Tracking chromosome number and DNA amount (2n, 4C...) through mitosis vs meiosis: where am I going wrong?
Mehak AroraDropper Sample
29 Sept 2026 · 1 min read
Sorry for the 1 am post, I can't sleep and this question is stuck in my head (sleep hygiene, I know, I know 😅).
I'm trying to track "n" (chromosome number) and "C" (DNA amount) for a human diploid cell (2n = 46, DNA content 2C in G1) at every stage. This is what I wrote:
Mitosis
- G1: 2n, 2C
- After S phase: 4n, 4C ← I'm guessing 4n?
- Metaphase: 2n, 4C
- Anaphase: 4n, 4C
- After telophase and cytokinesis: 2n, 2C
Meiosis
- After meiosis I: n, 2C
- After meiosis II: n, C
The "4n after S phase" one feels off, because I know the chromosome number doesn't change after replication. But then my table also says "4n at anaphase", and I can't reconcile the two.
So what is the rule? Do we count chromatids as chromosomes at any time? And in anaphase of mitosis, the cell has 92 of something... what is it, exactly? Please correct my table, and tell me how to avoid this in the exam. My brain is not working properly at this time of day.
7 answers & replies
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[+][–]Siddharth RaoDropperSample· 21d ago
Rule: count chromosomes by centromeres. A chromosome with two sister chromatids joined at one centromere is still 1 chromosome. So after S phase you still have 46 chromosomes (2n), each with two chromatids, and DNA amount is doubled (4C). That's the thing: DNA content changes, chromosome number does not.
Anaphase is where it changes: the centromeres split, each chromatid becomes its own chromosome, so for a moment the cell has 92 chromosomes (4n), 4C. After cytokinesis each daughter has 46 (2n), 2C.
[+][–]Mehak AroraDropperSample· 21d agoAuthor
Thank you!! "count centromeres" is the sentence I was missing. Going to sleep now. For real this time 🌙
[+][–]Siddharth RaoDropperSample· 21d ago
Good, sleep. The chromosomes will still be there tomorrow.
[+][–]Farhan ShaikhDropperSample· 21d ago
So her table: only "After S phase: 4n" is wrong. It should be 2n, 4C. Everything else is good, I checked. Meiosis part is also right. Just to complete it:
- Anaphase I: 2n, 4C (homologues separate but each stays as 2 chromatids; both sets still in the same cell)
- End of telophase I: n, 2C in each of two cells
- Anaphase II: sister chromatids separate, so 2n worth of chromosomes in the cell temporarily, 2C
- End of meiosis II: n, C
[+][–]Kavya NaikDropperSample· 21d ago
Wait, I thought after meiosis I the cells are 2n? Since the sister chromatids are still together, so it's still 46 chromatids... no sorry, hmm. In meiosis I the homologous chromosomes separate, 46 chromosomes become 23 in each cell, each with 2 chromatids. So n, 2C. OK I was confusing myself, your table is right, Mehak.
[+][–]Rahul DubeyDropperSample· 21d ago
Exam trick that I put in a corner of my notebook: "meiosis I halves the chromosome number, meiosis II halves the DNA amount." That's a one-line memory for n/C changes. Mitosis halves nothing from the parent's point of view: 2n, 2C goes back to 2n, 2C.
[+][–]Lakshmi TripathiDropperSample· 21d ago
When you see a question on "n vs C" a good first step is to draw a mini chromosome next to every row. A tiny X for replicated chromosome (2 chromatids), a tiny I for unreplicated. Count X's and I's for number, and count lines for DNA. Takes ten seconds and nobody can confuse you after that.
