Question

Which of the following best describes the $DNA$ content and the number of chromosomes at the end of $S$ and $M-$ phases of the cell cycle in mitosis, if the $DNA$ con tent of the cell in the beginning of cell cycle ($G_1$-phase) is considered as $C$ and the number of chromosomes $2N$?

• A. $2C$ and $2N$ for $S-$ phase : $2C$ and $2N$ for $M-$ phase
• B. $2C$ and $N$ for $S-$phase : $2C$ and $N$ for $M-$ phase
• C. $2C$ and $2N$ for $S-$ phase : $C$ and $2N$ for $M-$ phase
• D. $C$ and $N$ for $S-$ phase : $C$ and $2N$ for $M-$ phase

Answer 1

Answer: (A)

$\begin{array}{ccccc}DNA& G_1\to & S\to & G_2\to & M\\ \text{content}& C& 2C& 2C& C\\ \text{Number}& 2N& 2N& 4N& 2N\end{array}$
of chromosome
During cell cycle in the $G_1$ -phase, the cellular contents excluding the chromosomes are duplicated. In $S-$ phase, each of the $46$ chromosomes is duplicated by the cell but their ploidy is not disturbed.
In $G_2-$ phase, the cell double checks the duplicated chromosomes for error, making any needed repair. In $M-$ phase, the cell division occurs.