Biology 1 - Lesson 11: Cell Cycle and Mitosis

The cell cycle is the orchestrated series of events that lead to cell growth, DNA replication, and ultimately cell division. For many eukaryotic cells, this culminates in mitosis, a highly regulated process that ensures each daughter cell receives the correct complement of chromosomes. Understanding the cell cycle and mitosis helps explain how organisms grow, repair tissues, and maintain genetic stability across generations.

Overview of the Eukaryotic Cell Cycle

The cell cycle is divided into two major phases: interphase and the mitotic (M) phase. Interphase itself can be subdivided into three primary stages:

• G₁ Phase (First Gap): The cell grows, produces organelles, and carries out normal metabolic activities. Many cells can pause here and enter G₀ if conditions are unfavorable or if they are terminally differentiated (e.g., neurons).
• S Phase (DNA Synthesis): The cell replicates its nuclear DNA, producing two identical copies (sister chromatids). Centrosomes (microtubule-organizing centers) also replicate in preparation for cell division.
• G₂ Phase (Second Gap): The cell continues to grow, synthesizes proteins needed for mitosis, and checks for errors in DNA replication. By the end of G₂, conditions are assessed for entry into the M phase.

These phases comprise the bulk of the cell cycle, where cells spend most of their time growing, carrying out metabolic functions, and ensuring genomic integrity before dividing.

Key Cell Cycle Checkpoints

Cells rely on checkpoints to control progression through the cell cycle:

• G₁/S Checkpoint: Also known as the restriction point, it assesses cell size, nutrient availability, growth factor signals, and DNA integrity. If conditions are met, the cell commits to DNA replication.
• G₂/M Checkpoint: Confirms that DNA replication is complete and checks for DNA damage. The cell also ensures adequate resources for mitosis.
• Spindle Assembly Checkpoint (During M Phase): Verifies that all chromosomes are properly attached to the mitotic spindle before allowing sister chromatids to separate.

These checkpoints help prevent the transmission of genetic errors and maintain genomic stability. Abnormal checkpoint function can lead to uncontrolled cell division and cancer.

Mitosis: Dividing the Genetic Material

Mitosis distributes duplicated chromosomes into two daughter nuclei. It consists of several coordinated stages:

1. Prophase: Chromatin condenses into discrete chromosomes (each with two sister chromatids). Centrosomes migrate to opposite poles, and spindle fibers begin to form.
2. Prometaphase: The nuclear envelope fragments. Microtubules from each centrosome attach to the kinetochores on sister chromatids, forming the kinetochore microtubules.
3. Metaphase: Chromosomes line up at the cell’s equatorial plane (the metaphase plate), with each sister chromatid facing opposite spindle poles.
4. Anaphase: Cohesin proteins binding sister chromatids are cleaved, and sister chromatids (now individual chromosomes) move to opposite poles via the shortening of kinetochore microtubules.
5. Telophase: Chromosomes decondense, and nuclear envelopes reform around each set of chromosomes. The spindle apparatus disassembles.

Cytokinesis

Cytokinesis, the division of the cytoplasm, typically overlaps with the latter stages of mitosis. Its mechanism varies in different organisms:

• Animal Cells: A contractile ring of actin microfilaments and myosin motors pinches the cell membrane inward at the cleavage furrow, separating the cell into two daughter cells.
• Plant Cells: Membrane-enclosed vesicles carrying cell wall materials coalesce at the cell’s equator, forming a cell plate that expands outward. Ultimately, this plate fuses with the plasma membrane, producing two new cells separated by a new cell wall.

Successful cytokinesis ensures each daughter cell has the necessary cytoplasmic components, organelles, and membranes for independent survival.

Regulation and Cancer

Regulatory proteins, such as cyclins and cyclin-dependent kinases (CDKs), drive transitions between cell cycle phases. Elevated cyclin levels activate corresponding CDKs, which phosphorylate specific target proteins to trigger progression. Once their job is complete, cyclin levels often drop, inactivating the complex.

When cell cycle regulation malfunctions, cells can proliferate uncontrollably. Mutations in checkpoint proteins (e.g., p53), oncogenes (e.g., Ras), or tumor suppressors (e.g., RB) often lead to cancer. Therapies targeting these pathways seek to reinstate normal checkpoint function or selectively kill rapidly dividing cells.

Biological Significance

Mitosis underlies growth in multicellular organisms, tissue repair (e.g., wound healing, regeneration), and asexual reproduction in some species. By maintaining the precise distribution of duplicated chromosomes, mitosis ensures genetic consistency across cell generations—a fundamental requirement for organismal viability and development.

Conclusion

The cell cycle and mitosis exemplify the highly coordinated nature of cellular processes. Interphase allows cells to grow, replicate DNA, and prepare for division, while mitosis and cytokinesis precisely distribute genetic material and cytoplasmic components. Proper regulation of these events is paramount to healthy growth, tissue maintenance, and the prevention of diseases such as cancer.