15. A DNA nucleotide pair has an average mass of approximately 660 daltons. Write
down the picogram amount, rounded to the closest integer, of DNA in a diploid
human nucleus.
There are 6.4 × 109 nucleotide pairs in a human diploid nucleus
Avogadro’s number is 6 × 1023
a. 7 pg
b. 1 pg
c. 70 pg
d. 10 pg
The amount of DNA in a diploid human nucleus is 7 picograms (pg), option a. This result comes from multiplying the total nucleotide pairs by the mass per pair, then converting daltons to picograms using Avogadro’s number.
Step-by-Step Calculation
A diploid human nucleus contains 6.4 × 10⁹ nucleotide pairs, each averaging 660 daltons. Total mass in daltons equals 6.4 × 10⁹ × 660 = 4.224 × 10¹² daltons. Dividing by Avogadro’s number (6 × 10²³) gives mass in grams: 4.224 × 10¹² / 6 × 10²³ = 7.04 × 10⁻¹² g. Converting to picograms (1 pg = 10⁻¹² g) yields 7.04 pg, rounded to 7 pg .
This matches standard values for human diploid DNA mass, around 6-7 pg depending on exact base pair counts and molecular weights used. The calculation assumes average nucleotide pair mass without phosphates or waters, common in such problems.
Option Analysis
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a. 7 pg: Correct, as derived from precise computation (7.04 pg rounds to 7) .
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b. 1 pg: Too low; possibly from dividing by 10⁴ error or haploid assumption (human haploid ~3 pg).
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c. 70 pg: Overestimate; likely from misplaced decimal (×10 error) or using 6600 daltons per pair .
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d. 10 pg: Close but high; may use ~3.2 × 10⁹ haploid pairs doubled incorrectly or 1000 daltons/pair.
CSIR NET Relevance
This question tests biomolecule mass conversion and stoichiometry, key for molecular biology in CSIR NET Life Sciences. Similar problems appear in genome sizing and cell cycle DNA content. Practice reinforces unit conversions from daltons to pg for exam success.
