A sensitive instrument called a microspectrophotometer can accurately determine the exact mass of DNA in cells. In a culture of diploid cells, the instrument was used to measure DNA in the cells at various stages of the cell cycle. The lowest amount measured was 3.2 picograms (pg) of DNA, while the highest was 6.4 pg. In one cell, 4.8 pg was measured. In what stage of the cell cycle was this particular cell?
The amount of DNA in the cell was intermediate to the amount in a cell before DNA replication (3.2 pg, in G1) and a cell after DNA replication (6.4 pg, in G2). Therefore, the cell had to be undergoing replication of the DNA and therefore had to be in S phase. Cells in mitosis (M) would contain either 6.4 pg (prior to telophase) or 3.2 pg (just after telophase) of DNA, but never any amount in-between.