The onion cell has 16 chromosomes before S phase. How many chromosomes are present in the onion cell after S phase?

A) 16 chromosomes
B) 32 chromosomes
C) 64 chromosomes
D) 8 chromosomes

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Solution

The correct answer is (A).

Cell cycle includes a non-dividing phase, interphase and a dividing phase, mitotic phase.
Interphase has a long duration and during this phase the cell prepares to divide by undergoing cell growth and DNA replication. Interphase is divided into $G_{1}$ , S and $G_{2}$ phases.

During S phase, there is replication of DNA which results in duplication of the genetic material. The content of DNA gets doubled in the S phase. At the end of the $G_{1}$ phase, there is one chromatid in each chromosome. At the end of S phase, each chromosome now will have two chromatids. There is no change in chromosome number. It remains constant. So, the onion cell before and after S phase will have 16 chromosomes.

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