Internal & External Factors that Regulate Cell Division

Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

Cells can be incredibly active, carrying out processes like growth and division. To coordinate these activities, the cell uses a range of intracellular and extracellular factors.

Cell Growth and Division

Have you ever noticed the dust particles floating in a beam of sunlight? Much of that dust is made up of dead, sloughed-off skin cells, one of our most tangible examples of how important cell growth and division are to maintenance of the human body. We need the little stem cells in our epidermis to keep growing and dividing to provide us with cells to replace those we are constantly losing.

The Cell Cycle

The whole process of cell growth and division is usually mapped out as a cycle. One of the common analogies for this process is the cycle of a washing machine. Once you press start, the cycle proceeds from fill to wash to rinse to spin pretty much without fail. The steps don't get jumbled each time and each step has a specific objective to fulfill.

These are the major phases of the cell cycle:

  1. Growth Phase 1 or Gap 1 Phase: This is the phase most cells are in most of the time. This is a phase of growth, so the cell is manufacturing required components to remain functional (or alive) and increasing its mass.
  2. Synthesis of DNA or S Phase: The cell is getting ready to divide here. Before the cell can divide, it is crucial that each new cell gets a complete copy of the cellular instructions (the DNA) needed to function. Thus a whole new set of chromosomes are synthesized.
  3. Growth Phase 2 or Gap 2 Phase: The cell almost ready to divide, and now it's is going to enlarge a bit (grow) so that after the split each new cell is a decent size.
  4. Mitosis or M Phase: Here is where all that copied DNA gets neatly separated within the cell so each new cell will have a complete set of chromosomes.
  5. Cytokinesis: Now we are finally ready for division! The cell divides and one cell becomes two. These two new cells can then proceed through the cell cycle again or just hang out in a 'resting' phase carrying out their cellular duties.

Major steps in the cell cycle (numbered 1 through 5) along with major checkpoints (purple stars).
Cell cycle with major steps and checkpoints

The Major Checkpoints for Control of the Cell Cycle

So, as a cell progresses through the phases of division and growth, what happens if something malfunctions? We need some cellular factors that can essentially act like a braking system to slow things down if there is a problem.

There are three big questions to address as the cycle proceeds:

1. Is there DNA damage and are there enough nutrients?

This is the major checkpoint that occurs between the first growth phase and synthesis of the new set of chromosomes. The cell needs plenty of nutrients to carry out all the steps in the cycle and you only want to copy cells that aren't already damaged, so you want an undamaged set of chromosomes.

2. Did the DNA get copied right?

This happens between the second growth phase and mitosis. Basically, if the copying of the DNA and the preparation for mitosis did not go as planned, the cell can stop before the division process starts.

3. Are the spindle fibers properly built and attached?

Finally, one of the last things to happen before cell division is the bundle of DNA has to get pulled apart into two separate bundles, one for each new cell. There are spindle fibers, or proteins, that tether the chromosomes in place and then separate them out in an orderly way. If the spindle proteins are not ready then the brakes go on.

The idea of checkpoints is kind of like that moment in a wedding ceremony where the 'if any one has any objections'-question is asked. Instead of going through the long process of getting the go ahead from everyone in the wedding audience, all they want to know is if any one person has a reason they should stop. If so, then the process gets temporarily halted while things get figured out. Same idea in the cell, most of the pieces and parts will be in place for the process to proceed, but if one factor waves a red flag the whole process can get temporarily halted.

Intracellular Factors That Control Cell Division and Growth

When there's work to be done around a cell you can be sure that proteins are responsible for getting it done. In cell growth and division, different groups of proteins are responsible for each phase of the cycle. Since these proteins are found and act inside the cell they are considered intracellular factors. Instead of calling out all the troops right away, different platoons of proteins are activated and called upon at the appropriate time. As they finish their jobs, the next platoon is called up to action and the previous proteins are tagged for destruction.

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