Mitosis and cell cycle?

Question

Ok, I understand mitosis very clearly except for one significant detail. I think the problem is that the text never describes the diagrams. So after S phase, the cell has a complete replication of it's DNA, which is 23 pairs of chromosomes (in human) so 46 individual chromosomes (or in this case are the individuals called chromatids?). So after S phase, does that mean there are 92 chromatids total which will then align during mitosis to split into 46 and 46 (2 sets of 23 per daughter cell)? because in the text I'm reading each daughter cell then has 2N (a diploid) which would mean 46 (or a pair of 23). I think the confusing thing is that text books keep changing their terms as you read, from 23 sets to 46 total.

Also, to put it to diagram let's say we have this ----> ><

Is that considered 1 of the 23 chromosomes? so there are 23 ">" and 23 "<" (which I understand are all different from one another but for the point I'm asking, just say they look the same)

Would > be one chromatid and >< be the set of chromosomes?

And that would mean right after S phase there would be 46 "><" correct?

I hope after that ramble, you understand where my confusions lie and can help me sort it out for me. That would be very appreciated!

Answer 1

It is not hard to understand it at all.

First, we have 46 chromosomes, which consist of 23 pairs (for each pair, one was inherited from father, one from mother).

Second, each of the 46 chromosomes is attached to a centromere. So 1 centromere is counted as one chromosome. When chromosomes replicate themselves, instead of double from 46 to 92, the new copies are attached to the same centromeres as the old one, so each centromere has 2 copies attach to them, and each copy is call a sister chromatid. (They can&#039;t be called 2 chromosomes, because they are sharing the same centromere).

So, if you are counting for chromosomes itself, you are counting the # of centromeres. If you are counting the # of complete copies of the genome, you are counting the # of sets of sister chromatids.

Using your graph,

During interphase, each chromosome looks like &gt;, and there are 46 of them, 23 pairs, 2 complete copies of the genome (diploid).

During S phase, each one become &gt;&lt;, 2 sister chromatids on each centromere, so the total # of chromosomes are the same, but instead of 2 complete copies of the genome, we have 4 now.

and during anaphase, the sister chromatids are separated from each other, pulled to opposite pole of the cells. Each of the daughter cell has 2 complete sets of the genome, and they are exactly the same as the mother cell.

I hope this helps.

Answer 2

I had just this same problem some years ago, it&#039;s actually confusing if you don&#039;t get it right from the start.

You said it correctly. We have 23 diploid chromosomes, meaning 23 unique chromosomes (or chromatids) that happen to have a &quot;copy&quot; of themselves in the cell (which, in our case -humans- happens to be, one of  our mother, the other from our father, being that the reason for me to say &quot;copies&quot; and not just copies, because they might, and probably will, have differences)

So, to summarize.

We have 23 unique chromosomes, some times called chromatids (specially when referring to one of the &quot;unique copies&quot;)

We have 46 chromatids, being that, 2 of each &quot;unique&quot; of them in each diploid chromosome

When the cell is about to go into division, it has to have a copy of each unique chromatid (or chromosome), reason for which the cell ends up with 92 chromosome (this not being the case with meiosis, in which the division ends up with just 23 chromatids, and not the complete pair of chromosomes)

Hope that helps

Answer 3

I agree - it is really confusing, but you just have to remember the following:-

1.After S-phase, when the DNA has doubled each of the 46 chromosomes is made of 2 chromatids (2 DNA molecules).(92 chromatids in total).

2.After mitosis each chromosome is split into 2 separate chromatids.(One of each in each of the two cells formed by division).(46 per cell).

3.These chromatids are now called the daughter chromosomes.

4. These then become the double chromosomes again (2 chromatids) after S-phase.

5. And the cycle continues with each cell division.