Final answer:
Initially, a cell in the G1 phase with 12 chromosomes will contain 12 individual DNA molecules. After DNA replication in the S phase, the cell will have 24 chromatids (DNA molecules), still associated with the original 12 chromosomes. The G2 phase does not alter this number, and it remains the same until cell division at the end of mitosis.
Step-by-step explanation:
The question focuses on the number of chromosomes and DNA molecules (chromatids) a cell will have after progressing from G1 phase to other stages of the cell cycle. Given that a cell in G1 has 12 chromosomes, we know that this is the number of individual DNA molecules it initially has. During the S phase, which is a part of interphase, each chromosome is replicated, resulting in two sister chromatids for each original chromosome, joined together by a centromere.
By the end of the S phase, which is still before actual cell division, the cell will have 24 DNA molecules (12 pairs of sister chromatids) associated with still 12 chromosomes. During the G2 phase, the cell continues to grow but the number of chromosomes and chromatids remains the same as after the S phase because no further DNA replication occurs until the next cell cycle.
Finally, during the mitotic phase, particularly in metaphase, chromosomes align in the center of the cell and each consists of two chromatids. However, when cells divide at the end of the mitotic phase, these sister chromatids are separated, resulting in daughter cells with the original 12 chromosomes. Thus, during the mitotic phase, just before the actual cell division, the cell will still have 12 chromosomes and 24 chromatids.