​The figure represents the denaturation-renaturation profile of a double stranded DNA in citrate buffer.​

​The percent of DNA that remains denatured at 30°C after cooling from 100°C is:​

Correct option is A

Explanation-

At 30°C, compare the absorbance values for heating and cooling curves:
                                      Heating curve at 30°C: ~1.0 (fully double-stranded DNA).
                                      Cooling curve at 30°C: ~1.2 (significantly higher).
This means even after cooling from 100°C back to 30°C, the DNA has not fully renatured.

Y-axis: Absorbance at temperature T / Absorbance at 30°C
Higher absorbance indicates more single-stranded (denatured) DNA because single-stranded DNA absorbs more UV light (~260 nm).
X-axis: Temperature in °C.
Two curves are plotted:
                Heating curve: As temperature increases, DNA denatures → Absorbance increases.
                Cooling curve: As temperature decreases, DNA renatures → Absorbance decreases.

​To find the percent of DNA still denatured at 30°C after cooling:

$$\text{Denatured fraction} = \frac{\text{Abs}_{\text{cooled}} - \text{Abs}_{\text{native}}}{\text{Abs}_{\text{denatured}} - \text{Abs}_{\text{native}}}$$

Plug in the values:

$$\text{Denatured fraction} = \frac{1.05 - 1.00}{1.40 - 1.00} = \frac{0.05}{0.40} = 0.125 = 12.5\%$$

Only 12.5% of DNA remains denatured at 30°C after cooling from 100°C. This falls in the "< 25%" range.

So, the correct answer is option a - ​<25%

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