During S phase,the DNA replicates.By  M phase each chromosome is a coiled and super coiled strands of two identical strands of DNA called sister chromatids that makes up the chromosome.During anaphasic  movement the sister chromatids separate and each of these strands of DNA is called chromosome again.

The same applies to Anaphase II in meiosis.

Thanks for your answer. Is it correct that a supercoiled structure of DNA + proteins which have a CENTROMERE is called a chromosome irrespective of a mitosis stage?

A chromosome may have a have two sister chromatids (from S phase to metaphase) and become single stranded after anaphase (from G1 to S).The best way to sort this problem out is to count the centromeres.

Thanks to all for helping clarify the issue re chromosomes.

I am not an expert on cell biology and I want to clarify the terminology associated with when chromatids transform into chromosomes. Long time ago, as a student we learned that when any piece of chromatid have their own centromere they can be termed as an individual chromosome. As Susmita says" The best way to sort this problem out is to count the centromeres". I also follow the same definition of a chromosome.

For example, at prophase chromatids already have two CENTROMERES to which spindle fibers start to attach. However we still say they are a pair of chromatids, although by counting there are two centromeres.

So what I understand from this discussion is that the definition depends on whether the chromosomes are close together and not that each have their own independent centromere. Am I right?