A Levels Biology (9700)•9700/12/M/J/21
Explanation
Chromosome Component Quantities in Human Cells
Steps:
- Human white blood cells are diploid with 46 chromosomes.
- Each chromosome contains 1 DNA molecule, 1 centromere, and 2 telomeres.
- Each DNA molecule consists of billions of nucleotides and is wrapped around many histone proteins (forming nucleosomes with ~8 core histones each).
- Compare quantities: histones (~240 million), telomeres (92), DNA molecules (46), centromeres (46), nucleotides (~12 billion).
Why C is correct:
- Histones vastly outnumber telomeres and DNA molecules, as each chromosome requires millions of histones for DNA packaging into chromatin, per the nucleosome structure (146 bp DNA per octamer).
Why the others are wrong:
- A: Starts with centromeres (46, few) and ends with nucleotides (most numerous), reversing the order.
- B: DNA molecules (46) are fewer than telomeres (92), violating most-to-least sequence.
- D: Ends with nucleotides (most numerous), not least.
Final answer: C
Topic: Structure of nucleic acids and replication of DNA
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