DNA Base Pair Spacing and Helix Stability — NEET Biology
✅ Asked in NEET 2022If the length of a DNA molecule is 1.1 metres, what will be the approximate number of base pairs? (NEET 2022)
If the length of a DNA molecule is 1.1 metres, what will be the approximate number of base pairs? (NEET 2022)
Complete the flowchart on Central Dogma: (NEET 2021)
Answer & NCERT explanation
Correct answer: D — 3.3 × 10⁹ bp
Distance between consecutive base pairs = 0.34 nm. DNA length = 1.1 meters = 1.1 × 10⁹ nm. Number of base pairs = Total length ÷ Distance per bp = (1.1 × 10⁹ nm) ÷ (0.34 nm) = 3.24 × 10⁹ ≈ 3.3 × 10⁹ bp. This calculation is based on B-form DNA structure parameters from NCERT.
Read more NCERT concept on the PYQ
Consequently, the distance between a bp in a helix is approximately 0.34 nm. (v) The plane of one base pair stacks over the other in double helix. This, in addition to H-bonds, confers stability of the helical structure. Compare the structure of purines and pyrimidines. Can you find out why the distance between two polynucleotide chains in DNA remains almost constant? The proposition of a double helix structure for DNA and its simplicity in explaining the genetic implication became revolutionary. Very soon, Francis Crick proposed the Central dogma in molecular biology, which states that the genetic information flows from DNA→RNA→Protein.
NTA tests students' understanding that the distance between consecutive base pairs in DNA is 0.34 nm, which remains constant because purines always pair with pyrimidines (A-T and G-C). The common mistake is assuming all base pairs are identical in size—students forget that purine-pyrimidine pairing maintains uniform width in the double helix regardless of which bases pair together. Additionally, base stacking (van der Waals forces) plus hydrogen bonds stabilize the helix structure. To score correctly: memorize 0.34 nm for length calculations, recall that base pair complementarity ensures constant distance, and remember that multiple forces (H-bonds + base stacking) maintain helical stability.
What does NCERT say about Consequently distance between bp?
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