Number of Chromatids in G1
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:


Lodish, H. (2002). Molecular cell biology. 4th ed. New York, N.Y.: Freeman