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Molecular Basis of Inheritance — BIOLOGY STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 ScienceBIOLOGYMolecular Basis of Inheritance5 Marks
Question
Describe the molecular structure of DNA by making a labelled diagram.
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Answer

DNA is a long molecule. Its units are called nucleotides. Each nucleotide contains three compounds— pentose sugar, phosphoric acid and nitrogenous base
I. Pentose Sugar : It is a five carbon sugar which is called deoxyribose. There is one less oxygen molecule on carbon (See Fig.).
Image
II. Phosphoric Acid : This orthophosphoric acid is $H _3 PO _4$. In nucleic acids, phosphoric acid and pentose sugar are found in alternating order, that is, one molecule of phosphoric acid is found between two sugar molecules. Each phosphate group is $3^{\prime} C$ of one sugar and by connecting to $5^{\prime} C$ of the second sugar, it forms a phospho diester bond. The chain of nucleic acid starts from $5^{\prime} C$ and ends at $3^{\prime} C$ or starts from $3^{\prime} C$ and ends at $5^{\circ} C$.
III. Nitrogenous bases : These are of two types :
(i) Pyrimidine : One of these is a heterocyclic hexagonal ring which contains four carbons and two nitrogen. Two types of Pyrimidine—thymine (T) and cytosine (C) are found in DNA whereas RNA contains uracil (U) in place of thymine.
(ii) Purine : In this, an imidazole ring is combined with the fourth and fifth carbon of purine. Purines named Adenine (A) and Guanine (G) are found in both DNA and RNA.

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