4.7: Regulation of Cell Cycle

In the previous lesson, we learned about how cells grow and replicate in the cell cycle. But how does an organism keep the cell cycle under control so that there are always enough resources to go around? Topic 4.7 explains the restrictions cells place on their replication. Please keep note that this is not the same process that organisms use to mate (more on that in unit 5).

Vocab List

• G1 phase checkpoint
• G2 phase checkpoint
• M phase checkpoint
• Cyclin-dependent kinases (CDKs)
  • Growth factor
• Density-dependent inhibition
• Anchorage dependency
• Cancer
  • Tumor
    • Benign
    • Malignant
  • Metastasis

Written Explanation

Cell Cycle Recap:

Cell Cycle Checkpoints:

There are three main checkpoints that occur throughout the cell cycle to ensure the safe replication of a cell. The first of these checkpoints, known as the G1 phase checkpoint (because that's when it occurs), checks the cell's DNA for errors. If it detects a mistake that occurred from replication, this checkpoint sends the cell into G0.

The second checkpoint is the G2 phase checkpoint. This confirms that the DNA replication that occurred during S phase was accurate, so that when the cell divides, both daughter cells will have a correct genome.

The last checkpoint occurs during metaphase. The M phase checkpoint confirms that all chromatids have been properly attached to the microtubules on the metaphase plate. This is to ensure that both daughter cells properly receive a full genome, and that no chromatids are mixed around or missing in any cell.

Cyclin-dependent kinases (CDKs) are enzymes that control these checkpoints. When CDKs bind to certain cyclin proteins, those complexes give the go-ahead at the G1 and G2 checkpoints. When a cell fulfills certain requirements, it can produce these cyclin proteins to continue on the cycle towards replication. The factors that encourage the production of cyclin proteins are known as growth factors, like an abundance of space or nutrients.

There are two other checks made by cells before they continue towards division. First, density-dependent inhibition prevents crowded cells from continuing to divide. Second, anchorage dependency requires that cells are connected to something to divide. Anchorage dependency is important in making sure cells only divide in the parts of the body they originate in.

Cancer:

When something goes wrong in cell division and it isn't caught by any checkpoint (or a mutation causes the checkpoints to be ignored), a tumor may be formed. Tumors are masses of abnormal cells within normal tissue. These abnormal cells are called cancer cells, and have no limits on division, specifically in regards to density dependence. This means that tumors can grow outside of the bounds of the original tissue, both taking up the body's nutrients and messing with the body's shape.

A tumor which remains in its original site is called a benign tumor. While these tumors are still concerning, they do not always pose an immediate concern. Benign tumors are not cancerous (because they don't spread to other parts of the body).

Tumors which invade or spread to other parts of the body are called malignant tumors. These are cancerous, and quite deadly if untreated. When cells separate from the malignant tumor and travel to other parts of the body, we call that metastasis.