Summary
Highlights
The cell cycle is a process for continuously dividing cells, taking them from division to readiness for the next division. Some cells enter a resting phase called G0 temporarily or permanently. Most of the cell cycle is spent in interphase, which includes G1 (organelle replication), S-phase (DNA replication and error checking), and G2 (synthesis of cell division proteins and increased ATP production).
Mitosis produces two identical daughter cells with the same DNA and chromosome number. Meiosis produces four daughter cells. Chromosomes are visible only during cell division; they are formed during interphase's S-phase when DNA replicates. Before replication, a chromosome is one chromatid. After S-phase, it's an X-shaped structure of two identical sister chromatids joined by a centromere.
Changes in chromosome number or DNA quantity can be shown graphically. 'N' represents the normal amount of DNA, and '2N' represents double the amount after replication. A graph shows DNA quantity increasing from N to 2N during S-phase and then dropping back to N after mitosis and cell division.
Mitosis consists of prophase, metaphase, anaphase, telophase, and cytokinesis. In prophase, chromosomes condense, centrioles move to opposite poles (animal cells), and the nuclear envelope breaks down. In metaphase, chromosomes align at the cell's center, with spindle fibers attached to their centromeres. In anaphase, centromeres split, and sister chromatids move to opposite poles. In telophase, chromosomes uncoil, and new nuclear envelopes form around each set of DNA. Cytokinesis divides the cytoplasm and organelles, forming two new daughter cells.
Mitotic calculations help determine the duration of cell division or its stages. This involves knowing the number of cells undergoing mitosis, the total number of cells, and the duration of one cell cycle. For example, to calculate the time spent in anaphase, you find the proportion of cells in anaphase and multiply it by the total cell cycle duration.
The mitotic index is the proportion of cells undergoing mitosis. It's calculated by dividing the number of cells in any stage of mitosis (prophase, metaphase, anaphase, telophase) by the total number of cells. This index can then be used to determine the time cells spend dividing if the cell cycle length is known.