Question

Onion has 16 chromosomes in each cell. Can you tell how many chromosomes will the cell have in G1 phase, after the S phase, and after the M phase? What will be the DNA content of the cells at G1, after S, and at G2, if the content after the M phase is 2C?

Answer 1

Onion Chromosome:

1. A healthy onion cell has 16 chromosomes (2n = 16).
2. They are relatively big, making them ideal for detecting morphological changes.

Phases:

1. G1, S, G2, and M are the four stages of the cell cycle.
2. The three phases of the interphase, where the cell prepares for division, are G1, S, and G2.
3. The mitotic phase is when the actual division takes place.

Interphase

1. The interphase period between two consecutive M phases.
2. M phase (mitotic phase): The phase in which actual cell division or mitosis occurs.
3. Interphase is the time when cells prepare for cell division.
4. Cell division and DNA replication are orderly.
5. Furthermore, it can be divided into the following three phases .
6. G1 phase (gap 1): The interval from mitosis to the start of DNA replication.
7. Cells are metabolically active and grow continuously, but do not replicate DNA.
8. S phase (Synthesis): It is the phase during which the DNA replicates and the amount of DNA per cell doubles.
9. Phase 2 (Gap 2): Proteins are generated in preparation for mitosis during this phase, while cell development continues.
10. M Phase: It is the most dramatic phase of the cell cycle, involving the reorganization of the cell components.
11. It's further broken down into four steps of nuclear division (karyokinesis).
    A figure showing the cell cycle is given below:
    

M-Phase (Mitosis):

Prophase

1. Chromosomal material condenses to form defined chromosomes.
2. Centrosomes start producing spindle fibers as they migrate towards opposite poles.
3. The nuclear envelope and other organelles start to disintegrate.

Metaphase

1. Chromosomes move to the spindle equator and align along with the metaphase plate.
2. Spindle fibers attach to the centromere of sister chromatids.

Anaphase

1. Centromeres split and chromatids separate.
2. Chromatids move towards opposite poles.

Telophase

1. The identity of chromosomes is lost when they cluster at opposite spindle poles.
2. At each pole, the nuclear membrane forms around the chromosomes, generating two daughter nuclei.
3. The number of chromosomes in onion cells in the G1 phase will be 16, after the S phase it will be 16 and after the M phase also it will be 16.
4. The DNA content of the cell in the G1 phase will be 2C, it will be 4C after the S phase and in the G2 phase, the content after the M phase is 2C.

The following table shows the number of chromosomes and DNA content in onion cells at various stages of the cell cycle.

Phase	Events occurring in the phase	Number of chromosomes	DNA content
G1	The interval between mitosis and DNA replication, cell growth	16	2C
S	DNA replication	16	4C
G2	Synthesis of proteins and cells continues to grow	16	4C
M	Mitosis, cell division	16	2C