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?



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.


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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.