The purine & pyrimidine pairs of complementry strands of DNA are held together by -

Options

A. H- bonds
B. O- bonds
C. C- bonds
D. N- bonds

Answer

**Step-by-Step Solution:**

1. **Identify Purines and Pyrimidines:**
- Purines are nitrogenous bases that have a double-ring structure. In DNA, the purines are adenine (A) and guanine (G).
- Pyrimidines are nitrogenous bases that have a single-ring structure. In DNA, the pyrimidines are cytosine (C) and thymine (T).

2. **Understand the Base Pairing:**
- In DNA, adenine pairs with thymine (A-T), and cytosine pairs with guanine (C-G). This complementary pairing is crucial for the structure of DNA.

3. **Recognize the Structure of DNA:**
- DNA is a double-stranded molecule, where two strands run in opposite directions (antiparallel) and are held together by base pairing.

4. **Identify the Type of Bond Holding the Bases Together:**
- The pairs of purines and pyrimidines (A-T and C-G) are held together by specific types of bonds.

5. **Determine the Type of Bond:**
- The bonds that hold the complementary base pairs together are known as hydrogen bonds. Each A-T pair is connected by two hydrogen bonds, while each C-G pair is connected by three hydrogen bonds.

6. **Conclusion:**
- Therefore, the purine and pyrimidine pairs of complementary strands of DNA are held together by hydrogen bonds.

**Final Answer:**
The purine and pyrimidine pairs of complementary strands of DNA are held together by hydrogen bonds.

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