Cell division is a fundamental process in all living organisms. In order for cells to divide properly, they must ensure that each daughter cell receives an equal number of chromosomes. This process is monitored by a cellular checkpoint mechanism called the spindle assembly checkpoint (SAC).

The SAC works by preventing cells from dividing until all of the chromosomes are properly aligned on the spindle. The spindle is a cellular structure that helps to separate the chromosomes during cell division.

The SAC is activated by a protein called Mad1, which binds to unattached chromosomes. Mad1 then recruits other proteins to the SAC, which ultimately prevents the cell from dividing.

In a recent study, researchers at the University of California, San Francisco, discovered that the SAC can also be activated by attached chromosomes that are not properly tensioned. This finding upends the current model for how the SAC works.

The researchers believe that the SAC may be able to sense the tension of attached chromosomes by monitoring the activity of a protein called Aurora B. Aurora B is a kinase that is involved in chromosome segregation. The researchers found that Aurora B activity is decreased when chromosomes are not properly tensioned. This decrease in Aurora B activity may be what triggers the SAC.

The discovery that the SAC can also be activated by attached chromosomes that are not properly tensioned has important implications for understanding how cells divide. This finding may also lead to new insights into how errors in cell division can lead to cancer and other diseases.