Cell Cycle Overview

Cell Cycle

• The cell cycle is a series of events in a cell preparing it to divide into two daughter cells.
• Cell division is how pre-existing cells multiply.
• Cell multiplication is crucial for embryonic development.
• Cells require division after birth for growth and replacing dead cells.
• Daughter cells must have identical chromosome number and genetic content as the parent cell (mitosis).
• Meiosis is composed of two divisions, reducing the chromosome number to half the normal amount. Genetic information in gametes produced isn't identical.

The Cell Cycle: Major Events

• Interphase: A long period of time before cell division. It includes:
  • G1 phase (Gap 1): Cell growth, synthesis of macromolecules (RNA, regulatory proteins, enzymes) needed for DNA replication. Cell volume returns to normal. Centrioles begin to duplicate.
  • S phase (Synthesis): DNA replication occurs, doubling the amount of DNA in the cell (autosomal cells go from 2n to 4n).
  • G2 phase (Gap 2): The final stage of cell growth; cell prepares for mitosis by producing proteins and energy stores.
• Mitosis: A shorter period where the nucleus divides first, then the cytoplasm, creating two daughter cells.
  • This process is also known as karyokinesis, followed by cytokinesis.
  • Mitosis contains 4 stages: prophase, metaphase, anaphase, and telophase.
• Cells that stop dividing reside in the stable phase (G0).

Mitosis Stages

• Prophase: Chromosomes condense and become visible. Centrioles move to opposite poles, the nuclear envelope breaks down.
• Metaphase: Chromosomes align along the equator of the cell.
• Anaphase: Sister chromatids separate and move to opposite poles.
• Telophase: Chromosomes reach poles, decondense, and the nuclear envelope reforms. Cytoplasm divides (cytokinesis), creating two daughter cells.

Meiosis

• A special cell division creating gametes (sperm and egg). It reduces the chromosome number from diploid (2n) to haploid (1n).
• Two rounds of divisions (Meiosis I and Meiosis II).
  • Meiosis I (Reductional division): Homologous chromosomes separate. Diploid cells become haploid.
    • Lasts a long time as homologous chromosomes pair up for crossing over. Chiasmata (crossing over sites) appear during prophase I, exchanging genetic material.
    • Prophase I: Long process, homologous chromosomes pair up and exchange genetic material (crossing-over).
    • Metaphase I: Homologous chromosome pairs line up at the metaphase plate.
    • Anaphase I: Homologous chromosomes separate and are pulled to opposite poles.
    • Telophase I: Chromosomes reach the poles, the nuclear envelope may reform, and cytokinesis may occur, splitting the cell into two.
  • Meiosis II (Equational division): Similar to mitosis. Sister chromatids separate, creating four haploid daughter cells. Each daughter cell contains a haploid number of chromosomes and has unique genetic content.
• Resulting in four daughter cells containing unique genetic material.

Key Concepts

• Chromosomes and Chromatids: Chromosomes are condensed DNA, visible during mitosis and meiosis. Sister chromatids are identical copies of a chromosome, joined at the centromere.
• Genetic Variation: Meiosis through crossing-over in prophase I, and random assortment of chromosomes during metaphase I, leads to genetic variation in gametes.