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?
(ii) Also, what will be the DNA content of the cells at $G_{1}$ , after $S$ and at $G_{2}$ , if the content after $M$ phase is $2 C$ ?

Open in App

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 $G_{1}$ 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 $16.$

Step 1: $G_{1}$ phase.
This phase is also known as pre-DNA synthesis phase. In this phase, the cell prepares itself for DNA replication by forming proteins involved in the replication machinery.
Considering the DNA content present in a diploid onion cell to be $2 C,$ the DNA content per cell at $G_{1}$ phase is $2 C .$

Step 2: $S$ phase.
In this phase, the amount of DNA per cell doubles. Considering the initial amount of DNA to be $2 C$ per cell, it increases to $4 C$ after DNA replication

Step 3: $G_{2}$ phase.
Synthesis of proteins occurs in the $G_{2}$ phase to make the cell ready for M phase. Hence, the amount of DNA per cell at $G_{2}$ phase is $4 C$ .

Final answer:
The amount of DNA per cell at $G_{1}$ phase, after $S$ phase and at $G_{2}$ phase is $2 C, 4 C$ and $4 C$ respectively

Suggest Corrections

Similar questions

G_{1}

G_{1}

G_{1}

G_{2}

G_{1}

G_{1}

G_{2}

Join BYJU'S Learning Program

Explore more

Join BYJU'S Learning Program