You have studied mitosis in onion root tip cells. It has 16 chromosomes in each cell. Can you tell:
(i) How many chromosomes will the cell have at the $G_{1}$ phase, after S phase, and after M phase?

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Solution

Step 1: $G_{1}$ phase
In the $G_{1}$ phase, the cell is metabolically active.
Number of chromosomes per cell given = 16
Number of the chromosome per cell at G1 phase = 16

Step 2: S phase
S phase is also known as the synthesis phase. It is marked by DNA replication. During this time, the amount of DNA per cell doubles without any increase in the chromosome number. Number of the chromosome per cell after S phase = 16

Step 3: M phase
M phase represents the phase in which the actual cell division occurs. During mitosis, the chromosomes containing two chromatids each with one DNA molecule splits into two separate chromatids, and each chromatid behaves as a future daughter chromosome. At the end of the M phase, both daughter cells formed contain an equal number of chromosomes as was present in the parent cell. The number of chromosomes per cell at the beginning of the M phase = 16 The number of chromosomes after the M phase (here, mitosis) per daughter cell =16

Final Answer:
Onion is diploid with 16 chromosomes. During the cell cycle, the number of chromosomes at $G_{1}$ and after S are 16 each. Also, after the M phase (after mitotic cell division), the number of chromosomes in each daughter cell is 16.

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