A eukaryotic cell has a haploid number of n = 12. What is the total number of sister chromatids present in a single cell at the end of the S phase?

Core Concept

PILLAR 1 — MOLECULAR/CONCEPTUAL MECHANISM

Step-by-Step Analysis

During the S phase of interphase, eukaryotic cells execute a tightly regulated program of semi-conservative DNA replication. The molecular machinery responsible—including origin recognition complexes (ORCs), helicase, DNA polymerase δ/ε, and sliding clamp PCNA—unwinds the double helix at thousands of replication origins and synthesizes new complementary strands. The result is that every linear chromosome in the nucleus becomes duplicated into two identical DNA molecules called sister chromatids. These sister chromatids remain physically tethered at the centromere by cohesin protein complexes (specifically, the SMC1/SMC3 heterodimer bound to RAD21). Cohesin establishes topological entrapment of both sister DNA molecules, resisting the pulling forces generated by spindle microtubules until anaphase. This molecular tethering is the structural reason we count replicated chromosomes as single units (one chromosome, two sister chromatids) rather than as independent chromosomes. The cell also employs checkpoint kinase CHK1 and the ATR-ATRIP sensor complex to monitor replication fork progression; S phase does not conclude until every origin has fired and all replication forks have converged, ensuring that the full complement of genetic material has been copied before the cell transitions into G₂.

Why Other Options Are Wrong

The distinction between chromosome number and chromatid number is therefore rooted in DNA replication mechanics and cohesin-mediated cohesion. A diploid somatic cell (2n) carries two complete sets of homologous chromosomes—one set maternally derived, one paternally derived. After S phase, each homolog exists as a pair of sister chromatids. The total chromatid count equals twice the diploid chromosome number because every single chromosome, regardless of its homologous pairing status, has been duplicated.

PILLAR 2 — STEP-BY-STEP LOGIC

The stimulus provides a haploid number of n = 12. This means that one complete set of chromosomes in this organism contains 12 unique chromosomes, each carrying distinct genes at specific loci. A eukaryotic somatic cell in G₁ phase is diploid, possessing two homologous sets: 2n = 24 total chromosomes (12 maternal homologs + 12 paternal homologs).

When S phase concludes, DNA replication has produced an identical sister chromatid for every one of those 24 chromosomes. Therefore, the calculation proceeds as follows: 2n = 24 chromosomes → each replicated into 2 sister chromatids → 24 × 2 = 48 sister chromatids total. The correct answer is 48 (Option C).

A critical nuance: the number of centromeres has not changed. There are still 24 centromeric regions, each now bearing two chromatids. Anaphase of mitosis will later separate these sister chromatids into 48 independent chromosomes (24 per daughter cell), but at the end of S phase, cohesion remains intact.

PILLAR 3 — DISTRACTOR ANALYSIS

Option A (12) traps students who conflate the haploid number n directly with the chromatid count, ignoring both the diploid condition (2n) and the effect of replication. This reflects a fundamental confusion between ploidy levels and the structural product of S phase.

Option B (24) captures two common errors simultaneously. First, a student may correctly identify the diploid chromosome number (2n = 24) but fail to account for replication, reporting the chromosome count rather than the chromatid count. Alternatively, a student may reason that n = 12 chromosomes replicate into 24 chromatids, forgetting that the cell is diploid, not haploid, and thus only half the genetic material was considered.

Option D (96) ensnares students who overcount by doubling the correct answer—perhaps reasoning that 2n = 24 chromosomes become 48 chromatids, and then mistakenly doubling again. This error often stems from conflating the sister chromatid count with the total number of DNA molecules in a tetrad (bivalent) during meiosis I, where homologous pairing creates structures with four chromatids. Applying meiotic tetrad logic to a mitotic S-phase context introduces an unwarranted extra multiplication step.