Reproduction

2.1 The Cell Cycle and Mitosis
Learning Objectives

  • Describe the four phases of mitosis and the major events during each phase.

  • Identify the five stages of the cell cycle and the major events during each stage.

Overview of Cell Types

  • Diploid (2n): Autosomal cells, containing two copies of each chromosome (humans: 46 chromosomes).

  • Haploid (n): Germ cells containing one copy of each chromosome (humans: 23 chromosomes).

The Cell Cycle

  • The cell cycle is a highly regulated process composed of four stages: G₁, S, G₂, and M. It is critical for growth, repair, and reproduction in multicellular organisms.

  • Interphase: Comprised of the G₁, S, and G₂ stages; 90% of the cell's life is spent in interphase. This is a period of growth and preparation for division. Interphase is further divided into three distinct phases:

    • G₁ phase (First Gap): The cell increases in size, produces RNA, and synthesizes proteins necessary for DNA replication.

    • S phase (Synthesis): DNA replication occurs, resulting in the duplication of chromosomes.

    • G₂ phase (Second Gap): The cell continues to grow and produces proteins necessary for mitosis.

  • G₀ Stage: A non-dividing phase where cells perform their functions without preparation for division. Cells can enter this phase temporarily or permanently depending on signals from their surroundings, such as nutrient availability and growth factors.

G₁ Stage: Presynthetic Gap

  • Cells grow in size and create organelles necessary for energy production (e.g., mitochondria, ribosomes, endoplasmic reticulum). This stage is crucial as it sets the stage for DNA synthesis.

  • During this stage, the restriction point or G₁/S checkpoint assesses DNA conditions before the cell commits to DNA synthesis, checking for DNA damage and nutrient availability. If conditions are favorable, the cell progresses to the S phase.

S Stage: Synthesis Phase

  • DNA replication occurs; each chromosome consists of two sister chromatids connected at the centromere, effectively doubling the genetic content without increasing the chromosome number. This ensures that both daughter cells will receive the same genetic information.

  • The overall chromosome count remains 46, despite the duplication of the chromatids to 92, ensuring that upon division, each daughter cell will receive the correct amount of genetic material. The S phase also involves critical processes such as the unwinding of DNA and synthesis of new strands.

G₂ Stage: Postsynthetic Gap

  • At this stage, DNA has been duplicated; a quality control checkpoint (G₂/M checkpoint) verifies that DNA replication has been completed successfully and assesses organelles and cytoplasm to ensure readiness for mitosis. This ensures that any errors in DNA replication are corrected to prevent mutations.

M Stage: Mitosis

  • Mitosis includes the four phases: Prophase, Metaphase, Anaphase, and Telophase. Each phase has distinct roles in ensuring accurate division of genetic material.

  • Cytokinesis occurs concurrently with mitosis, dividing the cytoplasm and ensuring that each daughter cell is equipped with organelles and sufficient cytoplasm. In animal cells, this is accomplished through cleavage furrow formation, while plant cells form a cell plate.

Mitosis Phases

  1. Prophase: Chromatin condenses into visible chromosomes, centrioles move apart toward opposite poles, and the spindle apparatus begins to form. The nuclear membrane dissolves, allowing spindle fibers to interact with chromosomes. This phase is critical for proper chromosome segregation.

  2. Metaphase: Chromosomes align along the metaphase plate; kinetochores attach to spindle fibers, ensuring that each sister chromatid is ready to be pulled apart. This alignment is crucial for ensuring each daughter cell receives an identical set of chromosomes.

  3. Anaphase: Sister chromatids are pulled to opposite poles of the cell as the centromeres divide, ensuring genetic equality in daughter cells during division. This phase guarantees that each daughter cell will have the necessary genetic content.

  4. Telophase: Nuclear membranes reform around each set of chromosomes, the spindle fibers disappear, and chromosomes decondense back into chromatin. This phase marks the near end of mitotic division.

Control of the Cell Cycle

  • The cell cycle is carefully controlled by cyclins and cyclin-dependent kinases (CDKs), which work together to activate transcription factors necessary for progression to the next stage of the cell cycle. Cyclins are proteins whose levels fluctuate throughout the cell cycle, while CDKs are enzymes that only activate when bound to cyclins.

  • p53: A crucial tumor suppressor protein that regulates the G₁/S and G₂/M checkpoints. Mutations in p53 can lead to the development of cancer due to unregulated cell division, allowing defective cells to proliferate. This helps prevent the propagation of damaged DNA.

  • Types of cancer often correlate with mutations in oncogenes (which promote cell division) and tumor suppressor genes (which inhibit cell division). Understanding these mechanisms is critical for cancer therapies and interventions.