Cell Cycle, Mitosis, and Meiosis Notes

Cell Cycle Overview

Main Stages of the Cell Cycle

• Mitosis: Division of the nucleus, leading to two identical daughter cells.
• Meiosis: Reduction division forming gametes (sperm and eggs) with half the chromosome number.

Requirements for Cell Growth

• Cell Mass Increase: Essential for division.
• Growth and Metabolism: Supports cellular activities.
• DNA Duplication: Ensures two identical copies of chromosomes before division.

Phases of the Cell Cycle

• Interphase (G1, S, G2)
  • G1 Phase: Growth phase, where the cell grows and prepares for DNA synthesis.
  • S Phase: DNA is replicated; centrosome is also duplicated. Each chromosome now consists of two sister chromatids.
  • G2 Phase: Preparatory phase for mitosis.
• M Phase: Mitosis and cytokinesis occur, wrapping up the cell division process.

Detailed Phases of Mitosis

1. Prophase: Chromatin condenses into visible chromosomes; spindle apparatus forms.
2. Prometaphase: Nuclear envelope breaks down; spindle fibers attach to kinetochores on chromosomes.
3. Metaphase: Chromosomes align at the metaphase plate.
4. Anaphase: Sister chromatids separate and move towards opposite poles of the cell.
5. Telophase: Chromosomes de-condense back into chromatin; nuclear envelope re-forms around separated sets of chromosomes. Cytokinesis completes the division into two identical daughter cells.

Key Transition Points in the Cell Cycle

• G1 Checkpoint (Restriction Point): Critical checkpoint for determining if the cell will proceed to S phase.
  • Factors influencing this checkpoint include growth factors, nutrients, cell size, and DNA damage.
• G2 Checkpoint: Ensures everything is ready for mitosis, checking DNA integrity and completeness of replication.
• Metaphase Checkpoint: Ensures all chromosomes are properly attached to the spindle before proceeding to anaphase.

Differences Between Mitosis and Meiosis

• Purpose:
  • Mitosis: Produces two identical somatic cells.
  • Meiosis: Produces four genetically diverse haploid gametes.
• Cells Involved:
  • Mitosis: Occurs in all growing cells.
  • Meiosis: Occurs in germ cells.
• Chromosome Number:
  • Mitosis: Diploid cells (2n) resulting in diploid daughter cells.
  • Meiosis: Starting diploid cell undergoes two divisions to result in haploid gametes (1n).

Chromosome Structure and Terminology

• Chromatin: Loosely packed DNA present during interphase.
• Chromosomes: Tightly coiled structures, visible during mitosis and meiosis.
• Sister Chromatids: Two identical copies of a chromosome joined at the centromere after replication.
• Diploid vs. Haploid:
  • Diploid (2N): Cells with two sets of chromosomes.
  • Haploid (1N): Cells with one set of chromosomes; typical for gametes.

Genetic Recombination and Independent Assortment in Meiosis

• Crossing Over: Occurs during Prophase I of meiosis; exchange of genetic material between nonsister chromatids, leading to genetic variation.
• Independent Assortment: Random distribution of homologous chromosomes during metaphase I results in varied gametes.
• Significance of Genetic Variation: Promotes biodiversity and evolutionary adaptability.

Summary Points

• Mitosis Results: Two identical 2N daughter cells, exactly the same DNA as the parent cell.
• Meiosis Results: Four genetically different 1N cells (gametes), essential for sexual reproduction.