What shortens after each cell division?

Telomeres are specialized structures located at the ends of linear chromosomes, and their primary function is to protect chromosome ends from deterioration or from fusion with neighboring chromosomes. Each time a cell divides, a portion of the telomere is not fully replicated due to the limitations of DNA replication; this phenomenon is often referred to as the "end replication problem." As a result, telomeres become progressively shorter with each round of cell division. When telomeres reach a critically short length, the cell can enter a state known as senescence, where it stops dividing, or it may undergo apoptosis, or programmed cell death.

This process is a critical component of cellular aging and has implications for organismal lifespan as well as the development of age-related diseases. In contrast, while chromosomes themselves are recombined and segregated during cell division, their overall number does not shorten in a healthy dividing cell. The nucleus and cell membrane maintain their structural integrity throughout cell division, generally not undergoing changes that lead to a decrease in size or number.