Mitosis, Meiosis, and Cell Cycle Regulation Review

The Cell Cycle and Mitosis

• DNA and genes are organized into chromosomes.

• Mitosis: Produces two daughter cells with the same number of chromosomes as the parent cell.

• Eukaryotic Mitosis Stages: Interphase, Prophase, Metaphase, Anaphase, and Telophase.

• Chromosome Structure: Includes sister chromatids (dyad), monads, centromeres, and kinetochores.

• Centromere Location: Determines the specific designation of the chromosome and its shape during anaphase.

• Cytogenetics: Geneticists use Giemsa stain to identify and analyze chromosome structures.

Meiosis and Genetic Reduction

• Meiosis: Reduces genetic content to produce four haploid ($n$) gametes from one diploid ($2n$) cell.

• Meiosis I: Involves homologous chromosome separation; Meiosis II: Involves sister chromatid separation.

• Prophase I Substages:   1. Leptonema: Chromosome condensation.   2. Zygonema: Formation of the synaptonemal complex.   3. Pachynema: Crossing over occurs between nonsister chromatids.   4. Diplonema: Chiasma (sites of crossing over) become visible; synaptonemal complex dissolves.   5. Diakinesis: Final stage of Prophase I preparation.

• Tetrad: A pair of homologous chromosomes consisting of four chromatids.

Cell Cycle Regulation

• The cell cycle consists of Interphase ($G_1$, $S$, $G_2$) and the $M$ phase (Mitosis and Cytokinesis).

• $S$ Phase: The period of DNA replication.

• Checkpoints: Regulatory points that monitor the progression of the cell cycle.

Chromosome Variations and Nondisjunction

• Nondisjunction: The failure of chromosomes or sister chromatids to segregate properly during Meiosis I or II.

• Aneuploidy: Variations in individual chromosomes.   - Trisomy ($2n + 1$)   - Monosomy ($2n - 1$)

• Polyploidy: Variations in entire sets of chromosomes.   - Triploid ($3n$)   - Tetraploid ($4n$)

Spermatogenesis and Oogenesis

• Spermatogenesis: The process of sperm production in the testes.   - Sequence: Primordial germ cell $\rightarrow$ Spermatogonium $\rightarrow$ Primary spermatocyte (Meiosis I) $\rightarrow$ Secondary spermatocyte (Meiosis II) $\rightarrow$ Spermatids $\rightarrow$ Mature sperm.

• Oogenesis: The process of ovum production.   - Characteristic: Uneven cytoplasmic division produces one large oocyte and small polar bodies.   - Timing: Meiosis II is triggered only by fertilization to form a zygote.