Count joined chromosomes and copied DNA separately
A replicated chromosome contains joined sister chromatids before those sisters separate. The counting convention matters. In this unit, chromosomes are counted as still-undivided centromere units before sister separation. Copying DNA creates the paired chromatids; it does not by itself perform the later separation or a meiotic reduction. Worked model: a cell begins with 4 chromosomes. At the specified stage, each has two sister chromatids joined at an undivided centromere. The chromosome count remains 4, and the chromatid count is 4×2=8. Counting 8 chromosomes at that stage would count each joined copy as an already separated unit. Counting 2 would introduce a reduction that the model never describes. Write both the stage and the convention beside your numbers. A diagram that looks like anX does not mean that there are two already independent chromosomes under this convention. Conversely, a later stage after separation needs a fresh analysis rather than automatic reuse of this result. The diagnostic uses a model species and gives all counts; it does not ask you to diagnose a human chromosome condition or infer a medical outcome. Its goal is to distinguish copying from the events that change how chromosome units are counted.
A model species has 6 chromosomes before DNA replication. After replication but before sister chromatids separate, each chromosome consists of two joined sister chromatids. Count chromosomes by their still-undivided centromeres. Which count fits this stage?
- The 6 undivided chromosome units remain 6 under the stated counting convention. Each has 2 sister chromatids, giving 6×2=12. DNA copying does not itself perform the later separation or a meiotic reduction.
Compare the other answer choices
This counts each joined sister copy as a separate chromosome before the stated centromere separation.
This introduces a halving event that the stem does not describe.
This retains the chromosome count but omits the doubling into two sister chromatids per chromosome.
Try it, then explain it.
Complete a short set without looking at the example. After each answer, explain which evidence or rule supports your decision and why the alternatives do not fit.
Sources & authorship
Original Slope explanations and examples. References support the concepts; they are not the source of copied questions. Independent educator review is pending.
NHGRI—Chromatid