DNA Packaging in Nucleus — NEET Biology
✅ Asked in NEET 2020If the distance between two consecutive base pairs is 0.34 nm and total number of base pairs is 6.6 × 10⁹, then DNA length is approximately: (NEET 2020)
If the distance between two consecutive base pairs is 0.34 nm and total number of base pairs is 6.6 × 10⁹, then DNA length is approximately: (NEET 2020)
Answer & NCERT explanation
Correct answer: A — 2.2 meters
DNA length = Number of base pairs × Distance between consecutive base pairs = 6.6 × 10⁹ × 0.34 nm = 2.244 × 10⁹ nm = 2.244 meters ≈ 2.2 meters. This calculation uses the standard B-form DNA parameters where each base pair is separated by 0.34 nm as mentioned in NCERT.
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Taken the distance between two consecutive base pairs as 0.34 nm (0.34×10⁻⁹ m), if the length of DNA double helix in a typical mammalian cell is calculated (simply by multiplying the total number of bp with distance between two consecutive bp, that is, 6.6 × 10⁹ bp × 0.34 × 10⁻⁹ m/bp), it comes out to be approximately 2.2 metres. A length that is far greater than the dimension of a typical nucleus (approximately 10⁻⁶ m). How is such a long polymer packaged in a cell?
NTA tests your ability to understand the paradox: a 2.2-meter DNA molecule must fit into a nucleus that is only 10⁻⁶ m in diameter. This requires hierarchical packaging through chromatin and chromosome structures. Students often make the mistake of thinking DNA exists as a straight, uncoiled polymer in the nucleus, or they fail to calculate the actual length correctly using the given values (6.6×10⁹ bp × 0.34×10⁻⁹ m). Remember: DNA is compacted approximately 10,000-fold through nucleosomes (beads on a string), higher-order coiling, and supercoiling to fit efficiently. This concept bridges molecular structure with cellular organization, making it a favorite for calculation-based NEET questions.
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