Cell Division: Mitosis and Meiosis

Overview

• Adult human cells: Estimated 100 trillion cells; start as a single cell.
• Cell division: Essential for growth, development, and tissue repair.
• Mitosis: Process for producing identical cells for repair and growth.
• Meiosis: Process for producing genetically unique gametes for reproduction.

Mitosis

• Purpose: Replace lost cells, growth, and repair.
• DNA Replication: DNA must be replicated so each daughter cell gets an exact copy.
• Chromosome Condensation: DNA condenses around histone proteins into chromatin.
• Sister Chromatids: Replicated chromosomes.

Cell Cycle Phases

1. Interphase:
   • G1 (Gap 1): Replication of organelles and cytoplasmic components.
   • S (Synthesis): DNA replication.
   • G2 (Gap 2): Production of enzymes needed for cell division.
2. Mitosis: Consists of Prophase, Metaphase, Anaphase, and Telophase.
   • Prophase: Chromosomes condense, spindle formation begins, centrioles migrate, nuclear envelope breaks down.
   • Metaphase: Chromosomes align at the metaphase plate.
   • Anaphase: Sister chromatids separate.
   • Telophase: Cleavage furrow forms, chromosomes decondense, nuclear envelope reforms.
3. Cytokinesis: Division of the cytoplasm to form two daughter cells.

Checkpoints

• G1/S Checkpoint: Primary point where the cell cycle continues or stops.
• G2/M Checkpoint: Ensures the cell is ready for mitosis.
• Spindle Checkpoint: Ensures all chromosomes are attached to the spindle.

Meiosis

• Purpose: Production of haploid cells for reproduction, ensuring genetic diversity.
• Location: Occurs in germ cells within gonads.
• Outcome: Four genetically unique haploid cells.

Meiosis Process

1. Interphase: Similar to mitosis, with DNA replication in S phase.
2. Meiosis I: Reduction division.
   • Prophase I: Synapsis and crossing over occur, nuclear envelope fragments.
   • Metaphase I: Homologous chromosomes align independently at the metaphase plate (independent assortment).
   • Anaphase I: Homologous chromosomes separate.
   • Telophase I: Chromosomes decondense, nuclear envelope reforms, cytokinesis produces two haploid cells.
3. Meiosis II: Similar to mitosis without another round of DNA replication.
   • Prophase II: Chromatids condense, nuclear envelope disappears.
   • Metaphase II: Sister chromatids align at the metaphase plate.
   • Anaphase II: Sister chromatids separate.
   • Telophase II: Chromosomes decondense, nuclear envelope reforms, cytokinesis produces four haploid cells.

Genetic Diversity Sources

• Crossing Over: Exchange of DNA between nonsister chromatids during Prophase I.
• Independent Assortment: Random alignment and sorting of homologous chromosomes during Metaphase I.

Genetic Combinations

• Humans can produce over 8 million different unique gametes due to these processes, explaining genetic variability among siblings.