Cell Cycle and Cell Division

10.1 Cell Cycle 🌱

Every living thing starts as a single cell! Cells grow and split into two “daughter cells” through cell division. This cycle of growth → division → growth again creates millions of cells from just one starter cell.

The cell cycle is the step-by-step process where a cell:

  • Copies its DNA (replication)
  • Makes new cell parts
  • Splits into two identical daughter cells

DNA copying only happens at a specific time—other times, the cell just grows. All steps are controlled by genes 🧬.

10.1.1 Phases of Cell Cycle ⏳

Two main phases:

  • Interphase (prep time—takes >95% of the cycle!)
  • M Phase (actual cell division—about 1 hour in human cells)

Interphase = “Getting Ready” Phase

Divided into 3 sub-phases:

  • 🔸 G₁ phase (Gap 1): Cell grows and is super active. No DNA copying yet!
  • 🔸 S phase (Synthesis): DNA is copied! Amount doubles from 2C → 4C. Chromosome number stays the same (e.g., 2n stays 2n).
  • 🔸 G₂ phase (Gap 2): Cell keeps growing and makes proteins needed for division.

Fun fact: Animal cells also copy their centrioles (cell “anchors”) during S phase!

G₀ Phase: The “Break Time” 🛋️

Some adult cells (like heart cells) stop dividing and enter G₀. They’re still active but take a rest unless the body needs them (e.g., to heal a wound).

10.2 M Phase ⚡

The big show! Cells divide to make identical copies (same chromosome number). Called equational division. Divided into 4 stages:

Prophase: “Let’s Get Organized!”

First stage of nuclear division. After S and G₂, the cell:

  • Condenses tangled DNA into visible chromosomes (each has 2 sister chromatids).
  • Sends duplicated centrosomes to opposite poles. They shoot out spindle fibers (like ropes!).

By the end, chromosomes are clear and the mitotic apparatus (spindle + asters) is ready!

NEET Must-Knows! 🚀

  • 💡 DNA content changes: G₁ = 2C, S = 4C, G₂ = 4C, after M = 2C (e.g., onion root tip: 16 chromosomes throughout!)
  • 💡 G₀ phase: Why some cells (e.g., neurons) stop dividing.
  • 💡 Mitosis in haploid vs. diploid cells: Animals (mostly diploid), Plants (both!), and exceptions (male honey bees 🐝).