Question

The 4-amino or 4-keto group of pyrimidine bases is located in the

• A. major groove of the double stranded DNA
• B. major groove of the B form DNA but not the A form DNA
• C. minor groove of the B form DNA but not the A form DNA
• D. minor groove of the double stranded DNA

Hint:

In the major groove, the functional groups exposed are the C6 of purines and C4 of pyrimidines.
In the minor groove, the functional groups exposed are the C2 of purines and C2 of pyrimidines.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The double-helical structure of DNA has two distinct grooves running along its exterior: the major groove and the minor groove.
These grooves are formed because the glycosidic bonds connecting the base pairs to the sugar-phosphate backbone are not directly opposite each other.
The grooves expose different functional groups of the nitrogenous bases, allowing DNA-binding proteins (such as transcription factors) to recognize specific sequences without unwinding the double helix.

Step 2: Detailed Explanation:
Let us examine the structural geometry of the pyrimidine bases (cytosine, thymine, and uracil) within the double helix:
The atoms of a pyrimidine ring are numbered from 1 to 6, starting with the nitrogen atom linked to the deoxyribose sugar (N1).
The Watson-Crick hydrogen bonding interface faces inward, while other functional groups project outward into the grooves.
In a Watson-Crick base pair:
- The C2 carbonyl (keto) group of both cytosine and thymine/uracil projects toward the narrow angle of the glycosidic bonds, which faces the minor groove.
- The C4 position of the pyrimidine ring projects toward the wide angle of the glycosidic bonds, which faces the major groove.
Specifically, the 4-amino group of cytosine and the 4-keto (carbonyl) group of thymine or uracil are exposed within the major groove of double-stranded DNA.
These C4 functional groups act as hydrogen bond donors (in the case of the cytosine amino group) or acceptors (in the case of the thymine keto group).
Proteins read these chemical patterns to identify specific sequences in double-stranded DNA.
This spatial arrangement is a general feature of double-stranded DNA, including both the A and B structural forms.

Step 3: Final Answer:
The 4-amino or 4-keto groups of the pyrimidine bases are located within the major groove of double-stranded DNA.