The Cell Cycle Control System

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  • 0:02 Cell Division & the Cell Cycle
  • 1:24 The Cell Cycle Control System
  • 2:32 Checkpoint 1: G1 Sub-Phase
  • 3:29 Checkpoint 2: G2 Sub-Phase
  • 4:15 Checkpoint 3: M Phase
  • 5:11 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

The cell cycle is an important process that occurs in your cells. But, it's also important that controls are in place to regulate this cycle. Here, we'll learn about the cell cycle control system and how it ensures proper cell division takes place.

Cell Division & the Cell Cycle

You might think you know yourself pretty well, but do you really know what's going on in your body? One thing you might not be aware of is that your body is working non-stop to maintain an almost constant amount of cells—about 10 trillion of them! This occurs through cell division, which is how organisms reproduce, the body grows, and old or damaged cells are replaced.

Cell division is part of the cell cycle, which is a series of sequential steps from the beginning of a cell until it splits in two. The cell cycle has two different phases, interphase and the M phase. Interphase is the longest phase, lasting about 90% of a cell's life, and during this time, the cell is performing for the body. Maybe the cell's function is to help you absorb nutrients or protect you from disease, and during interphase, that's what it's doing. The other 10% of the time the cell is in M phase, or, mitotic phase, which is when the cell is dividing (think 'M' for 'multiplying').

Of course, there's a lot more that goes on during each phase, but for this lesson, we're more concerned with something else: how the body keeps the cell cycle in check. A cell cycle gone wild can actually cause serious problems. For example, when cells divide excessively, this may lead to cancer in the body. And, if the cell cycle doesn't run to completion, new cells may not be made to replace old or damaged cells.

The Cell Cycle Control System

Luckily, cells have the cell cycle control system. This is a set of molecules that sets the cell cycle in motion and coordinates its steps. There are three check points in the cell cycle control system that are a lot like a series of traffic lights. As you go down the street, a light will either be green, indicating that you can go through, or red, indicating that you must stop. It's important to note that just because you get to go through one light doesn't mean the next one will automatically be green when you arrive. In fact, for safety purposes the default setting at each checkpoint is 'stop.'

Because interphase takes up so much of the cell cycle, it makes sense that two of the three checkpoints occur here. During interphase, a cell will grow during a sub-phase called G1, which stands for 'first gap.' After this is the S phase, 's' because during this phase DNA is synthesized, or copied (the cell also grows during this phase). After the S phase, the cell grows even more during the G2 phase—can you guess what this stands for? That's right, 'second gap!' You could also think of the two G sub-phases as 'growth 1' and 'growth 2' since the cell does so much growing during those times.

Checkpoint 1: G1 Sub-Phase

The first checkpoint is found in the G1 phase, before DNA is replicated. This makes a lot of sense because if there's a problem with the cell, you definitely don't want to copy that problem during the S phase; you want to stop it short before it gets there. During G1, the cell is receiving all kinds of signals that tell it if both the internal and external environments are what they should be. These signals indicate if cellular processes are normal, and therefore, if the cell should proceed to the next step of the cell cycle. If yes, then it goes through the S and G2 sub-phases of interphase, but if no, then it stops during G1.

What's amazing is that this first checkpoint seems to be the most important to many cells. And in fact, many of our cells never proceed past this point, and forever sit in our bodies without dividing! Nerve cells and muscle cells are just a few examples of cells that do not divide after reaching 'maturity' in this phase.

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