Human genetics — counting chromatids before mitosis In a human diploid somatic cell (2n = 46), how many chromatids are present just prior to mitotic division, after DNA replication has been completed?

Introduction / Context:
Chromosome and chromatid counting often confuses learners. The key is distinguishing chromosome number (defined by centromeres) from DNA content (chromatids) after S phase but before anaphase.

Given Data / Assumptions:

• Human diploid (2n) number is 46 chromosomes.
• After S phase, each chromosome consists of two sister chromatids joined at one centromere.
• Chromatid count equals 2 * chromosome count before separation.

Concept / Approach:

Post-replication, the centromere count (chromosome number) remains 46, but each chromosome now has two identical chromatids. Therefore, chromatids total 92 until sister chromatids separate at anaphase, at which point each chromatid becomes an independent chromosome in the daughter cells.

Step-by-Step Solution:

Verification / Alternative check:

Cytogenetic spreads show duplicated metaphase chromosomes as X-shaped structures with two chromatids each, totaling 92 chromatids in human somatic cells.

Why Other Options Are Wrong:

23 and 46 are chromosome counts, not chromatid counts at this stage; 69 is not a normal value; 184 doubles the correct chromatid number.

Common Pitfalls:

Equating chromosome number with DNA content; remember the definition based on centromeres for counting chromosomes and chromatids.

Final Answer:

92