Separate the cell cycle from mitosis

The cell cycle describes the broader path in which a cell grows, copies DNA, and may divide or remain outside active division. DNA replication occurs during S phase of interphase. Mitosis divides the replicated chromosomes into two nuclei; cytokinesis then separates the cell. HHMI BioInteractive’s cell-cycle educator guide distinguishes the cell cycle from mitosis and discusses mature muscle cells and neurons in G0.

What checkpoints inspect

A G2 checkpoint monitors whether DNA replication and preparation are adequate before mitosis. The spindle-assembly checkpoint in M phase monitors whether kinetochores are attached to spindle microtubules; unmet conditions can delay anaphase. Rieder and colleagues’ original-paper abstract reports observations of 126 PtK1 cells in which one unattached kinetochore could delay anaphase onset. This is a result from that cell model; its delay should not be generalized to all cells. Read the original-paper abstract.

Worked observation: one chromosome is not attached

Imagine observing a dividing cell. Most chromosomes connect to spindle poles, but one kinetochore has not attached to microtubules. If the checkpoint signal remains active, separation of sister chromatids may be delayed. Once attachment is established, the cell can proceed into anaphase. This hypothetical observation illustrates the checkpoint; it does not mean all cells pause for the same duration.

Cell type and tissue matter

Some cells in renewing tissues such as skin or intestinal epithelium divide often. Mature neurons and skeletal-muscle cells can remain in G0 and typically do not divide. HHMI’s teaching material explicitly notes that mitotic frequency differs across tissues. Cycle duration and checkpoint response also depend on cell type, growth signals, DNA damage, and experimental conditions. Checkpoints reduce risk by detecting problems and delaying progression; they do not guarantee error-free division. Meiosis, which reduces chromosome number in the germ-cell lineage, is a separate process from mitosis.

Sources