Question

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

Answer 1

Hint: Cell division is a very important process, during which a cell grows and replicates its DNA. The sequence of events by which a cell duplicates its genome, synthesizes the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle.

Complete answer:
To answer this question we will first look at phases of the cell cycle.
The cell cycle is divided into two basic phases-
Interphase- phase between two successive M phases
M Phase (Mitosis Phase)- phase when the actual cell division or mitosis occurs
Interphase is the time during which the cell prepares itself for cell division. It undergoes cell division and DNA replication in an orderly manner. It is further divided into following three phases-
1. G1 phase (Gap 1)- It is the interval between mitosis and initiation of DNA replication. The cell is metabolically active and grows continuously but does not replicate DNA.
2. S phase (Synthesis)- It is the phase during which the DNA replicates and the amount of DNA per cell doubles. There is no increase in the number of chromosomes.
3. G2 phase (Gap 2)- In this phase proteins are synthesized in preparation for mitosis while cell growth continues.
4. M Phase
It is the most dramatic phase of the cell cycle, involving reorganization of the cell components. It is further divided into four stages of nuclear division (karyokinesis), which are-
Prophase
- Chromosomal material condenses to form defined chromosomes.
- Centrosomes move towards opposite poles and start forming spindle fibers.
- Nuclear envelope and other organelles start to disintegrate.
Metaphase
- Chromosomes move to the spindle equator and align along the metaphase plate.
- Spindle fibers attach to kinetochores of chromosomes.
Anaphase
- Centromeres split and chromatids separate.
- Chromatids move towards opposite poles.
Telophase
- Chromosomes cluster at opposite spindle poles and their identity is lost.
- Nuclear envelope develops around the chromosomes at each pole forming two daughter nuclei.
- All other cell organelles reform.
From the discussion we can answer that-
The number of chromosomes in onion cells in G1 phase will be 16, after S phase it will be 16 and after M phase also it will be 16.
The DNA content of the cell in G1 phase will be 2C, it will be 4C after S phase and at G2 phase, if the content after M phase is 2C.

Note:
Mitosis is equational division and usually restricted to diploid cells only. However, in some lower organisms haploid cells also undergo mitosis. Two diploid daughter cells having identical genomes are produced. Mitosis maintains the nuclear-cytoplasmic ratio and results in growth of multicellular organisms. It also contributes to cell repair.