Question

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

Answer 1

The correct option is A 16 chromosomes

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.