THE CELL CYCLE

 An Overview

The cell cycle is the sequence of events where a cell duplicates its genome, synthesizes other constituents, and eventually divides into two daughter cells.

1. Interphase (The Preparation Phase)

This is the "quiet" phase where the cell grows. It actually accounts for 95% of the total duration of the cell cycle.

• G1 Phase (Gap 1): The cell is metabolically active and grows continuously but does not replicate its DNA.

• S Phase (Synthesis): DNA replication occurs. The amount of DNA per cell doubles (from $2C$ to $4C$), but the chromosome number remains the same.

• G2 Phase (Gap 2): Proteins are synthesized in preparation for mitosis while cell growth continues.

• G0 Phase (Quiescent Stage): Cells that do not divide further exit G1 and enter an inactive stage (e.g., heart cells or nerve cells).

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2. M Phase (Mitosis)

This is the most dramatic period, involving a complete reorganization of virtually all components of the cell. It is divided into four stages of nuclear division (Karyokinesis):

Stage	Key Events
Prophase	Chromosomes condense; nuclear envelope breaks down; nucleolus disappears.
Metaphase	Chromosomes align at the metaphase plate (equator); spindle fibers attach to kinetochores.
Anaphase	Centromeres split; sister chromatids move toward opposite poles.
Telophase	Chromosomes de-condense; nuclear envelope reforms around each cluster.

Cytokinesis: The division of the cytoplasm itself. In animal cells, this happens via a furrow in the plasma membrane; in plant cells, it occurs via the formation of a cell plate.

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3. Meiosis: The Reduction Division

Meiosis is what happens in specialized cells to produce gametes (sperm and eggs). It reduces the chromosome number by half (from diploid $2n$ to haploid $n$).

Key Highlights of Meiosis I:

• Prophase I: The most complex stage. The most important sub-stage is Pachytene, where Crossing Over occurs between non-sister chromatids. This is why you don't look exactly like your siblings!

• Result: Four daughter cells are produced at the end of Meiosis II, all genetically distinct.

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Why should you care? (Significance)

1. Mitosis: Maintains the genetic continuity of organisms and helps in cell repair (like when you scrape your knee).

2. Meiosis: Maintains the constant number of chromosomes across generations and introduces genetic variation, which is the fuel for evolution.

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