Chromosome number constancy — Within an organism, which process ensures that daughter cells maintain the same chromosome number after cell division?

Introduction / Context:
Stable inheritance requires that cells maintain the correct chromosome number after each division. The fundamental mechanism is duplication of the entire genome before segregation, ensuring each daughter cell receives a complete set. This concept underlies mitosis in somatic tissues and the orderly reduction in meiosis.

Given Data / Assumptions:

• Cell cycle includes S phase, in which DNA is replicated.
• Mitosis segregates sister chromatids equally to daughter cells.
• Meiosis halves chromosome number in gametes; fertilization restores the diploid number across generations.

Concept / Approach:
Chromosome number constancy from one cell generation to the next relies on accurate replication prior to division. While independent assortment and crossing over generate genetic diversity during meiosis, they do not ensure numerical constancy within somatic divisions. Synapsis is a meiotic pairing event, not the cause of constant number in daughter somatic cells.

Step-by-Step Solution:

Verification / Alternative check:
Cytological observations show chromosomes replicate to form sister chromatids prior to metaphase; aneuploidy arises when replication or segregation fails.

Why Other Options Are Wrong:

Common Pitfalls:
Mixing the concept of organismal ploidy restoration (meiosis + fertilization) with cellular chromosome maintenance during mitosis.

Final Answer:
DNA duplication (replication) prior to cell division.