1. The Cell Cycle and Mitosis

Genetic Material

The genetic material of eukaryotic cells is organized into chromosomes, which are composed of DNA and associated proteins. Understanding the differences between chromosome structures is essential for grasping cell division processes.

• Chromosome vs. Chromatid: A chromosome is a single DNA molecule with associated proteins. After DNA replication, each chromosome consists of two sister chromatids joined at a centromere.

• Homologous Chromosomes: Chromosomes that have the same genes at the same loci but may have different alleles. One is inherited from each parent.

• Somatic Cells vs. Gametes: Somatic cells are body cells and are diploid (2n), containing two sets of chromosomes. Gametes (sperm and egg) are haploid (n), containing one set of chromosomes.

• Human Chromosome Number: Human somatic cells have 46 chromosomes (23 pairs); human gametes have 23 chromosomes.

• Definition of Mitosis: Mitosis is the process by which a cell divides its nucleus and genetic material to produce two identical daughter cells.

• Additional info: Chromatin is the less condensed form of DNA found during interphase; chromosomes are the condensed form visible during mitosis.

Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It consists of interphase (G1, S, G2) and the mitotic phase (mitosis and cytokinesis).

• Phases of the Cell Cycle:

  1. G1 phase: Cell growth

  2. S phase: DNA replication

  3. G2 phase: Preparation for mitosis

  4. M phase: Mitosis and cytokinesis

• Phases of Mitosis:

  1. Prophase: Chromosomes condense, spindle forms

  2. Metaphase: Chromosomes align at the cell equator

  3. Anaphase: Sister chromatids separate

  4. Telophase: Nuclear envelopes reform

  5. Cytokinesis: Division of the cytoplasm

• Comparison: Cytokinesis differs between animal and plant cells. Animal cells use a cleavage furrow; plant cells form a cell plate.

• Bacterial Cell Division: Bacteria divide by binary fission, not mitosis.

• Additional info: Binary fission is a simpler process involving DNA replication and division without mitotic spindle formation.

Cell Cycle Control

Cell cycle progression is tightly regulated by checkpoints and growth factors to ensure proper division and prevent errors.

• Checkpoints: The cell cycle has key checkpoints (G1, G2, M) that assess whether the cell is ready to proceed. For example, the G1 checkpoint checks for DNA damage before replication.

• Growth Factors: Proteins that stimulate cell division. They bind to receptors and trigger signaling pathways that promote progression through the cell cycle.

• Cancer: Uncontrolled cell division due to failure of cell cycle control mechanisms can lead to cancer.

• Additional info: Cyclins and cyclin-dependent kinases (CDKs) are key regulators of the cell cycle.

2. Meiosis and Sexual Life Cycles

Chromosomes

Meiosis is a specialized type of cell division that reduces the chromosome number by half, producing haploid gametes. This process is essential for sexual reproduction.

• Diploid vs. Haploid: Diploid (2n) cells have two sets of chromosomes; haploid (n) cells have one set.

• Homologous Chromosomes: Pairs of chromosomes with the same genes but possibly different alleles.

• Sister Chromatids: Identical copies of a chromosome connected at the centromere.

• Sex Chromosomes: Chromosomes that determine sex (X and Y in humans).

Sexual Life Cycles

Sexual life cycles vary among organisms, but all involve meiosis and fertilization. These cycles ensure genetic diversity in offspring.

• Life Cycles in Plants/Fungi vs. Animals:

  • Animals: Gametes are the only haploid cells; fertilization restores diploidy.

  • Plants/Fungi: Alternation of generations; both haploid and diploid multicellular stages exist.

• Major Phases of Meiosis:

  1. Meiosis I: Homologous chromosomes separate, reducing chromosome number by half.

  2. Meiosis II: Sister chromatids separate, similar to mitosis.

• Comparison to Mitosis: Meiosis involves two divisions and results in four non-identical haploid cells; mitosis produces two identical diploid cells.

• Additional info: Crossing over during meiosis I increases genetic variation.

Variation

Sexual reproduction introduces genetic variation through several mechanisms, which are crucial for evolution and adaptation.

• Major Aspects of Genetic Variation:

  1. Independent Assortment: Random distribution of maternal and paternal chromosomes during meiosis.

  2. Crossing Over: Exchange of genetic material between homologous chromosomes during prophase I of meiosis.

  3. Random Fertilization: Any sperm can fertilize any egg, increasing possible genetic combinations.

• Importance: Genetic variation is essential for natural selection and adaptation in populations.

Table: Comparison of Mitosis and Meiosis

Key Equations

• Chromosome Number in Gametes:

• Possible Chromosome Combinations (Independent Assortment): where n = haploid number of chromosomes