Study guides & original practice
Chromosome counting practice: find and fix the mistake
DNA replication doubles DNA molecules without doubling chromosome number. Sister separation changes the chromosome count within the dividing cell. To solve a counting question, first write the stage and whether you mean one cell, one pole or all products.
This free worksheet contains 10 original problems with explanations. It focuses on checking reasoning rather than memorizing a human-cell table. No login is needed. Review the mitosis vs meiosis comparison if the division stages are unfamiliar.
A four-line answer checklist
- Boundary: identify exactly what is inside your count.
- Stage: decide whether DNA replication or sister separation has occurred.
- Structures: an attached sister pair is one replicated chromosome containing two DNA molecules.
- Reason: name the event that changes the count.
Assume normal nuclear chromosome segregation and completed replication, with organelle DNA excluded. One DNA molecule here means one double-stranded molecule, not one strand. An X-shaped chromosome has two sister chromatids, not four. After separation, call the sisters daughter chromosomes. Some courses also use “one-chromatid chromosome” for an unreplicated chromosome; state that convention if reporting chromatids.
Worked diagnosis: three labels, one error
A species has 2n = 4. A student records these entries for a cell undergoing meiosis:
- Before replication: four chromosomes, four DNA molecules.
- After replication: four chromosomes, eight DNA molecules.
- After meiosis I, in each cell: four chromosomes, four DNA molecules.
The final chromosome count is wrong. Homologs have separated into two cells, so each contains two replicated chromosomes and four DNA molecules. The DNA count was correct. Repairing only the incorrect field matters: doubling or halving every number together would introduce another error.
For a second check, draw two X shapes in each cell. Each X is one chromosome; its two sisters explain the four DNA molecules. The two cells together still account for all eight post-replication molecules.
Ten problems with explained answers
For each item, write your answer and the evidence before opening the explanation. Questions 1–7 and 9 use the fictional 2n = 16 species; question 8 supplies its own drawing information.
1. Audit a laboratory notebook
A fictional species has 2n = 16. A notebook lists G1: 16 chromosomes, 16 DNA molecules; G2: 32 chromosomes, 32 DNA molecules. Correct the wrong entry and explain the event.
Show answer 1
G2 should say 16 chromosomes and 32 DNA molecules. S phase copies DNA, producing two attached sisters for each chromosome. The chromosome count stays 16 until those sisters separate. The G1 entry is correct.
2. Interpret a change without inventing replication
In one mitotic cell, a count changes from 16 chromosomes and 32 DNA molecules to 32 chromosomes and 32 DNA molecules. Which event explains this?
Show answer 2
Sister chromatids have separated at anaphase. Each separated sister is now a daughter chromosome. The unchanged DNA-molecule total rules out another round of replication as the explanation.
3. Repair an incomplete answer
A question asks for chromosomes in each daughter cell after normal mitosis and completed cytokinesis in that species. A student answers 32, using the previous question. Rewrite the answer.
Show answer 3
Each daughter cell has 16 chromosomes and 16 DNA molecules. The 32-chromosome figure described the entire undivided anaphase cell. Cytokinesis places half in each daughter cell; always attach a counting boundary to your answer.
4. Read a meiosis-I result
After meiosis I and cytokinesis, one cell from the 2n = 16 species contains eight replicated chromosomes. A student calls it diploid because each chromosome has two sisters. What is wrong?
Show answer 4
It is haploid: one homolog from each of the eight homologous pairs is present. It contains eight chromosomes and 16 sister chromatids, hence 16 DNA molecules. Two copies of a chromosome do not constitute two homologous sets.
5. Use a conservation check
The four final products of one normal meiosis each contain eight nuclear DNA molecules. How many were present in the original cell after S phase? State your assumption.
Show answer 5
32: four products × eight molecules each. This assumes normal segregation and completed divisions without chromosome loss or extra DNA replication. The number matches 16 replicated chromosomes in the original post-S cell.
6. Reject an impossible stage label
A normal cell is labeled metaphase II. It contains eight chromosomes and eight nuclear DNA molecules. For the 2n = 16 species, is the label consistent?
Show answer 6
No. Metaphase II should have eight replicated chromosomes and 16 DNA molecules in each cell. Eight chromosomes with eight molecules fits an unreplicated haploid product after meiosis II, but the counts alone do not prove that exact stage. A diagram or additional context is needed.
7. Separate amount from sets
Two samples have equal DNA mass. Sample A is a diploid G1 cell; sample B is a cell after meiosis I. Must they have the same chromosome count? Use 2n = 16.
Show answer 7
No. A has 16 unreplicated chromosomes; B has eight replicated chromosomes. Both have 16 nuclear DNA molecules and, in this idealized comparison, 2C DNA amount. Their chromosome-set counts differ. Equal DNA mass is insufficient to identify ploidy or stage.
8. Decode a drawing
A metaphase-I drawing contains four groups, each showing two X-shaped homologs paired together. How many chromosomes and attached sister chromatids are represented?
Show answer 8
Eight chromosomes and 16 sister chromatids. Each group is a tetrad: two replicated homologs, each with two sisters. The four groups therefore represent four homologous pairs. Count the whole chromosomes before counting their copies.
9. Name the missing boundary
An anaphase-II question for the 2n = 16 species gives the answer 16 chromosomes. When is that correct, and when is it wrong?
Show answer 9
It is correct for one whole anaphase-II cell after sister separation. Each pole receives eight chromosomes. Across both simultaneously dividing meiosis-II cells the total would be 32. The answer must specify which of those regions is being counted.
10. Write a better question
A worksheet asks: “How many chromatids are present after division?” List three details needed for a reliable answer.
Show answer 10
Specify the organism’s chromosome number, which division and whether cytokinesis is complete, and whether the boundary is one cell or all products. Also define the course’s chromatid convention: separated sisters are normally called daughter chromosomes, while some worksheets use “one-chromatid chromosome.” Prefer chromosome and DNA-molecule counts when comparing conventions.
Turn an error into your next exercise
Mark each answer for both its count and its explanation. A correct number without a correct boundary needs another attempt.
- Errors on 1–3: draw the same chromosome before replication, after replication and after separation.
- Errors on 4, 7 or 8: label homologs and sisters with different symbols. Explain which relationship determines chromosome sets.
- Errors on 5, 6 or 9: write a per-cell total and an all-products total side by side.
- Errors on 10: rewrite an ambiguous question so another student can answer it without guessing.
Then repeat questions 1–5 for a species with 2n = 20. Your check values are: G2 20 chromosomes/40 molecules; mitotic anaphase whole cell 40/40; each mitotic daughter 20/20; each meiosis-I cell 10/20; four final products total 40 molecules. Explain each change before checking these numbers.
Save your corrected prompts in the free manual flashcard maker. Include the stage and boundary on the question side so the answer stays testable.
Sources and related study
Check the underlying processes in OpenStax Biology 2e: the cell cycle and meiosis. The numerical cases, notebook audit and explanations above are original practice, not official exam questions.
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