Base-Pair Rule in DNA: Adenine pairs with Thymine, Guanine pairs with Cytosine, held by hydrogen bonds.

Actually, there is a mistake in the text on the image that needs to be corrected. Let’s solve and explain this step by step.

---

✔ Correct Base-Pair Rule (Complementary Base Pairing)

In DNA:

- Adenine (A) pairs with Thymine (T) → via 2 hydrogen bonds
- Guanine (G) pairs with Cytosine (C) → via 3 hydrogen bonds

✔ This part of the image is correct.

---

✘ Mistake in the Image: “The sides of the DNA ladder are phosphate & sugar held together by hydrogen bonds”

This statement is incorrect.

#### 🔍 Let’s break it down:

- The sides (backbone) of the DNA double helix are made of alternating deoxyribose sugar (D) and phosphate (P) groups.
- These sugar and phosphate molecules are covalently bonded to each other — specifically, through phosphodiester bonds.
- Hydrogen bonds are what hold the nitrogenous base pairs (A-T and G-C) together across the middle of the ladder — not the sugar-phosphate backbone.

---

🧬 Corrected Statement:

> “The sides of the DNA ladder are phosphate & sugar, held together by phosphodiester bonds. The rungs (base pairs) are held together by hydrogen bonds.”

---

📌 Summary of Corrections:

| Part of Statement | Original (Incorrect) | Corrected |
|-------------------|----------------------|---------|
| Base Pairing | ✔️ Adenine ⇔ Thymine<br>✔️ Guanine ⇔ Cytosine | ✔ Correct as written |
| Backbone Bonds | ✘ “held together by hydrogen bonds” | ✔ “held together by phosphodiester bonds” |
| Hydrogen Bonds | ✘ Implied to hold backbone | ✔ Actually hold base pairs together |

---

🧪 Why This Matters:

Understanding the difference between covalent bonds (strong, in the backbone) and hydrogen bonds (weaker, between bases) is crucial for understanding:
- How DNA replicates (hydrogen bonds break easily for unzipping)
- How enzymes like DNA polymerase work
- Why mutations occur more easily in base pairing than in the backbone

---

✔ Final Answer:

The base-pair rule shown (A-T and G-C) is correct. However, the statement that “the sides of the DNA ladder are phosphate & sugar held together by hydrogen bonds” is incorrect. The sugar-phosphate backbone is held together by phosphodiester bonds, while hydrogen bonds connect the complementary nitrogenous bases (A-T and G-C) in the center of the helix.