Nucleotide Structure and Components — NEET Biology
✅ Asked in NEET 2019Let us recapitulate the chemical structure of a polynucleotide chain (DNA or RNA). A nucleotide has three components – a nitrogenous base, a pentose sugar (ribose in case of RNA, and deoxyribose for DNA), and a phosphate group. There are two types of nitrogenous bases – Purines (Adenine and Guanine), and Pyrimidines (Cytosine, Uracil and Thymine). Cytosine is common for both DNA and RNA and Thymine is present in DNA. Uracil is present in RNA at the place of Thymine. A nitrogenous base is linked to the OH of 1'C pentose sugar through a N-glycosidic linkage to form a nucleoside, such as adenosine or deoxyadenosine, guanosine or deoxyguanosine, cytidine or deoxycytidine and uridine or deoxythymidine. When a phosphate group is linked to OH of 5'C of a nucleoside through phosphoester linkage, a corresponding nucleotide (or deoxynucleotide depending upon the type of sugar present) is formed. Two nucleotides are linked through 3'-5' phosphodiester linkage to form a dinucleotide. More nucleotides can be joined in such a manner to form a polynucleotide chain. A polymer thus formed has at one end a free
NTA tests whether students can identify the three components of a nucleotide (nitrogenous base, pentose sugar, phosphate group) and distinguish between purines and pyrimidines. Students often confuse which bases are purines (Adenine, Guanine) versus pyrimidines (Cytosine, Thymine, Uracil), or forget that Thymine appears only in DNA while Uracil appears only in RNA. The key is memorizing: purines have two rings, pyrimidines have one ring; DNA has Thymine, RNA has Uracil instead. Understanding 3'-5' phosphodiester linkages and N-glycosidic bonds between components is crucial for answering structure-based questions in NEET.
Purines found both in DNA and RNA are: (NEET 2019
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