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I know that in G1, the number of chromosomes is 23 pairs, so 46.

I assumed there were 46 chromatids too.

Why is the number of chromatids in G1 actually 0, not 46?

Thanks.

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These terms are quite similar and, for many, confusing:

  • Chromatin
  • Chromosome
  • Chromatid

But they are not synonyms. According to Molecular Cell Biology, Lodish, 4th ed:

Chromatin: Complex of DNA, histones, and nonhistone proteins from which eukaryotic chromosomes are formed.

Which brings us to:

Chromosome: In eukaryotes, the structural unit of the genetic material consisting of a single, linear double-stranded DNA molecule and associated proteins.

So, you can think of chromatin as the building material of chromosomes.

But chromatid has a very different definition:

Chromatid: One copy of a duplicated chromosome, formed during the S phase of the cell cycle, that is still joined at the centromere to the other copy; also called sister chromatid.

So, one should not use the term chromatid as synonym of chromosome. Chromatid is the name of each copy of the chromosome after the S (synthesis) phase. According to that terminology, there is no chromatid before the duplication.

I have to confess that I personally don't follow that terminology: I like to say to my students that, before S phase, each chromosome has 1 chromatid and, after the S phase, each chromosome has 2 chromatids. Even this not being the most correct terminology, it has some advantages: it makes the student understand that the number of chromosomes doesn't double after the S phase.


Source:

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    $\begingroup$ Classic example of too many names spoiling the meaning. $\endgroup$ – Roni Saiba Sep 1 '17 at 9:27

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