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Analysis

During DNA renaturation (as temperature decreases after denaturation):

• At high temperature, almost all DNA is single-stranded (high ssDNA concentration).

• As temperature decreases, complementary strands reassociate, so ssDNA concentration drops.

• The transition is cooperative, resulting in a sigmoidal (S-shaped) decrease in ssDNA concentration.

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Examining the Options

• Graph (1): Shows a sigmoidal decrease in ssDNA concentration as temperature decreases.

• Graph (2): Shows a sigmoidal increase, which does not fit the process.

• Graph (3): Shows a straight-line decrease, not reflecting the cooperative nature.

• Graph (4): Shows a sigmoidal increase, which is incorrect.

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Conclusion

Graph (1) best represents the process:

• Y-axis: Single-stranded DNA concentration

• X-axis: Decreasing temperature

• Curve: Sigmoidal decrease

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Correct answer:
(1)

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Keywords

DNA renaturation graph, single-stranded DNA vs temperature, DNA melting curve, sigmoidal decrease, DNA hybridization, DNA stability

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Explanation:
The concentration of single-stranded DNA decreases in a sigmoidal fashion as temperature drops and renaturation occurs, which is best illustrated by graph (1).