cell-division

The Chromosome

  • Genetic Material: Living organisms contain genetic material known as DNA (deoxyribonucleic acid), serving as instructions for cellular activities and functions.

  • Hereditary Transmission: DNA is passed from one generation to the next, ensuring the continuity of life.

  • Chromosomes in Eukaryotic Cells: In eukaryotic cells (cells with organelles), DNA binds with proteins called histones, organized as "beads on a string" to form chromosomes.

  • Species-Specific Chromosome Number: Each species has a characteristic number of chromosomes (e.g., humans have 46 chromosomes; rice has 12).

  • Visual Representation: Diagrams depict the structure of chromosomes, nucleosomes, DNA double helix, and supercoiling.

The Cell Cycle

  • Cell Cycle Overview: The chromosomes change form as the cell transitions through different stages of the cell cycle.

  • Phases of the Cell Cycle:

    • Interphase: Chromosomes are long and extended, referred to as chromatin.

      • G1 Phase: First gap period, initial cell growth.

      • S Phase: Synthesis stage, period of DNA replication and chromosome duplication.

      • G2 Phase: Second gap, rapid cell growth in preparation for cell division.

    • Cell Division Phase (M Phase): Chromosomes condense, leading to two types of cell division: mitosis and meiosis.

Mitosis

  • Definition: Type of cell division in somatic cells resulting in two identical daughter cells with the same chromosome number.

  • Stages of Mitosis:

    • Prophase: Chromatin condenses into double-stranded chromosomes; nuclear envelope breaks down; centrioles move apart and spindle forms.

    • Metaphase: Spindle fibers attach to centromeres; sister chromatids align at the equator (metaphase plate).

    • Anaphase: Sister chromatids separate as the spindle fibers shorten, moving them to opposite poles.

    • Telophase: Chromosomes uncoil forming chromatin; spindle apparatus breaks down; nuclear membranes reform; cytokinesis occurs, forming two daughter cells.

Meiosis

  • Definition: Special type of cell division that produces gametes (sperm/egg cells) through two rounds of division, resulting in four daughter cells with half the chromosome number and unique genetic material.

  • Stages of Meiosis:

    • Meiosis I:

      • Prophase I: Homologous chromosomes pair, condense, and crossing-over occurs (exchange of chromosome segments).

      • Metaphase I: Paired chromosomes align at the equatorial plate.

      • Anaphase I: Homologous chromosomes separate, moving towards cell poles; sister chromatids remain together due to intact centromeres.

      • Telophase I: Chromosomes reach poles; cytokinesis results in two haploid daughter cells.

    • Meiosis II (similar to mitosis):

      • Consists of prophase II, metaphase II, anaphase II, telophase II, resulting in four haploid cells from the two daughter cells formed in meiosis I.

      • Each cell has one sister chromatid for each chromosome, illustrating the reduction in chromosome number from diploid to haploid.