Q.26 The 4-amino or 4-keto group of pyrimidine bases is located in the
(A) major groove of the double stranded DNA
(B) minor groove of the double stranded DNA
(C) minor groove of the B form DNA but not the A form DNA
(D) major groove of the B form DNA but not the A form DNA
Location of 4-Amino or 4-Keto Group in Pyrimidine Bases of DNA
The 4-amino or 4-keto group of pyrimidine bases, such as those in cytosine, thymine, and uracil, resides in the major groove of double-stranded DNA. This positioning enables specific interactions with proteins like transcription factors. The correct answer to the query is option (A).
Correct Answer Explanation
In B-form DNA, the predominant structure in cells, pyrimidine bases (cytosine and thymine) position their C4-amino (in cytosine) or C4-keto (in thymine) groups toward the major groove. The major groove, wider at about 22 Å, exposes these functional groups for hydrogen bonding with regulatory proteins. This accessibility distinguishes sequence-specific recognition sites.
Option Analysis
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(A) Major groove of the double stranded DNA: Correct, as C4 and C5 of pyrimidines face the major groove, allowing H-bonding with proteins.
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(B) Minor groove of the double stranded DNA: Incorrect; the minor groove (12 Å wide) primarily exposes N3 and the 2-position keto/amino groups of pyrimidines.
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(C) Minor groove of the B form DNA but not the A form DNA: Incorrect; the 4-position group stays in the major groove across forms, though groove dimensions vary (A-form has a deeper minor groove).
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(D) Major groove of the B form DNA but not the A form DNA: Incorrect; major groove exposure persists in A-form, despite conformational shifts.
DNA Groove Recognition in Molecular Biology
DNA grooves facilitate protein-DNA interactions critical for gene regulation. Purine N7 and C6, plus pyrimidine C4/C5, project into the major groove for precise binding. Minor groove binders, like some antibiotics, target narrower edges. Understanding this aids biotechnology applications, such as designing CRISPR guides or enzyme inhibitors.
