Question 1: Which type of bonds are represented by the dotted lines in the image? (AIIMS Nov 2017)
- A. Hydrogen bond (Correct Answer)
- B. Covalent bond
- C. Ionic bond
- D. Phosphodiester
Explanation: ***Hydrogen bond***
- The dotted lines in the image represent the weak, non-covalent interactions between the **nitrogenous bases** on opposite strands of the DNA double helix.
- Specifically, these are **hydrogen bonds** formed between complementary base pairs (Adenine with Thymine via two hydrogen bonds, and Guanine with Cytosine via three hydrogen bonds).
*Covalent bond*
- **Covalent bonds** involve the sharing of electron pairs between atoms and are much stronger than hydrogen bonds.
- In DNA, covalent bonds form the **sugar-phosphate backbone** of each strand and link the nitrogenous bases to the deoxyribose sugars.
*Ionic bond*
- **Ionic bonds** involve the electrostatic attraction between oppositely charged ions, formed by the complete transfer of electrons.
- While ions (like magnesium or sodium) interact with DNA, the dotted lines specifically represent the inter-strand forces between bases, which are not ionic.
*Phosphodiester*
- A **phosphodiester bond** is a specific type of covalent bond that links the 3' carbon of one deoxyribose sugar to the 5' carbon of the next deoxyribose sugar via a phosphate group, forming the backbone of a single DNA strand.
- The dotted lines are between the two DNA strands, not within a single strand's backbone.
