Cell Cycle and Mitosis

Introduction to the Cell Cycle

The cell cycle is a series of events that cells go through as they grow and divide. It ensures that genetic material is accurately duplicated and distributed to daughter cells. The cell cycle is fundamental for growth, development, tissue renewal, and asexual reproduction in eukaryotic organisms.

• Asexual reproduction: Single-celled organisms divide to produce genetically identical offspring.

• Growth and development: Multicellular organisms grow by increasing cell number through division.

• Tissue renewal: Damaged or dead cells are replaced by new cells.

Cellular Organization of Genetic Material

Genome and Chromosomes

The genome is the complete set of a cell’s DNA, including all of its genes and noncoding regions. In eukaryotes, DNA is organized into linear structures called chromosomes. Chromosomes are composed of chromatin, a complex of DNA and proteins (mainly histones) that condense during cell division.

• Chromatin: The combination of DNA and histone proteins that forms chromosomes.

• Condensation: Chromatin condenses to form visible chromosomes during cell division.

Chromosome Structure and Terminology

Each duplicated chromosome consists of two sister chromatids joined at a region called the centromere. The kinetochore is a protein structure at the centromere that attaches to spindle fibers during mitosis. Chromosome number is species-specific; humans are diploid (2n), meaning they have two sets of chromosomes.

• Sister chromatids: Identical copies of a chromosome connected by a centromere.

• Centromere: The region where sister chromatids are joined and where spindle fibers attach.

• Kinetochore: Protein complex on the centromere essential for chromosome movement.

• Diploid (2n): Two sets of chromosomes; haploid (n): one set.

Chromosome Duplication and Distribution

During cell division, chromosomes are duplicated and then distributed equally to daughter cells. This process ensures genetic continuity between generations of cells.

• Duplicated chromosomes are separated into unduplicated chromosomes during mitosis.

The Cell Cycle: Phases and Events

Phases of the Cell Cycle

The cell cycle consists of interphase (G1, S, G2) and the M phase (mitosis and cytokinesis). Interphase is the period of cell growth and DNA replication, while the M phase is when the cell divides.

• G1 phase: Cell growth and metabolic activity.

• S phase: DNA synthesis and chromosome replication.

• G2 phase: Preparation for cell division, synthesis of proteins.

• M phase: Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Events of Mitosis

Mitosis is the process by which a eukaryotic cell separates its duplicated chromosomes into two identical sets. It consists of several stages:

• Prophase: Chromatin condenses, nucleolus disappears, spindle apparatus forms.

• Prometaphase: Nuclear envelope fragments, spindle fibers attach to kinetochores.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Sister chromatids separate and move to opposite poles.

• Telophase: Nuclear envelopes reform, chromosomes decondense, nucleoli reappear.

• Cytokinesis: Division of the cytoplasm, forming two daughter cells.

Special Features of Mitosis in Animal and Plant Cells

Animal cells use a contractile ring to divide the cytoplasm (cleavage furrow), while plant cells form a cell plate from vesicles. Plant cells lack centrosomes but still organize spindle fibers.

Binary Fission in Prokaryotes

Prokaryotic cells (such as bacteria) divide by binary fission, a simpler process than mitosis. The single, circular DNA molecule is replicated, and the cell splits into two genetically identical daughter cells.

Regulation of the Cell Cycle and Cancer

Cell Cycle Control and Checkpoints

The cell cycle is regulated by checkpoints (G1, G2, M) that ensure proper division. Regulatory proteins such as cyclins and cyclin-dependent kinases (Cdks) control progression through these checkpoints. If DNA is damaged, the cell may undergo apoptosis (programmed cell death).

• G1 checkpoint: Most important; determines if the cell will divide.

• G0 phase: Non-dividing state entered if conditions are not met.

• G2 checkpoint: Ensures DNA replication is complete before mitosis.

• MPF (Maturation Promoting Factor): Controls passage through G2 checkpoint.

External and Internal Signals

Cell division is also regulated by external signals such as growth factors, and by internal signals like density-dependent inhibition (cells stop dividing when crowded) and anchorage dependence (cells must be attached to a substrate to divide).

Cancer: Loss of Cell Cycle Control

Cancer results from uncontrolled cell division due to loss of cell cycle regulation. Mutations in proto-oncogenes (which become oncogenes) or tumor suppressor genes can lead to cancer. Cancer cells may form tumors, which can be benign (localized) or malignant (invasive and metastatic).

• Benign tumors: Do not spread; usually not life-threatening.

• Malignant tumors: Invade tissues and can metastasize (spread to distant sites).

• Metastasis: Spread of cancer cells to other parts of the body.

Treatment of Cancer

Cancer treatments include surgery, radiotherapy, and chemotherapy. Chemotherapy drugs often target rapidly dividing cells by interfering with mitosis (e.g., blocking spindle formation). Side effects occur because normal dividing cells are also affected. Precision medicine uses genetic testing to select the most effective drugs for individual patients.

Summary Table: Key Terms and Concepts