Question

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) G1 phase.
b) G0 phase.
c) M phase.
d) G2 phase.
e) S phase.

Answer 1

Answer:

e) S phase

Explanation:

The cell cycle is divided into various phases. These are in accordance with their order:

1.$G_{1}$ phase

In this phase, the cell grows in size and doubles its organelles. This phase is in fact a preparatory phase for cell division.

2.S phase

In this phase, the cell doubles its nuclear material (DNA). As cell divides into 2 daughter cells in mitosis,  therefore it is necessary for cell to double its genetic material (DNA) which takes place in S phase. This genetic material is equally distributed among the daughter cells.

3.$G_{2}$ phase

In this phase, the cell further increases its store of proteins and organelles and prepares for the next phase i.e. M phase in which cell divides.

4.M phase

It consists of 2 processes i.e. mitosis and cytokinesis. In mitosis, the cell divides its genetic material (DNA) and in cytokinesis, the cell divides its cytoplasm resulting in 2 daughter cells.

$G_{0}$ phase

This is an inactive stage of cell cycle. In this stage cell does not divide further but remains quiescent. It performs its normal function in this phase. This phase is not a feature of all cells. Some cells like neurons (nerve cells) enter Go phase and remain there permanently whereas epithelial cells (cells lining body surfaces) do not enter Go phase and continuously divide. Cells enter Go phase after going through $G_{1}$ phase.

Back to the question,

If interval A contains half as much DNA as interval C then it implies that interval A is $G_{1}$ phase as in this phase of cell cycle, cell increases its size, and organelles but does not double its DNA content. It is further understood that interval C is $G_{2}$ phase as it contains double the DNA amount as in interval A. It can be understood that B interval is S phase, as in S phase cell doubles its DNA content and this phase occurs in between $G_{1}$ and $G_{2}$ phases.