Chapter 12: Cell Cycle
BIO 111 – Summary: The Cell Life Cycle
Key Terminology
Diploid (2n)
A diploid cell has two sets of chromosomes (one from each parent).
Example: Human body cells are diploid with 46 chromosomes (23 pairs).
Haploid (n or 1n)
A haploid cell has one set of chromosomes.
Example: Human gametes (sperm and egg) are haploid with 23 chromosomes.
The Cell Cycle – Overview
The cell cycle is the series of stages a cell goes through from one division to the next.
All cells follow the same cycle but spend different amounts of time in each stage depending on their function.
Two major parts:
Interphase – non-dividing stage (cell growth, DNA replication)
Division stage – mitosis or meiosis + cytokinesis
Function of Cell Division
Growth: allows an organism to develop from one cell to many.
Maintenance: replaces old, worn-out, or dead cells.
Example: Skin cells divide constantly.
Repair of damaged tissues.
Example: Healing a cut.
Prevents cells from becoming too large to function efficiently.
Production of gametes through meiosis.
Prokaryotic Cell Cycle
Interphase
Cell grows and duplicates its single circular chromosome.
Cell Division
Occurs by binary fission (simple splitting of the cell).
Example function: Bacteria reproduce extremely fast using this method.
Eukaryotic Cell Cycle
Interphase (Most of the cell’s life)
Cells spend 90% of their life in interphase.
Stages of Interphase
1. G1 Phase (Gap 1 / Growth 1)
Cell grows in size, performs normal functions, and makes new organelles.
Example: Liver cells actively producing proteins during G1.
2. S Phase (Synthesis)
DNA replication occurs—each chromosome is copied.
Results in two sister chromatids held together at a centromere.
Example function: Ensures each daughter cell receives identical genetic information.
3. G2 Phase (Gap 2 / Growth 2)
Final preparation before division: cell makes proteins needed for mitosis.
Example: Spindle fibers begin forming.
Cell Division (M Phase + Cytokinesis)
M Phase
Mitosis – used for growth, repair, and asexual reproduction (somatic cells).
Meiosis – produces haploid gametes (sex cells).
Karyokinesis
Division of the nucleus (occurs during mitosis or meiosis).
C Phase (Cytokinesis)
Division of the cytoplasm into two new daughter cells.
Example: In animal cells, membrane pinches inward (cleavage furrow).
G0 Phase (Optional Resting/Non-Dividing Stage)
Stage where the cell exits the cycle and stops dividing.
Cell becomes specialized (differentiates).
Some cells remain here permanently:
Example: Neurons, cardiac muscle cells.
Others can re-enter the cycle when needed:
Example: Liver cells after injury.
Cell Cycle Regulation
Controlled by checkpoints that verify:
Cell size
DNA integrity
Proper chromosome replication
Failing these checkpoints can lead to uncontrolled cell division → cancer.
Time Frame
In many adult mammalian cells:
Interphase = ~20 hours
This makes up about 90% of the entire cell cycle.