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The percentage of nitrogen bases in four different dsDNA samples are given. Study the given data and choose the option that shows the mutated DNA sample.

NUCLEIC ACIDS
NEET
1

A = 32%, C = 18%, T = 32%, G = 18%

2

G = 24%, A = 26%, T = 26%, C = 24%

3

C = 20%, T = 28%, G = 24%, A = 28%

4

T = 17%, G = 33%, A = 17%, C = 33%

Solution:

According to Chargaff's rules for double-stranded DNA (dsDNA), the amount of adenine (A) is always equal to the amount of thymine (T), and the amount of guanine (G) is always equal to the amount of cytosine (C). That is, A=T and G=C.

Let's check each option:

  1. A = 32%, T = 32%, C = 18%, G = 18%. Here, A=T and G=C. This is a normal dsDNA sample.

  2. A = 26%, T = 26%, C = 24%, G = 24%. Here, A=T and G=C. This is a normal dsDNA sample.

  3. A = 28%, T = 28%, C = 20%, G = 24%. Here, A=T (28%=28%), but C is not equal to G (20% ≠ 24%). This violates Chargaff's rule, indicating a mutated DNA sample.

  4. A = 17%, T = 17%, C = 33%, G = 33%. Here, A=T and G=C. This is a normal dsDNA sample.

Therefore, option (3) represents a mutated DNA sample.

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