Question

Which bonds are indicated by X and Y in the given diagram?

	X	Y
(a)	Glycosidic bond	Hydrogen bond
(b)	Phosphodiester bond	Hydrogen bond
(c)	Glycosidic bond	Phosphodiester bond
(d)	Phosphodiester bond	Glycosidic bond

• A. a
• B. b
• C. c
• D. d

Answer 1

Answer: (B)

Given figure represents a part of $DNA$ molecule, in which phosphoric acid groups and deoxyribose sugars are linked together by means of phosphodiester bonds. The two $DNA$ chains are antiparallel, i.e., they run parallel but in opposite directions. In one chain the direction is $5' \to 3'$ while in the opposite one it is $3' \to 5'$. The two chains are held together by hydrogen bonds between their bases. Adenine $(A)$, a purine of one chain lies exactly opposite to thymine $(T)$, a pyrimidine of the other chain. Similarly, cytosine $(C)$, a pyrimidine lies opposite to guanine $(G)$, a purine. Three hydrogen bonds occur between cytosine and guanine $(C \equiv G)$ at positions $1' - 1'$, $2' - 6'$ and $6' - 2'$. There are two such hydrogen bonds between adenine and thymine $(A = T)$ which are formed at positions $1' - 3'$ and $6' - 4'$. Hydrogen bonds occur between hydrogen of one base and oxygen or nitrogen of the other base.