Chapter 19 Cell Division

Cell Division

The process where one cell divides to form new cells.

• Growth: Mitosis increases the number of cells as an organism grows.

• Repair and maintenance: Mitosis replaces damaged, worn-out, or dead cells.

• Reproduction: Mitosis produces genetically identical cells for asexual reproduction, while meiosis produces sex cells (sperm and eggs) with half the usual number of chromosomes for sexual reproduction.

Stages of the Cell Cycle & Interphase:

Interphase: The cell grows, performs normal functions, and prepares to divide. Chromatin is loose inside the nucleus.

• G1 phase: Cell grows and duplicates organelles.

• S phase: DNA is copied; each chromosome has two identical sister chromatids.

• G2 phase: Cell makes proteins needed for division.

Mitotic phase (M phase)

The cell divides through mitosis and cytokinesis.

• Cytokinesis: Cytoplasm divides, forming two genetically identical daughter cells.

DNA Replication

During the S phase of interphase, DNA makes an identical copy of itself. Each chromosome now contains two identical sister chromatids joined together. During mitosis, the sister chromatids separate into two new nuclei so each daughter cell receives the same DNA.

Major Stages of Mitosis

• Prophase: Chromosomes condense and become visible.

• Metaphase: Chromosomes line up in the middle of the cell.

• Anaphase: Sister chromatids separate and move to opposite ends.

• Telophase: Two new nuclei form.

Cytokinesis

Division of the cytoplasm and organelles to form two separate daughter cells.

• Animal cells: A cleavage furrow forms as a ring of actin and myosin tightens, pinching the cell in half from the outside inward.

• Plant cells: A cell plate forms in the center from Golgi vesicles and grows outward, eventually creating a new cell wall and membrane between the two daughter cells.

Key difference: Animal cells pinch apart; plant cells build a cell plate because their rigid cell wall prevents pinching.

Genetic Continuity in Mitosis

DNA is copied before mitosis, and the sister chromatids separate so each daughter cell receives a complete set of chromosomes. This produces two diploid (2n) daughter cells with the same number and types of chromosomes as the parent cell. The daughter cells are genetically identical to each other and the parent cell, except for rare mutations.