M Phase of the Cell Cycle

Overview

• M phase is the fourth and final phase of the cell cycle.
• Comprises cell division, including:
  • Mitosis: Division of the nucleus.
  • Cytokinesis: Division of the cytoplasm.

Phases of Mitosis

1. Prophase

   • Longest phase of mitosis.
   • Chromatin condenses into chromosomes.
   • Nucleolus disappears.
   • DNA coils into sister chromatids attached at centromeres.
   • Spindle fibers form from centrioles moving to opposite poles.
   • Nuclear membrane disappears allowing spindle fibers to attach to centromeres.

2. Metaphase

   • Centrioles reach poles of the cell.
   • Spindle fibers align chromosomes along the equator of the cell.
   • Sister chromatids align on either side of the equator.

3. Anaphase

   • Spindle fibers separate sister chromatids at centromeres.
   • Chromatids are now called chromosomes.
   • Chromosomes move to opposite poles in a V-shape as spindle fibers shorten.

4. Telophase

   • Nuclear membrane reforms around chromosome sets.
   • Chromosomes disperse into chromatin.
   • Nucleolus becomes visible again.

Cytokinesis

• Final step of M phase.
• Division of the cytoplasm.
  • Animal Cells: Membrane pinches inward forming two daughter cells.
  • Plant Cells: Cell plate forms along the equator, growing until it joins the cell membrane, resulting in two daughter cells.

Key Takeaways

• M phase results in two identical daughter cells.
• Both daughter cells are identical to each other and to the original cell.
• Understand the distinction between centrioles (at poles) and centromeres (connecting chromatids).