Final answer:
The mass of DNA in the cell's nucleus when chromosomes cannot be distinguished is called chromatin. This form consists of DNA and associated proteins and occurs when the cell is not dividing. During division, chromatin condenses into visible chromosomes, each composed of two sister chromatids. The correct option is a).
Step-by-step explanation:
During the time the chromosomes cannot be distinguished, the mass of DNA in the cell's nucleus is called chromatin. Chromatin is the unwound, jumbled bunch of threads made up of DNA and proteins that are present when the cell is not dividing. It represents the chromosomes in their extended and relaxed form.
During cell division, chromatin condenses to form visible chromosomes. Histone proteins and nucleosomes play a crucial role in this DNA packaging process, with nucleosomes being the unit where DNA is wrapped around histone proteins. When chromosomes become distinguishable, they are in their compact form just before cell division.
Identical copies of chromatin held together by cohesin at the centromere are called chromatids, and can be seen during cell division. Within the nucleolus, one might find chromatin and associated histones.
Nucleosomes, which are made up of DNA wrapped around histone proteins, form a 'beads on a string' structure and are key components of chromatin. Option a) is the correct one.