Cell Division: Mitosis and Cancer

• Purpose of Cell Division: To produce two genetically identical daughter cells. This process is essential for various physiological functions such as growth, wound healing, regeneration, and tissue maintenance. Each daughter cell inherits an exact copy of the parent cell's genetic material, ensuring continuity in the genetic information required for functioning.

Reasons for Cell Division

• Growth of an Organism:

  • Embryonic Development: Following fertilization, a single fertilized egg, known as a zygote, undergoes extensive divisions to develop into a multicellular organism. These divisions are not just rapid, but also unlock the potential for differentiation, where cells specialize into various tissues and organs.

• Replace Damaged Cells:

  • Wound Healing: In response to injury, the body triggers cell division to replace cells lost during the healing process. This includes various stages of repair, where different cell types come into play, such as fibroblasts that aid in forming new connective tissue.

• Replace Old Cells:

  • Cell Replacement: Most somatic (body) cells have a predetermined lifespan and must be regularly replaced to maintain overall tissue health and function. For example:

  • Stomach lining: 2 days, highlights the rapid turnover due to the harsh acidic environment.

  • Epidermal skin cells: Replace every 2-4 weeks, essential in protecting the body and facilitating healing.

  • White blood cells: Live for about 1 month, crucial for immune responses.

  • Neurons: Many can last from age 2 until death, showcasing a unique survival aspect among cell types.

Cell Division Process

• Cell Cycle: The process of cell division is intricately organized into distinct phases:

  • Interphase: The preparatory stage for cell division, divided into three sub-phases:

  • G1 Phase: The cell increases in size, produces RNA, and synthesizes proteins in preparation for DNA replication. It also checks for any DNA damage before proceeding.

  • S Phase: DNA is replicated, producing two copies of each chromosome in the form of sister chromatids, doubling the genetic material in preparation for division.

  • G2 Phase: The cell continues to grow and finalizes its preparations for division, including the condensation of DNA and checking for any replication errors.

  • M Phase (Mitotic Phase): This phase is where the actual mitosis occurs, which includes four stages:

  • Prophase: Chromatin condenses into visible sister chromatids. The nucleolus disappears and microtubules align and attach to the kinetochores of the chromosomes at the centrioles.

  • Metaphase: Sister chromatids line up along the metaphase plate. This phase is the longest in mitosis and serves as a critical checkpoint to ensure all chromosomes are correctly aligned before segregation.

  • Anaphase: The sister chromatids are pulled apart at the centromeres and moved to opposite poles of the cell, ensuring each daughter cell will receive an identical set of chromosomes.

  • Telophase: New nuclear envelopes form around the separated chromatids, now individual chromosomes. Telophase is followed closely by cytokinesis, the physical division of the cytoplasm.

Cytokinesis

• Cytokinesis: This final step involves the process of splitting the cytoplasm and organelles, resulting in two genetically identical daughter cells. In animal cells, this occurs through the formation of a cleavage furrow that pinches the cell into two, while in plant cells, a cell plate forms to divide the cell wall.

Cancer and Cell Division

• Cancer: Cancer emerges as a result of unregulated cell division, often due to genetic mutations that impact the normal mechanisms of the cell cycle. Tumors can form as cells multiply uncontrollably, leading to various health complications.

• Types of Tumors:

  • Benign Tumor: These are usually non-invasive, grow slowly, and can often be removed surgically without the risk of spreading. They do not pose a significant threat to health unless they exert pressure on vital organs.

  • Malignant Tumor: These tumors are characterized by rapid growth and the ability to invade neighboring tissues, often leading to metastasis—the spread of cancer cells to other parts of the body.

Genetic Control of Cancer

• Key Genes:

  • Proto-oncogenes: These genes normally promote cell division and growth; however, when mutated, they can transform into oncogenes that drive excessive division, contributing to cancer development.

  • Tumor Suppressor Genes: These genes typically inhibit cell growth and division. Mutations can lead to their inactivation, allowing uncontrolled cell proliferation (e.g., BRCA1, BRCA2 mutations are linked to breast and ovarian cancers).

Treatment of Cancer

• Chemotherapy: This treatment involves the use of chemicals (e.g., Taxol), which are designed to interrupt cell division—primarily by targeting microtubules and preventing their formation, thus halting the cell cycle at metaphase. This can shrink tumors and eliminate cancerous cells, though it often also affects healthy cells, leading to side effects.

Summary

• A comprehensive understanding of mitosis and the cell cycle is crucial for comprehending normal cellular functions, tissue maintenance, and the pathological mechanisms leading to cancerous cells. Treatments focus on genetic abnormalities to restore normal regulation of cell division, with the hope of halting tumor progression and returning the body to homeostasis.