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?

Phases of the Cell Cycle

The cell cycle has two main phases:

1. Interphase – the period between two successive M phases, where the cell grows and prepares for division.
2. M Phase (Mitosis Phase) – the phase where actual cell division takes place.

Interphase

During interphase, the cell prepares itself by growing, metabolizing, and duplicating DNA in an orderly sequence. It is further divided into three stages:

1. G1 Phase (Gap 1):
   • Interval between mitosis and the start of DNA replication.
   • Cell is metabolically active and grows but does not replicate DNA.
2. S Phase (Synthesis):
   • DNA replication occurs, and the DNA content doubles.
   • The number of chromosomes remains the same.
3. G2 Phase (Gap 2):
   • Proteins are synthesized in preparation for mitosis.
   • Cell continues to grow.

M Phase (Mitosis)

This is the most dramatic phase, involving complete reorganization of cell components. It includes karyokinesis (nuclear division) in four stages:

• Prophase: Chromatin condenses into visible chromosomes. Centrosomes move to opposite poles and spindle fibers begin to form. Nuclear envelope disintegrates.
• Metaphase: Chromosomes align at the equatorial plate. Spindle fibers attach to kinetochores.
• Anaphase: Centromeres split, and sister chromatids separate, moving to opposite poles.
• Telophase: Chromosomes decondense at poles. Nuclear envelopes reform, and two daughter nuclei are produced. Cell organelles reappear.

Chromosome and DNA Content in Onion Cells

• In G1 phase: Chromosomes = 16; DNA content = 2C
• After S phase: Chromosomes = 16; DNA content = 4C
• In G2 phase: Chromosomes = 16; DNA content = 4C
• After M phase: Chromosomes = 16; DNA content = 2C (restored in daughter cells)