Which of the following statements are correct? Explain your answers. A. A DNA strand has a polarity because its two ends contain different bases. B. G-C base pairs are more stable than A-T base pairs.
The Correct Answer and Explanation is:
Let’s break down the two statements:
A. A DNA strand has a polarity because its two ends contain different bases.
This statement is incorrect. The polarity of a DNA strand is not because of different bases at each end, but because of the directionality of the sugar-phosphate backbone. In a DNA molecule, each strand has two ends: a 5′ (five prime) end and a 3′ (three prime) end. The 5′ end has a phosphate group attached to the fifth carbon of the sugar molecule, and the 3′ end has a hydroxyl group (-OH) attached to the third carbon. This gives the strand its polarity, meaning one end is different from the other in terms of the chemical structure, which allows DNA to be synthesized and replicated in a specific direction (from 5′ to 3′). The actual bases (A, T, C, and G) do not contribute directly to the polarity but rather to the genetic code.
B. G-C base pairs are more stable than A-T base pairs.
This statement is correct. G-C (guanine-cytosine) base pairs are more stable than A-T (adenine-thymine) base pairs because G-C pairs form three hydrogen bonds, while A-T pairs only form two. The extra hydrogen bond in G-C pairs contributes to their stronger interaction, making them more stable, especially under conditions like higher temperatures. This increased stability is one reason why regions of DNA with higher G-C content tend to have higher melting points, meaning they require more energy to separate the strands compared to regions with more A-T pairs.
In summary:
- Statement A is incorrect because DNA strand polarity arises from the sugar-phosphate backbone, not the bases.
- Statement B is correct because G-C base pairs are more stable than A-T pairs due to the additional hydrogen bond.
