JEE Chemistry 2025 Question with Solution

Given: One strand of the DNA double helix has sequence

$$5^{\prime}-\mathrm{G}-\mathrm{G}-\mathrm{C}-\mathrm{A}-\mathrm{A}-\mathrm{A}-\mathrm{T}-\mathrm{C}-\mathrm{G}-\mathrm{G}-\mathrm{C}-\mathrm{T}-\mathrm{A}-3^{\prime}$$

Find: Total number of hydrogen bonds in the double helix strand.

In DNA, Guanine–Cytosine base pairs form

$$3$$

hydrogen bonds, while Adenine–Thymine base pairs form

$$2$$

hydrogen bonds.

Count the bases in the given strand:

• $\mathrm{G} = 4$
• $\mathrm{C} = 3$
• $\mathrm{A} = 4$
• $\mathrm{T} = 2$

So total hydrogen bonds are

$$(4+3)\times 3 + (4+2)\times 2$$ $$= 7\times 3 + 6\times 2$$ $$= 21 + 12$$ $$= 33$$

Therefore, the total number of hydrogen bonds is $33$.

The solution also lists the base-pairwise count:

$$3 + 3 + 3 + 2 + 2 + 2 + 2 + 3 + 3 + 3 + 3 + 2 + 2 = 33$$

Given: The sequence contains only the four DNA bases.

Find: The total number of hydrogen bonds without writing the full complementary strand.

Why this works: Every G or C in one strand contributes one G–C pair, so each contributes

$$3$$

hydrogen bonds. Every A or T contributes one A–T pair, so each contributes

$$2$$

hydrogen bonds.

Thus,

$$\text{Total bonds} = 3\times (\mathrm{G}+\mathrm{C}) + 2\times (\mathrm{A}+\mathrm{T})$$

Substitute the counts from the sequence:

$$= 3\times 7 + 2\times 6 = 21 + 12 = 33$$

Therefore, the required numerical answer is $33$.