Chromosome and Chromatid Counting: Mitosis and Meiosis Practice
A cell has six chromosomes before it copies its DNA. After copying, it still has six chromosomes. Later, during mitotic anaphase, the same cell briefly contains twelve. No extra DNA was made at that point. The sister chromatids separated, and each now counts as a chromosome.
That change is where chromosome and chromatid counting gets confusing. A second trap is answering for one future daughter cell when the question asks about the whole dividing cell. Write down the boundary first, then count what is inside it.
What exactly are you counting?
These examples follow ordinary diploid eukaryotic cells with normal chromosome segregation. Count nuclear DNA only, excluding mitochondrial and chloroplast DNA. A DNA molecule means one complete double-stranded DNA molecule, not each of its two strands. We use stages before or after replication, rather than trying to count molecules partway through S phase.
An unreplicated chromosome contains one such DNA molecule. A replicated chromosome has two sister chromatids, each containing one DNA molecule. The sisters are the copies produced by replication. They remain parts of one replicated chromosome until they separate. The NHGRI definition of a chromatid describes this relationship.
Homologous chromosomes are a different pair: the corresponding chromosomes inherited from the two parents. They carry corresponding genes but can carry different versions of those genes. Replicating each homolog produces two sisters per homolog. Put the two replicated homologs together and you have two chromosomes, four chromatids, and four DNA molecules.
For this article, use chromatid for a member of an attached sister pair. Once sisters separate, call them daughter chromosomes. Some worksheets also describe an unreplicated chromosome as a “one-chromatid chromosome.” Under that convention, count one chromatid per unreplicated chromosome, including each daughter chromosome after separation. That's why two answer keys can use different chromatid counts for the same stage. Check the course's convention; the chromosome and DNA-molecule counts remain the same.
The useful part of “count the centromeres”
In a standard chromosome diagram, one X-shaped structure is one replicated chromosome. Its two long halves are sister chromatids; its four visible arms aren't four chromatids.
“Count the centromeres” is shorthand for following chromosome units as sisters remain joined or separate. It doesn't mean a replicated chromosome has only one copy of its centromeric DNA. Each sister has its own centromeric region. Count the joined pair as one chromosome before separation and each separated sister as a chromosome afterward. OpenStax shows both the sister kinetochores and their separation during mitosis.
Once you've chosen the cell, nucleus, or pole you're counting, give each replicated chromosome two DNA molecules and each unreplicated chromosome one. This lets you calculate the DNA total from the structures instead of remembering a separate number for every phase.
Follow a cell with 2n = 6
Imagine a species with three kinds of chromosome. A diploid cell has two of each kind, giving 2n = 6. The haploid number is n = 3.
Start with three homologous pairs in G1, before DNA replication. There are six chromosomes and six DNA molecules. During S phase, each DNA molecule is copied. At the end of S phase, the cell has six replicated chromosomes containing twelve sister chromatids and twelve DNA molecules.
For the tables below, “per pole” means the group moving toward one end of the dividing cell. It isn't a separate cell yet. “After division” includes completed cytokinesis, the physical division of the cell.
Mitosis: sisters separate
| Stage and counting boundary | Chromosomes | DNA molecules | How to describe the structures |
|---|---|---|---|
| G1, one whole cell | 6 | 6 | Six unreplicated chromosomes |
| G2 through metaphase, one whole cell | 6 | 12 | Six replicated chromosomes; twelve sister chromatids |
| Anaphase, one whole cell, after sisters separate | 12 | 12 | Twelve daughter chromosomes |
| Anaphase, the group moving toward each pole | 6 | 6 | Six daughter chromosomes per pole |
| After mitosis and cytokinesis, each daughter cell | 6 | 6 | Six unreplicated chromosomes per cell |
The metaphase-to-anaphase transition changes 6 chromosomes / 12 DNA molecules into 12 chromosomes / 12 DNA molecules across the whole cell. You're changing how the existing DNA molecules are packaged and counted. You're not running another S phase.
Cytokinesis completes the physical split into two cells, each with six of those daughter chromosomes. If a question says “telophase,” check whether it asks about each new nucleus or the whole cell before cytokinesis finishes. In this example, each new nucleus contains six chromosomes; the still-undivided cell contains twelve across both nuclei.
Meiosis: homologs separate first
Meiosis I separates homologous chromosomes while the sister chromatids stay joined. Meiosis II separates the sisters. There is no S phase between meiosis I and meiosis II. These are the counting-relevant events in OpenStax's explanation of meiosis.
| Stage and counting boundary | Chromosomes | DNA molecules | How to describe the structures |
|---|---|---|---|
| G1 before meiosis, one whole cell | 6 | 6 | Three homologous pairs, unreplicated |
| After S phase through metaphase I, one whole cell | 6 | 12 | Six replicated chromosomes; twelve chromatids |
| Anaphase I, one whole cell | 6 | 12 | Homologs moving apart; sisters still joined |
| Anaphase I, the group moving toward each pole | 3 | 6 | Three replicated chromosomes; six chromatids |
| After meiosis I and cytokinesis, each cell | 3 | 6 | Three replicated chromosomes; six chromatids |
| Metaphase II, each of the two cells | 3 | 6 | The same three replicated chromosomes per cell |
| Anaphase II, one whole dividing cell | 6 | 6 | Six daughter chromosomes after sister separation |
| Anaphase II, the group moving toward each pole | 3 | 3 | Three daughter chromosomes per pole |
| After meiosis II and cytokinesis, each of four cells | 3 | 3 | Three unreplicated chromosomes per cell |
The answer to “How many DNA molecules are there after meiosis I?” is six per cell in this example. Each cell has three chromosomes, and each still contains two DNA molecules. Across both cells, the total is twelve. The phrase per cell does real work here.
At metaphase I, the three paired groups are called bivalents, or tetrads when emphasizing their four chromatids. Three tetrads therefore contain six chromosomes and twelve chromatids. Pairing homologs doesn't merge them into one chromosome.
Normal crossing over exchanges corresponding DNA segments between nonsister chromatids. It changes the combinations of genetic variants carried by the chromatids, without changing these completed-stage chromosome or DNA-molecule totals. Sisters needn't remain genetically identical after crossing over. OpenStax describes that exchange during prophase I.
Keep chromosome sets separate from DNA amount
Ploidy describes chromosome sets. Haploid means one set; diploid means two. NHGRI's haploid glossary entry explains the distinction. Copying DNA doesn't change a diploid cell into a tetraploid cell: G2 still has both homologs of each kind, now replicated.
If your class uses C, define 1C as the DNA amount in one unreplicated haploid chromosome set. C measures DNA amount, not the number of DNA molecules; Columbia University's meiosis notes make that distinction explicit. In our example, 1C is the combined DNA amount of three unreplicated chromosomes. Those three molecules can have different lengths, so don't assign 1C to each molecule.
For our 2n = 6 species, use these completed stages:
| Boundary | Chromosome sets | DNA amount | Chromosomes | DNA molecules |
|---|---|---|---|---|
| One G1 diploid cell | 2n | 2C | 6 | 6 |
| The same cell in G2 | 2n | 4C | 6 | 12 |
| Each cell after meiosis I and cytokinesis | n | 2C | 3 | 6 |
| Each cell after meiosis II and cytokinesis | n | 1C | 3 | 3 |
This makes a useful comparison: a G1 diploid cell and a cell just after meiosis I both contain 2C of DNA. One has six unreplicated chromosomes; the other has three replicated chromosomes. Equal DNA amount doesn't establish equal ploidy.
During anaphase, report explicit whole-cell and per-pole counts instead of trying to make a temporary whole-cell chromosome total serve as a ploidy label.
Seven counting questions to try on paper
Cover the answers while you work. For each question, write boundary → chromosome count → DNA-molecule count → reason. Add the chromatid count where the question asks for it. All seven assume normal replication and segregation.
1. Replication has finished
A diploid species has 2n = 10. One cell has completed S phase but hasn't entered mitosis. How many chromosomes, chromatids, and DNA molecules does it contain?
Answer: 10 chromosomes, 20 sister chromatids, and 20 DNA molecules. Each of the ten chromosomes now has two sisters. Doubling the DNA didn't double the chromosome count.
2. One anaphase cell, two possible boundaries
A cell from a species with 2n = 8 is in mitotic anaphase. All sisters have separated, but cytokinesis hasn't finished. Give the chromosome and DNA-molecule counts for the whole cell and for the group moving toward each pole.
Answer: The whole cell contains 16 chromosomes and 16 DNA molecules. Each pole receives 8 chromosomes and 8 DNA molecules. An answer of eight is correct only if your boundary is one pole or one future daughter nucleus.
3. Count a tetrad
A metaphase I diagram shows five tetrads in one cell. What is the species' diploid number, and how many chromatids and DNA molecules are shown?
Answer: 2n = 10, with 20 chromatids and 20 DNA molecules. Each tetrad contains two replicated homologous chromosomes: 5 × 2 = 10 chromosomes and 5 × 4 = 20 chromatids. Five is the number of homologous pairs, not the diploid number.
4. The first meiotic division is complete
For a species with 2n = 12, give the chromosome, chromatid, and DNA-molecule counts in each cell after meiosis I and cytokinesis. Are those cells haploid or diploid, and do they contain 1C or 2C of DNA?
Answer: Each is haploid, with 6 chromosomes, 12 sister chromatids, and 12 DNA molecules: n, 2C. Each cell has one homolog from each original pair, so it has one chromosome set. That set is still replicated, giving twice the DNA amount of an unreplicated haploid set. The two sisters within each replicated chromosome don't make the cell diploid.
5. Read the boundary in meiosis II
A species has 2n = 14. Look at just one of its two cells in anaphase II, after sister separation. How many chromosomes and DNA molecules are inside that one dividing cell? How many head toward each pole?
Answer: That one cell contains 14 chromosomes and 14 DNA molecules; each pole receives 7 of each. Before sister separation, the cell had 7 replicated chromosomes containing 14 DNA molecules. Counting both anaphase II cells together would give 28 chromosomes, but that isn't the requested boundary.
6. Work backward from the products
Each of the four final cells from one normal meiosis contains 9 chromosomes and 9 nuclear DNA molecules. How many chromosomes and DNA molecules were in the starting diploid cell in G1? What about metaphase I?
Answer: Here n = 9, so the starting cell had 18 chromosomes and 18 DNA molecules in G1. At metaphase I it had 18 chromosomes and 36 DNA molecules. As a cross-check, the four final cells contain 4 × 9 = 36 DNA molecules altogether, matching the post-replication starting total.
7. Find the two mistakes
A student writes: “A species with 2n = 6 has six chromosomes after meiosis I because every chromosome has two chromatids. It must copy its DNA again before meiosis II.” Correct both statements for each post-meiosis-I cell.
Answer: Each cell has 3 chromosomes containing 6 chromatids and 6 DNA molecules. Counting sisters as separate chromosomes too early produced the first mistake. There is no intervening S phase; meiosis II separates the sisters that are already present. At the end, each final cell has 3 chromosomes and 3 DNA molecules.
Turn the mistake into a small flashcard
If you missed a question, save the decision that went wrong. A card asking “Explain mitosis and meiosis” makes it hard to tell whether you can handle the specific boundary that caused the error.
| Front | Back |
|---|---|
| A 2n = 10 cell finishes S phase. How many chromosomes does it have? | 10. Replication produces sister chromatids without separating them into daughter chromosomes. |
| In mitotic anaphase, should a whole-cell chromosome count equal a per-pole count? | No. After normal sister separation, the whole cell contains twice the number moving toward either pole. |
| What separates in anaphase I? | Homologous chromosomes. Sister chromatids remain joined. |
| Is a cell just after meiosis I haploid even though its chromosomes have two chromatids? | Yes. It has one homologous set, with each chromosome still replicated. |
| Does DNA replicate between meiosis I and II? | No. Meiosis II separates the existing sister chromatids. |
Put the phase and boundary on the front of every counting card. “How many chromosomes?” is an incomplete prompt. “For 2n = 8, how many chromosomes move toward each pole in mitotic anaphase?” has a checkable answer.
You can keep these prompts on paper or add the ones you need to a Nibomo deck. Turning practice questions into flashcards explains how to preserve the reasoning without copying a whole worksheet, and making better flashcards covers keeping each prompt focused.
On your next review, change the starting chromosome number and solve a fresh question before looking at the table. Write the boundary beside the answer. If you can explain what separated and whether DNA was copied, you can rebuild the counts without memorizing a row of numbers.