Cell cycle

Cell Cycle Overview

  • The cell cycle is divided into Interphase and Mitotic Phase.

Interphase

  1. G1 Phase (Gap 1): Cell grows and synthesizes proteins for DNA replication.

  2. S Phase (Synthesis): DNA is replicated, resulting in two sister chromatids for each chromosome.

  3. G2 Phase (Gap 2): Further cell growth and preparation for mitosis.

Mitotic Phase

  • Mitosis: Division of the nucleus, includes prophase, metaphase, anaphase, and telophase.

  • Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

Meiosis Overview

  • Meiosis consists of two rounds of division: Meiosis I and Meiosis II.

Meiosis I

  1. Prophase I: Homologous chromosomes pair and exchange genetic material (crossing over).

  2. Metaphase I: Paired chromosomes line up at the equatorial plane.

  3. Anaphase I: Homologous chromosomes are pulled apart to opposite poles.

  4. Telophase I: Two new nuclei form; cell divides into two haploid cells (each with half the number of chromosomes).

Meiosis II

  1. Prophase II: Chromosomes condense again, and new spindle fibers form in each haploid cell.

  2. Metaphase II: Chromosomes line up at the equatorial plane of each cell.

  3. Anaphase II: Sister chromatids are separated to opposite poles.

  4. Telophase II: Nuclei reform; four haploid daughter cells are produced after cytokinesis, each genetically distinct from one another and from the original cell.

Key Differences Between Meiosis I and Meiosis II

  • Purpose:

    • Meiosis I: Reduction of chromosome number by half; homologous chromosomes separate.

    • Meiosis II: Similar to mitosis; sister chromatids separate.

  • Chromosome Configuration:

    • Meiosis I: Homologous chromosomes pair and exchange genetic material (crossing over).

    • Meiosis II: Chromosomes align individually at the metaphase plate, with no pairing.

  • End Result:

    • Meiosis I: Produces two haploid cells from one diploid cell.

    • Meiosis II: Produces four genetically distinct haploid cells from the two haploid cells created in Meiosis I.

What to Remember

  • Meiosis I is about halving the chromosome number and generating genetic diversity through crossing over.

  • Meiosis II is about separating sister chromatids and is similar to mitotic division.

  • Each of the four daughter cells from Meiosis II is unique due to the processes of crossing over and independent assortment that occur in Meiosis I.

Key Differences Between Meiosis and Mitosis

Purpose

  • Meiosis: Produces gametes (haploid cells) and reduces chromosome number by half.

  • Mitosis: Produces two identical diploid cells for growth and repair.

Processes

  • Meiosis: Involves two rounds of division (Meiosis I and Meiosis II).

  • Mitosis: Involves one round of division.

Genetic Variation

  • Meiosis: Includes crossing over during Prophase I and independent assortment; results in genetically diverse cells.

  • Mitosis: Produces genetically identical cells.

Outcome of Divisions

  • Meiosis: Results in four genetically distinct haploid cells.

  • Mitosis: Results in two identical diploid cells.

Chromosome Configuration

  • Meiosis: Homologous chromosomes pair during Meiosis I; no pairing in Meiosis II.

  • Mitosis: Chromosomes line up individually at the metaphase plate during division.