Detailed explanation-2: -Three of the four nitrogenous bases that make up RNA-adenine (A), cytosine (C), and guanine (G)-are also found in DNA. In RNA, however, a base called uracil (U) replaces thymine (T) as the complementary nucleotide to adenine (Figure 3).

Detailed explanation-3: -DNA (top) includes thymine (red); in RNA (bottom), thymine is replaced with uracil (yellow).

Detailed explanation-4: -In RNA, uracil base-pairs with adenine and replaces thymine during DNA transcription. Methylation of uracil produces thymine. In DNA, the evolutionary substitution of thymine for uracil may have increased DNA stability and improved the efficiency of DNA replication (discussed below).

Detailed explanation-5: -Uracil is one of four nitrogenous bases found in the RNA molecule: uracil and cytosine (derived from pyrimidine) and adenine and guanine (derived from purine). Deoxyribonucleic acid (DNA) also contains each of these nitrogenous bases, except that thymine is substituted for uracil.