A student was studying cell growth using cells grown in laboratory cultures. The cultures were synchronized so that all of the cells passed through the same stage of the cell cycle at the same time. The cells were examined during five different periods of time, intervals (A-E). The amount of DNA present per cell was determined for each interval. The graph shows the result of this study.

1. Most of the cells examined during interval A have half as much DNA as those measured in interval C. Therefore, most of the cells at interval B must have been in:

A. G0.

B. M phase.

C. S phase.

D. G1.

E. G2

C) S phase.

Explanation:

The cell cycle is a series of sequential events taking in a cell which represents how the cellular and nuclear contents of the cells are distributed in the new daughter cells.

The cell cycle takes place in two phases: Interphase in which cell prepares itself for nuclear division and Mitotic phase in which the cell divides its nuclear content especially the DNA. The interphase is divided into three phases that is G₁, S and G₂.

In the given question, since it is mentioned that at A interval the cell has half DNA than C, therefore, it could be concluded that a process is going on which doubles the amount of DNA in the cell which is known as DNA replication.

The DNA replication occurs during the S phase or synthesis phase thus the cell observed at interval B is in S phase.

Thus, C) S phase is the correct answer.