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Nucleic acids are biologically important polymers which are present in all living cells, also called as polynucleotides because repeating structural unit is nucleotide.

There are two types of nucleic acids

(i) DNA (deoxyribonucleic acid)

(ii) RNA (Ribonucleic acid)

Composition of nucleic acids

1.   A phosphate group

2.   Sugar

3.   Nitrogenous bases

1.   Phosphate Group: Phosphate group in nucleic acid is:

Phosphate group is bonded to a hydroxyl group of sugar.

2.   Sugar: Two types of sugars present in nucleic acids.

       RNA (D – ribose)

DNA (D – deoxyribose)

3.   Nitrogenous bases: Two types

(i) Purines – Adenine (A) and Guanine (G)

(ii) Pyrimidines – Cytosine (C), thymine (T) and uracil (U)

Illustration 1. Chargaff’s rule states that in an organism.

(A)  Amount of adenine (A) is equal to that of thymine (T) and the amount of guanine (G) is equal to that of cytosine (C).

(B)  Amount of adenine (A) is equal to that of guanine (G) and amount of thymine (T) is equal to that of cytosine.

(C)  Amount of adenine (A) is equal to that of cytosine (C) and the amount of thymine (T) is equal to that of guanine (G).

(D)  Amounts of all bases are equal.             

Solution:  According to Chargaff’s rule.

 Total amount of purines is equal to the total amount of pyrimidines (A + G = C + T), hence (A) is correct

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