Q How are the concepts of chromosome, chromatids and chromatin related? In which phase of the cell cycle does the DNA duplicate?

Chromatin is a set of filamentous the DNA molecules dispersed in the karyoplasm forming heterochromatin and euchromatin portions. Each chromatin filament is a complete chromosome a DNA molecule, or double helix the chromatin of the human somatic cell is formed by 46

The DNA molecules 22 homologous chromosomes and 1 pair of sex chromosomes in interphase the cell prepares itself for duplication and division of DNA molecules occurs. The duplication of every DNA⁰ molecule forms two identical DNA double helix bound by a structure know as centromere. In this phase each identical chromosome of these pairs is known as chromatid. It is also during the interphase that the chromatids start to condensate assuming the thicker and shorter shape typical of chromosome illustrations. So the phase of the cell cycle in which the DNA duplicates is the interphase.

Some Biology textbooks call the chromosome a unique filament of chromatin as well as the condensed structure made of two identical chromatids after the DNA duplication. Rigorously the pair of identical chromatids bound in the centromere is two copies of the same chromosome and so they are two identical chromosomes (and not only one).