细胞分裂

The cell cycle is the sequence of events throughout a cell's life involving growth, DNA replication, and preparation for division. For multicellular organisms, cell division produces new cells for development and replaces damaged cells from injury. For unicellular organisms, cell division generates a completely new organism, making the cell cycle essential for growth, reproduction, and tissue maintenance.

In eukaryotes, double-stranded DNA is wrapped tightly around histone proteins, forming nucleosomes. These nucleosomes, along with linker DNA, create coiled chromatin fibers. Additional fibrous proteins then compact the chromatin further, packing long DNA lengths into tightly-condensed units called chromosomes, which are visible during cell division phases.

Interphase comprises about 78% of a cell's life and contains three stages: G1, S, and G2. During G1, newly-formed cells grow and prepare for DNA replication. In S phase, cells duplicate their nuclear DNA and centrosomes. In G2, cells continue growing, multiply organelles and proteins needed for mitosis, and replenish energy stores before entering division.

Mitosis consists of prophase, prometaphase, metaphase, anaphase, and telophase. During prophase, chromatin condenses into chromosomes and spindle fibers form. Prometaphase dissolves the nuclear envelope and attaches kinetochores to chromosomes. Metaphase aligns chromosomes at the cell's equator. Anaphase separates sister chromatids to opposite poles. Telophase reforms nuclear envelopes around each chromosome set.

Cytokinesis is a distinct process occurring during telophase that physically divides the cell's cytoplasm, producing two separate daughter cells. While mitosis partitions genetic material into two nuclei, cytokinesis completes cell division by separating the cytoplasm around each nucleus, resulting in two genetically identical daughter cells.

External regulation ensures division necessity, such as replacing damaged stomach lining cells. Internal regulation maintains daughter cell health through checkpoints that verify DNA integrity before S phase, complete replication before mitosis, and proper chromosome attachment before anaphase. Cells failing checkpoints may undergo apoptosis if unable to correct errors.

Mutations in tumor-suppressor genes prevent cells from uncontrolled division, relieving normal growth inhibition. When these mutations accumulate, cells divide unchecked without external signals, producing daughter cells with increasing damage. This aberrant growth regulation ultimately forms tumors and can lead to cancer, one of the leading causes of death in the United States.