IB Biology Topic D2.1.13: Phases of the Cell Cycle Notes

The Cell Cycle Runs in a Fixed Order: Interphase Then M Phase

1. The cell cycle is the repeating sequence of events a cell passes through from one division to the next.
2. It has two parts: a long interphase followed by a short M phase.
3. During interphase the cell grows and copies its DNA ready to divide.
4. During M phase the cell divides into two, made up of mitosis and cytokinesis.

Interphase Is Split Into Three Stages: G1, S and G2

1. Interphase is the longest part of the cell cycle.
2. It runs in the fixed order G1, then S, then G2.
3. G1 Phase (First Gap)
   1. The cell grows in size and carries out its normal functions.
   2. It prepares the materials needed to copy its DNA in the next stage.
4. S Phase (Synthesis)
   1. This is the stage where DNA replication happens.
   2. Each chromosome is copied, forming two identical sister chromatids.
5. G2 Phase (Second Gap)
   1. The cell keeps growing and finishes preparing to divide.
   2. It makes the proteins and structures needed to separate the chromosomes in M phase.

M Phase Divides the Cell Through Mitosis Then Cytokinesis

1. M phase follows interphase and splits the cell into two.
2. Mitosis divides the nucleus so each new cell gets one identical copy of the DNA.
3. Cytokinesis then divides the cytoplasm, producing two separate daughter cells.
4. Each daughter cell can then begin the cycle again at G1.

The Cycle Seen As a Loop

1. The diagram below shows the phases as a loop, with interphase (G1, S, G2) taking up most of the cycle.
2. M phase is the short slice where the cell actually divides.