Cells Coordinate Growth and Proliferation

Cell size is a crucial factor in most fundamental processes, such as nutrient transport.

In an abnormally large cell, nutrients have to move longer distances to spread throughout the cell. As a result, nutrient diffusion becomes slow, causing the cell to die from nutrient starvation. 

Therefore, a healthy cell regulates its growth at cell cycle checkpoints- usually at the G1/S phase transition or the G2/M phase transition.

These checkpoints enable the cell to monitor its size and regulate the timing of cell division, thereby ensuring the daughter cells have a consistent size- a phenomenon called size homeostasis.

Unicellular organisms, such as yeast, are frequently used as model organisms to study size homeostasis.

A budding yeast cell divides asymmetrically, producing a larger mother cell and a smaller daughter cell.

The larger mother cell quickly grows to its critical size and passes the size checkpoint at the G1/S phase transition.

In contrast, the smaller daughter cell has a large size gap to achieve and therefore spends more time growing in the G1 phase.

At the early G1 phase, protein complexes SBF and MBF, the cell cycle-promoting transcription factors are usually bound and i