Cell Reproduction and Interphase

Cell Reproduction

• Cells reproduce through a process called the cell cycle.
• The cell cycle is divided into four stages: G1, S, G2, and M.
• G1, S, and G2 comprise interphase, the period when the cell is not dividing.
• Mitosis occurs during the M phase and involves chromosome duplication followed by separation, ultimately creating two identical daughter cells.
• Interphase, specifically the G1 phase, is the longest phase, where the cell determines whether to duplicate its chromosomes or exit the cell cycle.

Interphase (G1 + S + G2)

• G1: Growth and synthesis; begins after mitosis and cytokinesis end. The cell grows and prepares for DNA replication.
• G1: The longest phase. The cell determines if it will duplicate its chromosomes, committing to the S phase or leaving the cell cycle.
• S: Chromosomes duplicate, creating a pair of sister chromatids joined at a centromere. In animal cells, the centriole pairs are also replicated.
• G2: The cell prepares for division including final preparations for mitosis.

Mitosis

• Mitosis is the process by which a eukaryotic cell divides its chromosomes into two identical sets.
•  Mitosis is followed by cytokinesis, the division of the cytoplasm and organelles, resulting in two separate daughter cells.
•  Mitosis is divided into four phases: prophase, metaphase, anaphase, and telophase (PMAT). These phases ensure accurate distribution of chromosomes to the daughter cells.
• Daughter cells are genetically identical.

Reasons for Mitosis

• Growth of an organism by increasing the size or number of cells.
• Replacement of damaged or dying cells. This is essential in various parts of the body, tissues, like skin and the digestive tract for continuous renewal and repair.
• Regeneration, in which some organisms (sea stars, lizards, and lobsters) can regrow lost body parts.
• Asexual reproduction in various organisms (hydra, sponges, planaria). This results in genetically identical offspring.

Mitosis Phases

• Prophase:

• Nuclear membrane and nucleolus disappear.

• Chromatin condenses into chromosomes.

• Chromosomes have two sister chromatids bound at the centromere.

• Centrioles create spindle fibers that align with kinetochores on chromosomes.

• Chromosomes are visible under a microscope.

• Metaphase:

  • Spindle fibers attach to chromatids.
  • Chromosomes line up down the middle of the cell forming “the metaphase plate”.
  • This organization ensures an equal distribution of chromosomes to each daughter cell.

• Anaphase:

  • Sister chromatids split and become separate daughter chromosomes.
  • Spindle fibers pull the chromosomes to opposite ends of the cell. Centromeres split.

• Telophase:

  • Spindle fibers continue pulling chromosomes.
  • Nuclear membranes form around both sets of chromosomes and begin to break up the chromosomes.
  • Chromosomes become less condensed.
  • Mitosis is complete at this point.

Cytokinesis

• Cleavage furrow (in animal cells) pinches off two separate cells.
• Cell plate forms (in plant cells) to become new cell walls for two new cells.

Mitosis in Plant Cells

• Preprophase stage, a sheet of cytoplasm (phragmosome) bisects the cell where the division will occur.
• Centrioles are not present.
• Cytokinesis occurs by formation of a cell plate.

Summary

• The cell cycle, composed of G1, S, G2, and M phases, completes cell division.
• Interphase describes the non-dividing phase.
• Mitosis involves the duplication and separation of chromosomes.
• Mitosis is crucial for growth, repair, and asexual reproduction.
• Cytokinesis marks the end of cell division, producing two new daughter cells.