Mitosis Cytokinesis Cell Checkpoints

Chapter 9: Cellular Reproduction

Overview of Mitosis and Interphase

Cellular reproduction is crucial for growth, development, and maintenance of all living organisms. Mitosis is a fundamental process that results in the division of a single cell into two genetically identical daughter cells. This process is divided into several phases that ensure proper distribution of the genetic material.

Phases of Mitosis:

• Interphase: This phase is where the cell prepares for division and consists of three sub-phases:

  • G1 Phase: The cell grows and synthesizes proteins necessary for DNA replication.

  • S Phase: DNA replication occurs, resulting in the duplication of chromosomes, forming sister chromatids.

  • G2 Phase: The cell continues to grow and prepares for mitosis. The DNA is further condensed into visible chromosomes.

• Prophase: The chromatin, which is the relaxed form of DNA, condenses into visible chromosomes. Each chromosome is composed of two sister chromatids joined at a region called the centromere. The mitotic spindle begins to form, originating from the centrosomes that move apart.

• Prometaphase: The nuclear envelope dissolves, allowing microtubules to contact the chromosomes. Spindle fibers attach to the kinetochores, structures on the centromere of each sister chromatid, facilitating their movement.

• Metaphase: The sister chromatids align at the metaphase plate, an imaginary line that is equidistant from the two spindle poles. This alignment is critical for ensuring that each daughter cell will receive an identical set of chromosomes.

• Anaphase: The centromeres split, and the spindle fibers pull the sister chromatids apart towards the opposite poles of the cell. This separation ensures that each future daughter cell will have the correct number of chromosomes.

• Telophase: Chromosomes reach the cell poles and begin to uncoil back into chromatin. Nuclear membranes reform around each set of chromosomes, and nucleoli reappear, preparing the cell for the final stages of division.

• Cytokinesis: This is the final step in cell division, where the cytoplasm divides. In animal cells, a contractile ring forms, made of microfilaments, which constrict to pinch the cell in two. In plant cells, a cell plate forms along the center of the cell, eventually leading to the development of a new cell wall.

Transitioning to Mitosis

At the end of Interphase, DNA replication occurs in the S phase, resulting in two identical copies of each chromosome. The G2 Phase sets the stage for mitosis by condensing the DNA into sister chromatids, each consisting of an original strand and a newly replicated strand. This ensures that both daughter cells will inherit an exact copy of the parent cell's genetic material.

Cell Cycle Regulation (9.3)

• Normal Cell Cycle Control: The cell cycle is tightly regulated by the interaction of specific proteins called cyclins and cyclin-dependent kinases (CDKs). These combinations trigger activities necessary for DNA replication, protein synthesis, and nuclear division, thereby regulating the cell cycle's progression.

Cellular Reproduction Quality Control

• Checkpoints: The cell cycle includes several checkpoints that monitor its progression. If errors such as DNA damage or incomplete replication are detected, the cycle is temporarily halted.

• Spindle Checkpoints: These checkpoints are critical during mitosis, ensuring that chromosomes are accurately separated and that each daughter cell is given an identical set of chromosomes.

Detailed Phases of Mitosis

• Prophase: In addition to chromatin condensation, spindle fibers begin to radiate from the centrosomes, preparing for chromosomal movement.

• Telophase: The reformation of the nuclear envelope around each set of chromosomes is essential for restoring the normal nuclear function in daughter cells.

Apoptosis (9.3)

• Programmed Cell Death (Apoptosis): Apoptosis is vital for maintaining healthy tissue homeostasis and removing unnecessary or damaged cells. This process involves a series of biochemical events leading to characteristic morphological changes, including cell shrinkage and detachment, which play a crucial role in organizing cellular turnover and development.

Mitosis Summary Flow

1. Interphase: Cells prepare for division.

2. Prophase: Chromatin condenses; spindle fibers emerge and begin to form.

3. Prometaphase: Nuclear envelope dissolves; spindle fibers attach to kinetochores.

4. Metaphase: Chromatids align at the equator of the cell, ensuring proper separation.

5. Anaphase: Chromatids separate and migrate toward opposite poles of the cell.

6. Telophase: Chromosomes arrive at poles; new nuclear membranes form around each set.

7. Cytokinesis: Final division of the cytoplasm occurs, resulting in two genetically identical daughter cells.