Cell Cycle Phases and Checkpoints

The Cell Cycle
The cell cycle is a set of steps cells go through to grow, replicate, divide, and start the process again.

The cell cycle is a series of events that cells go through to grow, replicate their DNA, and divide. This process is vital for the growth, development, repair, and maintenance of living organisms. A consistent and regulated progression through the cell cycle ensures the proper duplication and distribution of a cell’s genetic material.

Overview of the Cell Cycle Phases

The two broad phases of the cell cycle are interphase and mitosis. During interphase, cells grow, replicate their DNA and organelles, and prepare for division. Interphase steps are the first gap phase (G1), the synthesis phase (S), and the second gap phase (G2). Cells divide during mitosis (M). The final step of mitosis, or the following step (depending on your source) is cytokinesis. Cytokinesis is the division of the cell’s cytoplasm, which forms two new cells. Some cells exit the cycle and enter G0.

  • Interphase (I): Interphase is the phase where the cell grows, replicates its DNA and prepares for division.
    • G1 phase: Cells grow following division and produce proteins and organelles.
    • S phase: The synthesis phase is where DNA replication occurs.
    • G2 phase: Cells continue growing and preparing for mitosis and the cell checks that DNA replicated without errors.
  • Mitosis (M phase): A cell divides and forms two new daughter cells during mitosis.
    • Prophase
    • Metaphase
    • Anaphase
    • Telophase
    • Cytokinesis
  • G0: Some cells exit the cell cycle and perform their function, without preparing for a new division.

Interphase – The Longest Step

Interphase, the period preceding mitosis, is the longest phase of the cell cycle and has three distinct sub-stages.

  1. G1 Phase (Gap 1): This is the phase right after cell division. Cells increase in size, produce RNA and synthesize proteins. Importantly, this phase ensures that everything is in place for DNA synthesis to occur in the next phase.
  2. S Phase (Synthesis): During this phase, the cell’s DNA replicates. At the end of the S phase, each chromosome consists of two chromatids attached at the centromere.
  3. G2 Phase (Gap 2): Here, the cell continues growing and prepares for mitosis. It ensures that all the DNA has been replicated without any errors.

Mitosis – Division of the Nucleus

Mitosis Definition
Mitosis is the process by which a single eukaryotic cell divides into two genetically identical daughter cells.

In mitosis or the M phase, one parental cell gives rise to two identical daughter cells. This phase has multiple steps:

  • Prophase: Chromosomes condense and become visible, the nuclear envelope starts to disintegrate, and the mitotic spindle begins to form.
  • Metaphase: Chromosomes line up along the cell’s equatorial plate, and spindle fibers attach to the centromeres.
  • Anaphase: Sister chromatids are pulled apart towards opposite poles of the cell.
  • Telophase: The chromatids or chromosomes move to opposite ends of the cell and two nuclei form.

Cytokinesis – Division of the Cytoplasm

Following mitosis (or as its final step), the cell undergoes cytokinesis where the cytoplasm divides, creating two daughter cells.

G0 Phase

The G0 phase is a “resting” phase where the cell exits the cell cycle and stops dividing. Some cells, like neurons and muscle cells, enter this phase semi-permanently and may never undergo division again. This phase is crucial for:

  • Conserving energy and resources in non-dividing cells.
  • Specializing cells for specific functions.

Regulation of the Cell Cycle

Checkpoints tightly regulate the cell cycle to prevent errors. These checkpoints include:

  1. G1 Checkpoint: This checkpoint ensures that the cell has adequate energy resources and that the surrounding environment is favorable for DNA replication. If conditions aren’t right, the cell can exit to G0 phase.
  2. G2 Checkpoint: Before entering mitosis, this checkpoint confirms that DNA has replicated properly.
  3. M Checkpoint (Spindle Assembly Checkpoint): This checkpoint occurs during metaphase in mitosis and ensures that all chromosomes properly align and attach to the spindle fibers.

Not all cells go through all checkpoints. Some fast-track through certain phases. Also, the time it takes for cells to complete the cycle varies. In humans, it ranges from two to five days for epithelial cells to an entire lifetime for certain neurons and cardiac cells. Disruption in these regulatory checkpoints can lead to cells with damaged or missing genetic material.

Tumor Formation and the Cell Cycle

Deregulation of the cell cycle can have grave consequences. When the checkpoints fail, it can result in:

  • Cells with incomplete or damaged DNA.
  • Uncontrolled cell division.

This uncontrolled division and growth of cells leads to the formation of tumors. Not all tumors are malignant, but those that are can invade nearby tissues and spread to other parts of the body (metastasis), leading to cancer.


The cell cycle is a critical and complex series of events ensuring the proper growth and replication of cells. Its tight regulation ensures the maintenance of the genetic material across generations of cells. Disruption of this process can lead to diseases, the most notable being cancer. Understanding the intricacies of the cell cycle is fundamental in cell biology and has vast implications in medical research and treatment.

Quiz Yourself


  • Cooper, G.M. (2000). The Cell: A Molecular Approach (2nd ed.). Washington, D.C: ASM Press. ISBN 978-0-87893-106-4.
  • De Souza, C.P.; Osmani, S.A. (2007). “Mitosis, not just open or closed”. Eukaryotic Cell. 6 (9): 1521–7. doi:10.1128/EC.00178-07
  • Morgan, D.O. (2007). The Cell Cycle: Principles of Control. London: Published by New Science Press in association with Oxford University Press. ISBN 978-0-87893-508-6.
  • Smith, J.A.; Martin, L. (1973). “Do cells cycle?”. Proceedings of the National Academy of Sciences of the United States of America. 70 (4): 1263–7. doi:10.1073/pnas.70.4.1263
  • Wang, J.D.; Levin, P.A. (November 2009). “Metabolism, cell growth and the bacterial cell cycle”. Nature Reviews. Microbiology. 7 (11): 822–7. doi:10.1038/nrmicro2202