Question

Given graphs $P, Q, R$ and $S$ show four stages of cell cycle i.e., $G_{1}, S, G_{2}$ and $M$, but in random order. Identify the stages and match with the activities of the cell.
I. Taxol treatment, which prevents microtubule depolymerization, arrests the cell at this stage.
II. With a mitogen treatment, such as an epidermal growth factor, an arrested cell at this stage proceeds to the next stage of the cell cycle.
III. The cell cycle check point at this stage confirms that $DNA$ duplication is complete before the cell proceeds to the next stage.

• A. (I) - (P), (II) - (Q), (III) - (R)
• B. (I) - (Q), (II) - (S), (III) - (R)
• C. (I) - (R), (II) - (Q), (III) - (S)
• D. (I) - (P), (II) - (S), (III) - (Q)

Answer 1

Answer: (A)

The given graphs $P, Q, R$ and $S$ illustrate the cell stages as following:
$ P - M$-phase, showing reduction in $DNA$ content from $4C$ to $2C; Q - G_{1}$ phase, the amount of $DNA$ is $2C; R - G_{2}$-phase, the amount of $DNA$ has doubled after $S$-phase i.e., $4C; S - S$-phase, showing duplication of $DNA$ content.
Taxol treatment prevents microtubule depolymerisation at $'M-$ phase'. Cell growth is assessed at $G_{1}$, checkpoint, if conditions are favourable for division, the cell begins to copy its $DNA$, initiating $S$-phase or else the cell cycle will get arrested in $G_{0}$ phase. This $G_{1}$ checkpoint is regulated by mitogens which affect cycle in - $CDK$ complexes which further perform checkpoint mechanism. The second checkpoint lies between $G_{2}$ and $M$. When the duplication is complete, Mitogen promoting factors $(MPF)$ triggers the start of $M$ phase. If this checkpoint is passed, the cell initiates the many molecular processes that signal the beginning of mitosis.