Summary
Highlights
If the G1 checkpoint is passed, the cell enters the S phase (synthesis), where DNA replication occurs. Each chromosome is copied, resulting in two identical sister chromatids joined at the centromere. The number of chromosomes remains the same (46 in human somatic cells), but the amount of genetic material doubles.
Mitosis involves karyokinesis (separation of replicated DNA into two distinct nuclei) and cytokinesis (separation of the daughter cells). Mitosis has four subphases: prophase, metaphase, anaphase, and telophase, followed by cytokinesis. In prophase, the nuclear membrane disintegrates, and chromosomes condense into visible 'X' shapes. Centrosomes migrate to opposite poles. In metaphase, chromosomes align at the metaphase plate, and spindle fibers attach to their centromeres.
During anaphase, spindle fibers pull sister chromatids apart towards opposite poles, effectively separating them. In telophase, a nuclear membrane forms around each new set of 46 single-chromatid chromosomes at the poles.
Cytokinesis immediately follows telophase, where the cell membrane pinches in, ultimately separating the cell into two distinct daughter cells. These daughter cells can either re-enter the cell cycle or enter the G0 phase, a non-dividing state. Cells like hepatocytes can re-enter G1 if needed (e.g., liver damage), while neurons spend their entire lives in G0.
In the G2 phase (Gap or Growth 2), the cell duplicates its organelles to prepare for division. By the end of G2, the cell is packed with cytoplasm and organelles. A G2 checkpoint verifies that there's no DNA damage after replication before the cell proceeds to mitosis.
The cell cycle describes the events somatic cells undergo from their formation until they divide into two identical daughter cells. The duration of this cycle varies, from less than a day for rapidly dividing skin cells to years for liver cells. The two main phases of the cell cycle are interphase and mitosis.
Interphase is the longest phase, a state of preparation where the cell grows, performs its functions, and replicates its DNA. It's divided into G1, S, and G2 phases. G1 (Gap or Growth 1) is where the cell grows, and organelles carry out metabolic functions. Each chromosome in G1 consists of a single chromatid. A G1 checkpoint ensures DNA integrity and proper protein synthesis; if issues exist, the cell can enter a non-dividing G0 phase to repair or undergo apoptosis.