Detailed Notes on Cell Cycle and Mitosis/Meiosis

Cell Cycle Phases

• G1 Phase

  • This is the first gap phase of the cell cycle.

  • Cells grow and synthesize proteins necessary for cell division.

  • The cell prepares itself for DNA synthesis.

• S Phase (Synthesis Phase)

  • The cell's DNA is replicated, producing two copies of each chromosome.

  • By the end of this phase, the cell will have double the amount of DNA.

• G2 Phase

  • The second gap phase; further cell growth.

  • More proteins are synthesized, and the cell prepares for mitosis.

  • Any errors in DNA replication are checked and repaired.

Phases of Mitosis

• Prophase

  • Chromatin condenses into visible chromosomes.

  • The nuclear envelope breaks down.

  • Spindle fibers begin to emerge from the centrosomes.

• Prometaphase

  • The nuclear envelope completely disappears.

  • Spindle fibers attach to the kinetochores on the chromosomes.

• Metaphase

  • Chromosomes line up along the metaphase plate (cell equator).

  • This ensures each daughter cell will receive one copy of each chromosome.

• Anaphase

  • Sister chromatids are pulled apart toward opposite poles of the cell.

  • Each pole will now have an identical set of chromosomes.

• Telophase

  • Chromosomes begin to de-condense back into chromatin.

  • Nuclear envelopes re-form around each set of chromosomes.

  • Cytokinesis usually begins during this phase to split the cell into two.

Meiosis I and II

• Meiosis I

  • The first round of division producing two daughter cells with half the original chromosome number.

  • Consists of prophase I, metaphase I, anaphase I, and telophase I.

• Meiosis II

  • Follows meiosis I and resembles a typical mitosis.

  • Consists of prophase II, metaphase II, anaphase II, and telophase II.

  • Results in four unique gametes (sperm or egg cells).

Stages of Meiosis II

1. Prophase II

   • Chromosomes condense again, and a new spindle apparatus forms.

2. Metaphase II

   • Chromosomes align at the metaphase plate in each daughter cell.

3. Anaphase II

   • Sister chromatids are pulled apart toward opposite poles.

4. Telophase II/Cytokinesis II

   • Chromatids reach the poles and de-condense.

   • Nuclear envelopes reform, resulting in four haploid cells.