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Of S Phase:

Existing DNA molecule acts as a template to synthesize new DNA molecule. The cell doubles it's DNA, yet the number of chromosomes remain the same

Of Prophase:

The chromatin condenses and during condensation, DNA strands untangle to form compact mitotic chromosomes

So in the S Phase, we're talking about chromosomes which contain double the number of initial DNA molecules, while the chromosomal number still being the same. And in Prophase, there's condensation of chromosomes! But the chromosomes are already formed in the S Phase as per the sentence I have quoted from the textbook.

That doesn't make sense to me. I'm very confused.

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  • $\begingroup$ This is actually an interesting question $\endgroup$
    – Always Confused
    Apr 28, 2020 at 7:42
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    $\begingroup$ @AlwaysConfused first of, your explanations back there and here were quite wonderful. Thank you. Secondly, I have one more question. By chromosomes in the S phase, do we mean actual "rod-shaped" structures that are formed from chromatin condensation or it's just the chromatin threads taking the shape of chromosomes (I'm sorry if that sounds a bit naive). Also, can you elaborate on "two chromatids per chromosomes?" Does it have anything relating with visibility in the microscope: that chromosomes with sister chromatids are formed in the S phase itself and they're visible that way in Prophase? $\endgroup$
    – yena shah
    Apr 28, 2020 at 16:08
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    $\begingroup$ Welcome to SE Biology. When you post a question with a query about a textbook it's always useful to give details of the textbook so if one has access to it one can check for mistakes in transcription or see the context. Is there a graphic to go with this explanation? $\endgroup$
    – David
    Apr 28, 2020 at 16:22
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    $\begingroup$ @AlwaysConfused Please use comments for clarification or for checking the OP's thinking; if you have an answer to post, use the answer box. $\endgroup$
    – Bryan Krause ♦
    Apr 28, 2020 at 18:10
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    $\begingroup$ @BryanKrause I thought an answer already exists so I didn't went for a duplicate answer. but since the question is not totally a duplicate one as Maximillian said, I'll make the comment into an answer and then will delete the comments. $\endgroup$
    – Always Confused
    Apr 29, 2020 at 5:15

2 Answers 2

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A comment by user AlwaysConfused has answered one part of the question, but I think that the OP's confusion is related to a point of language or teaching. The number of chromosomes doubles during S phase (C number), but the ploidy of the cell does not change (N number). That's what your textbook is trying to tell you, it's just stating it fairly confusingly.

When we say that there are the same number of chromosomes before and after S phase, that's just a lie that we tell ourselves to keep from getting confused. We have invented the C vs. N terminology that AlwaysConfused mentions to try to handle this: we say that the 2N of a diploid cell remains constant across the cell cycle, while the C number alternates between 2C (before S phase) and 4C (after S phase).

For you, this confusion seems to be worse because how we represent chromosomes is not what chromosomes actually look like, and this has gotten mixed up with the definitions of chromosomes.

When we draw pictures of chromosomes we draw them neatly like they are in mitosis, but that is not at all what they look like in interphase. They are still chromosomes during that time, they just aren't pretty and neat ("condensed").

To sum it up more, here is a diagram of chromosomes in interphase (most of the time) vs. during mitosis: enter image description here

It is only during a very specific part of the cell cycle (mitosis) that the chromosomes are packaged up ("condensed") in such a way that we can readily see them as discrete entities. The rest of the time they're all glommed together and are organized instead according to transcriptional activity etc. rather than according to which molecule they're part of. Remember: chromosomes are very big molecules and if they are "loose" they have a total length that is many orders of magnitude larger than the diameter of the nucleus; so the nucleus is like a big bowl of noodles in interphase.

In S phase it will just be a bowl with twice as many noodles in it as before, because there are twice as many DNA molecules ("chromosomes"; "chromatid" is a term that only applies to the condensed form in mitosis).

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    $\begingroup$ Noodles. Great analogy! Thanks a lot! $\endgroup$
    – yena shah
    Apr 29, 2020 at 17:29
  • $\begingroup$ @yenashah Glad it helps- I think that using familiar objects to get a physical intuition is one of the best ways to understand biology, which is at best a pretty confusing field. $\endgroup$
    – Maximilian Press
    Apr 29, 2020 at 17:32
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For mitosis What happens, in G1 phase the DNA content is 2C (for diploid organism). The DNA content indeed gets 4C after S phase that is why you get 2 chromatids per chromosome before next metaphase. So after S phase is completed, the DNA content is actually 4C although the chromosome number is said to be 2n. after metaphase the chromosomes are split and then daughter cells get one of each sister chromatid, then each daughter cell gets DNA content of 2C.

Further reading:

Genotypes in diploid/haploid cells under mitotic/meotic cell divisions

here is a diagram from there (highly diagrammatic) about what happens with chromosome numbers during cell cycle

enter image description here

"By chromosomes in the S phase, do we mean actual "rod-shaped" structures that are formed from chromatin condensation or it's just the chromatin threads taking the shape of chromosomes?"

it is just the Chromatin threads (DNA + some proteins) loosely spread in nucleoplasm (karyolymph) in an entangled, noodle like manner. Yes, there is a visibility issue, during interphase (G1 S, G2) the euchromatin regions of the chromatin fibres stays in a decondensed form of 30 nm or somewhat more condensed 60 or 130 nm (The Cell, G.M. Cooper, ed-4) whereas maximum resolution (d) achieved by light microscope is somewhere about 200 nm

two chromatids per chromosomes

Each chromatid stands for 1 linear DNA molecule; regardless of condensed or decondensed. It is mostly decondensed in interphase, and mostly condensed in M phase (P-M-A-T).

Here is an image of 2 chromatids in 1 chromosome :

enter image description here

This image from shutterstock well explained the configuration of chromosome just after telophase (1 chromatid per chromosome) and after replication to metaphase (2 chromatids per chromosome)

enter image description here

In the following sketch; the changes through chromosome number and form is shown diagrammatically. The terms A and B here are just for labelling purpose, they aren't any technical terms.

enter image description here

image A (1-5) shows what happens wth DNA; and Image B (1-5) shows somewhat realistic appearance of chromosomes (and chromatids); during cell cycle.

see also: Interpretation of picture of human chromosomes

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  • $\begingroup$ Thank you so so much for such an amazing explanation. This made me love the topic even more. $\endgroup$
    – yena shah
    Apr 29, 2020 at 18:00

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