S-Phase, Mitosis, Meiosis - Chromosome/Chromatid Count

[theyellowking]

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Hey, everyone. Although this question must be asked frequently, I am still confused. I've looked at not only the past threads from this website, but also other websites that give either the same or different answers.

- For S-Phase, when the DNA is replicated, I know that the chromosome # (46) stays the same, but that the chromatid number is to 2. Does this mean that before the S-Phase, the chromatid # was 1, that the chromatid can be considered the chromosome itself?

- For the Mitosis phase, would the chromosome count technically be 92 during the anaphase since technically a complete separation of cells hasn't occurred yet? (I am aware that at the telophase, each cell has 2n = 46 chromosomes)

- For the prophase of Meiosis I, would there still be 92 chromatids (And would this = 92 tetrads?)?
- For the anaphase of Meiosis I, would the same situation occur where there would be technically 92 chromosomes? (I am aware that at telophase I, each of the two cells is 1n = 23)
- Would it be at the anaphase II of Meiosis where the chromatids are now considered chromosomes?

I apologize for asking what's seemingly a simple question, but the EK textbook does not cover these types of questions, and as I said earlier, the internet is providing me a mixed bag of answers

 

[Czarcasm]

So a human somatic or germ line cell has 46 chromosomes, 23 from mom, 23 from pops. Together, we have 23 pairs (22 autosomal chromosomes, 1 sex chromosome). These pairs are referred to as homologous chromosomes. During S phase, realize that this is when replication is occuring. So the 23 pairs of chromosomes are each being replicated. By the end of S phase, each chromosome is now duplicated and we refer duplicated DNA (for a given chromosome) as sister chromatids. Generally, the word chromatid applies to duplicated DNA, so I'm not entirely sure if you would necessary refer to each chromosome as a single chromatid before S-phase. Generally the terminology is referring to duplicated chromosomes. A more likely comparison that I see frequently asked is what the chromosomal number is during the S-phase. Some people make the mistake of saying 92, when in fact it's still 46. The fact that each chromosome is duplicated does not change the chromosome count.

This is also true during anaphase of mitosis. Each individual chromosome (all 46) lines up during metaphase, and by anaphase, the sister chromatids are pulled apart. We still have 46 chromosomes by the time the cell separates, but the replicated DNA is basically split in two.

By comparison, during meiosis (specifically anaphase I and II), during anaphase 1, homologous chromosomes pair up, and their sister chromatids (the duplicated DNA) can cross over. In this case, we have a line of 23 pairs along the metaphase plate. By anaphase I, each homologous chromosome is pulled apart. So by the end of anaphase I, we actually end up with 23 chromosomes (and therefore the cell is haploid). By anaphase II, we are pulling apart the recombinant chromatids - I avoid saying 'sister' chromatids here simply because they are no longer identical due to recombination, but the idea is, the replicated chromosomes are now separated apart.

 

[theyellowking]

Ah, thank you so much. This will make me sleep easier 🙂