If the checkpoint mechanisms detect problems with the DNA, the cell cycle is halted, and the cell attempts to either complete DNA replication or repair the damaged DNA. If the damage is irreparable, the cell may undergo apoptosis, or programmed cell death 2.
When aging cells stop dividing, they become “senescent.” Scientists believe one factor that causes senescence is the length of a cell's telomeres, or protective caps on the end of chromosomes. Every time chromosomes reproduce, telomeres get shorter. As telomeres dwindle, cell division stops altogether.
The cell cycle is controlled by many cell cycle control factors, namely cyclins, cyclin-dependent kinases (Cdks) and cyclin-dependent kinase inhibitors (CKIs). Cyclins and Cdks, which are positive regulators of the cell cycle, activate cell cycle factors that are essential for the start of the next cell cycle phase.
External Limits Cells only divide when they receive specific signals from proteins called mitogens, thus the presence of mitogens can limit cell division. Mitogens are needed for cell division in healthy cells and are released depending on what cells the body needs.
What would occur in a cell if the production of cyclins halted during the cell cycle? The cell would remain in the G2 phase and would not enter into mitosis. plant cells need to build a cell wall while animal cells do not.
A checkpoint is one of several points in the eukaryotic cell cycle at which the progression of a cell to the next stage in the cycle can be halted until conditions are favorable. These checkpoints occur near the end of G1, at the G2/M transition, and during metaphase (Figure 1). Figure 1.
To survive, every cell must have a constant supply of vital substances such as sugar, minerals, and oxygen, and dispose of waste products, all carried back and forth by the blood cells. Without these substances, cells would die in a very short period of time.
The central components of the cell-cycle control system are cyclin-dependent protein kinases (Cdks), whose activity depends on association with regulatory subunits called cyclins. Oscillations in the activities of various cyclin-Cdk complexes leads to the initiation of various cell-cycle events.
So how does growth factor influence the cell cycle? The presence of growth factors induces cell cycle progression. The presence of growth factors induces the cells to exit the G0 cell cycle phase or quiescent phase and enter the succeeding cell cycle stages for cellular proliferation.
The important point is that the surface area to the volume ratio gets smaller as the cell gets larger. Thus, if the cell grows beyond a certain limit, not enough material will be able to cross the membrane fast enough to accommodate the increased cellular volume.
As the cell divides, the telomeres on the ends of chromosomes shorten. The Hayflick limit is the limit on cell replication imposed by the shortening of telomeres with each division. This end stage is known as cellular senescence.
Why don't cells just keep getting bigger instead of multiplying? Cells are limited in size because the outside (the plasma membrane) must transport food and oxygen to the inside of the cell. This can be represented by what is known as the surface to volume ratio.