AP Bio Study Guide

Big Idea 3.A

What are the phases of the cell cycle?

The cell cycle has five major phases: G1, S, G2, mitosis, and cytokinesis.

Interphase consists of G1, S, and G2. The G1 phase is a period of intense growth. S stands for the synthesis or replication or DNA. G2 is the phase when the cell continues to grow and prepares for mitosis. More than 90% of the life of a cell is spent in interphase. During interphase, the chromatin is threadlike, not condensed.

Mitosis is the actual dividing of the nucleus. It consists of prophase, metaphase, anaphase, and telophase.

  • Prophase — nuclear membrane begins to disintegrate, chromosomes begin to condense.
  • Metaphase — chromosomes line up at the center of the cell.
  • Anaphase — centromeres of each chromosome separate as spindle fibers pull apart the sister chromatids.
  • Telophase — chromosomes cluster at opposite ends of the cell, nuclear membrane reforms, two individual nucleoli form.

Cytokinesis is the dividing of the cytoplasm. In animal cells, a cleavage furrow forms down the middle of the cell as the cytoplasm is pinched together. In animal cells, a cell plate forms and eventually becomes a new cell wall.