Each chromosome is made up of a pair of sister chromatids which remain attached to each other at the centromere.
During DNA replication the DNA duplicates, which means that if initially in the parent cell 'n' chromosomes were present then, after duplication, the number of chromosomes in the cell will be '2n'.
As mentioned above, each chromosome has a pair of sister chromatids, that is, two chromatids per chromosome. So, 'n' chromosomes will have '2n' chromatids. Hence, '2n' chromosomes will have '4n' chromatids.
In a parent cell,
Before duplication, number of chromosome = n, number of chromatid = 2n.
After duplication, number of chromosome = 2n, number of chromatid = 4n.
The cell cycle proceeds as, G0, G1, S, G2, M.
G0 represents the interphase. It occurs before replication or duplication of the chromosomes, which occurs in the S phase.
So, in G0 phase, the chromosome number is 'n' and chromatid number is '2n'.
