Mitosis and Meiosis Processes PDF

Mitosis and Meiosis Mitosis:\ The process by which a cell replicates its chromosomes and then segregates them, producing two identical nuclei in preparation for cell division. There are 5 stages: - Interphase - Prophase - Metaphase - Anaphase - Telophase Stage What happen...

Mitosis and Meiosis Mitosis:\ The process by which a cell replicates its chromosomes and then segregates them, producing two identical nuclei in preparation for cell division. There are 5 stages: - Interphase - Prophase - Metaphase - Anaphase - Telophase Stage What happens ------------- ----------------------------------------------------------------------------------------------------------------------- Interphase The cell spends most of its life in this phase. The DNA in chromosomes copies itself ready for mitosis. Prophase The DNA in chromosomes and their copies condenses to become more visible. The membrane around the nucleus disappears. Metaphase Chromosomes and their copies line up in the middle of the cell. Anaphase Chromosomes and their copies are pulled to different ends of the cell. Telophase New membranes form around the chromosomes at each end of the cell. Cytokinesis The cell membrane pinches in and eventually divides into two daughter cells. Interphase Duration: The longest phase of the cell cycle. Activities: The cell grows, performs normal functions, and prepares for mitosis. Sub-phases: - (Gap 1): The cell grows, produces RNA and proteins, and ensures everything is ready for DNA replication. - (Synthesis): DNA replication occurs, doubling the genetic material so that each new cell will receive an exact copy. - (Gap 2): The cell continues to grow and produces proteins necessary for mitosis. The cell also checks for DNA replication errors and makes necessary repairs. Prophase - Chromosome Condensation: The chromatin (loose DNA and proteins) condenses into visible chromosomes. Each chromosome is made up of two identical sister chromatids, joined at a region called the centromere. - Nuclear Envelope Breakdown: The nuclear membrane (nuclear envelope) begins to break down, allowing chromosomes to move freely in the cytoplasm. - Spindle Formation: Centrioles (in animal cells) move to opposite poles of the cell and start forming spindle fibers, which help guide chromosome movement. Metaphase - Chromosome Alignment: The chromosomes, each consisting of two sister chromatids, align along the metaphase plate (the center of the cell). - Spindle Fiber Attachment: Spindle fibers attach to the centromere of each chromosome via structures called kinetochores. - Checkpoint: The cell checks that all chromosomes are properly attached to the spindle fibers before proceeding to the next stage. This prevents unequal division of genetic material. - Anaphase - Chromatid Separation: The sister chromatids are pulled apart by the shortening spindle fibers, moving toward opposite poles of the cell. - Chromosome Migration: Once separated, each chromatid is now considered an individual chromosome. The cell ensures that each new daughter cell receives an identical set of chromosomes. - Telophase - Chromosomes Decondense: The individual chromosomes begin to unwind and return to their less compact chromatin state. - Nuclear Envelope Reformation: New nuclear membranes start forming around each set of chromosomes at opposite ends of the cell. - Spindle Breakdown: The spindle fibers dissolve as their job is completed. - Cytokinesis - Final Division: The cytoplasm divides, creating two genetically identical daughter cells. - Mechanism: - Animal Cells: A contractile ring of actin filaments pinches the cell membrane inward, forming a cleavage furrow until the cell splits. - Plant Cells: A cell plate forms between the two nuclei, eventually developing into a new cell wall that separates the two daughter cells. - End Result: Two daughter cells enter interphase and start the cycle again. Meiosis - What is Meiosis? - Meiosis is a type of cell division that reduces the chromosome number by half, producing four genetically unique haploid cells (gametes: sperm and egg cells). - It occurs in sexually reproducing organisms to ensure genetic diversity. - Unlike mitosis (which produces identical cells), meiosis introduces genetic variation through crossing over and independent assortment. - Involves two divisions: Meiosis I and Meiosis II. - Steps of Meiosis - Meiosis I (Reduction Division - Diploid to Haploid) 1. Prophase I - Chromatin condenses into visible chromosomes. - Homologous chromosomes pair up to form tetrads (synapsis). - Crossing over occurs at chiasmata, where homologous chromosomes exchange genetic material, increasing genetic diversity. - Spindle fibers form, and the nuclear membrane breaks down. 2. Metaphase I - Homologous chromosome pairs (tetrads) align at the metaphase plate. - Independent assortment occurs, where homologous pairs randomly align, leading to genetic variation. - Spindle fibers attach to centromeres. 3. Anaphase I - Homologous chromosomes (each still made of two sister chromatids) are pulled to opposite poles. - Sister chromatids remain attached at this stage. 4. Telophase I and Cytokinesis - Chromosomes arrive at opposite poles. - The nuclear envelope reforms, and the cytoplasm divides, forming two haploid daughter cells. - Cells enter a short interphase-like stage (interkinesis) but do not replicate DNA. - Meiosis II (Similar to Mitosis - Sister Chromatids Separate) 5. Prophase II - Chromosomes condense again. - The nuclear membrane dissolves, and spindle fibers form. 6. Metaphase II - Chromosomes line up at the metaphase plate. - Spindle fibers attach to centromeres. 7. Anaphase II - Sister chromatids separate and are pulled to opposite poles. 8. Telophase II and Cytokinesis - Nuclear envelopes reform around separated chromosomes. - Cytokinesis occurs, resulting in four genetically unique haploid cells. - End Result of Meiosis - 4 haploid (n) gametes (sperm or egg), each with half the chromosome number of the original cell. - Each gamete is genetically unique due to crossing over (Prophase I) and independent assortment (Metaphase I). - In humans, meiosis produces sperm (in males) and eggs (in females), each containing 23 chromosomes. - - Key Differences: Mitosis vs. Meiosis +-----------------------+-----------------------+-----------------------+ | - Feature | - Mitosis | - Meiosis | +=======================+=======================+=======================+ | - Purpose | - Growth, repair, | - Sexual | | | asexual | reproduction | | | reproduction | | +-----------------------+-----------------------+-----------------------+ | - Number of | - 1 | - 2 | | Divisions | | | +-----------------------+-----------------------+-----------------------+ | - Number of Cells | - 2 | - 4 | | Produced | | | +-----------------------+-----------------------+-----------------------+ | - Genetic Variation | - No (identical | - Yes (crossing | | | cells) | over & | | | | independent | | | | assortment) | +-----------------------+-----------------------+-----------------------+ | - Chromosome Number | - Diploid (2n) → | - Diploid (2n) → | | | Diploid (2n) | Haploid (n) | +-----------------------+-----------------------+-----------------------+