Cell Cycle and Cell Division
QUICK REVISION NOTES & HIGH-YIELD CONCEPTS
1. Introduction to Cell Cycle
The growth and reproduction of all organisms are dependent on the division of cells. 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 the Cell Cycle.
• Cell growth (cytoplasmic increase) is a continuous process.
• DNA synthesis occurs only during one specific stage in the cell cycle.
• The replicated chromosomes (DNA) are then distributed to daughter nuclei by a complex series of
genetically controlled events.
2. Phases of Cell Cycle
The typical eukaryotic cell cycle (e.g., human cell dividing once every 24 hours) is divided into two basic
phases:
A. Interphase (95% of total cycle duration)
Also called the resting phase, it is the time during which the cell prepares for division by undergoing both cell
growth and DNA replication in an orderly manner. It is subdivided into three phases:
• G₁ Phase (Gap 1) : Interval between mitosis and initiation of DNA replication. The cell is metabolically
active and continuously grows but does not replicate its DNA.
• S Phase (Synthesis) : Marks the period during which DNA synthesis or replication takes place.
Important Note: The amount of DNA per cell doubles (from 2C to 4C), but the chromosome number
remains exactly the same (2n). In animal cells, centrioles duplicate in the cytoplasm while DNA replicates
in the nucleus.
• G₂ Phase (Gap 2) : Proteins (like tubulin) are synthesized in preparation for mitosis while cell growth
continues.
Quiescent Stage (G₀ Phase): Some cells in adult animals do not divide (e.g., heart cells, nerve cells).
Cells that do not divide further exit G₁ phase to enter an inactive stage called quiescent stage (G₀). These
cells remain metabolically active but no longer proliferate unless called upon to do so depending on the
requirement of the organism.
Biology Notes: Cell Cycle & Cell Division 1
QUICK REVISION NOTES & HIGH-YIELD CONCEPTS
1. Introduction to Cell Cycle
The growth and reproduction of all organisms are dependent on the division of cells. 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 the Cell Cycle.
• Cell growth (cytoplasmic increase) is a continuous process.
• DNA synthesis occurs only during one specific stage in the cell cycle.
• The replicated chromosomes (DNA) are then distributed to daughter nuclei by a complex series of
genetically controlled events.
2. Phases of Cell Cycle
The typical eukaryotic cell cycle (e.g., human cell dividing once every 24 hours) is divided into two basic
phases:
A. Interphase (95% of total cycle duration)
Also called the resting phase, it is the time during which the cell prepares for division by undergoing both cell
growth and DNA replication in an orderly manner. It is subdivided into three phases:
• G₁ Phase (Gap 1) : Interval between mitosis and initiation of DNA replication. The cell is metabolically
active and continuously grows but does not replicate its DNA.
• S Phase (Synthesis) : Marks the period during which DNA synthesis or replication takes place.
Important Note: The amount of DNA per cell doubles (from 2C to 4C), but the chromosome number
remains exactly the same (2n). In animal cells, centrioles duplicate in the cytoplasm while DNA replicates
in the nucleus.
• G₂ Phase (Gap 2) : Proteins (like tubulin) are synthesized in preparation for mitosis while cell growth
continues.
Quiescent Stage (G₀ Phase): Some cells in adult animals do not divide (e.g., heart cells, nerve cells).
Cells that do not divide further exit G₁ phase to enter an inactive stage called quiescent stage (G₀). These
cells remain metabolically active but no longer proliferate unless called upon to do so depending on the
requirement of the organism.
Biology Notes: Cell Cycle & Cell Division 1
