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 $5^{\prime }\to 3^{\prime }$ while in the opposite one it is $3^{\prime }\to 5^{\prime }$. 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 $1^{\prime }-1^{\prime }$, $2^{\prime }-6^{\prime }$ and $6^{\prime }-2^{\prime }$. There are two such hydrogen bonds between adenine and thymine $(A=T)$ which are formed at positions $1^{\prime }-3^{\prime }$ and $6^{\prime }-4^{\prime }$. Hydrogen bonds occur between hydrogen of one base and oxygen or nitrogen of the other base.