Cell Cycle Regulation - Online Tutor, Practice Problems & Exam Prep

The cell cycle is tightly regulated through checkpoints that act as stop signs, ensuring proper cell division. Key checkpoints include the G1 checkpoint, which verifies DNA integrity before replication; the S checkpoint, confirming accurate DNA replication; the G2 checkpoint, ensuring necessary proteins for mitosis are present; and the M checkpoint, checking chromosome alignment during metaphase. Disruption of these checkpoints can lead to unregulated cell division and cancer development, highlighting their critical role in maintaining cellular health.

Cell Cycle Regulation

In this video, we're going to begin our introduction to cell cycle regulation. So, the cell cycle, which includes cell division, is actually really highly controlled and regulated. A cell is not supposed to divide whenever it wants; a cell is only supposed to divide whenever it receives the appropriate signals for it to divide. Cell division is actually activated by a variety of cellular signals in the form of proteins called growth factors. Growth factors can be defined as a biological substance that promotes cell division. Once again, a cell is not supposed to divide whenever it wants. A cell is only supposed to divide whenever it receives the appropriate signals that promote cell division. In addition to the growth factors that can trigger cell division, there are also multiple cell cycle checkpoints that are very important in the regulation of the cell cycle. As we move forward in our course, we are going to talk more about these cell cycle checkpoints. Ultimately, these cell cycle checkpoints act like little stop signs for a cell. The cell will actually stop at these checkpoints, and the checkpoints are going to prevent the cell from entering the next phase prematurely. The checkpoints are a way for the cell to stop and check to make sure everything is okay and ensure that there are no errors before it proceeds into the next phase. If an error is detected at any checkpoint, then a protein called p53 can either trigger the repair of the error or, if the error cannot be repaired, trigger another process called apoptosis. Apoptosis is programmed cell death, which can actually be a good thing, even though it may seem like a bad thing when cells die. Apoptosis, or cell death, would be triggered if too many errors accumulate, and by the cell undergoing apoptosis, it's preventing the accumulation of errors. Apoptosis can be a good thing under the right scenario. Really, it's a cell that ignores these cell cycle checkpoints that can lead to problems, specifically, the development of cancer. Later in our course, we will also talk more about cancer. As we move forward and talk more about these cell cycle checkpoints, it's important for you to keep in mind that these checkpoints are very important for regulating the cell cycle to ensure that cancer does not develop. This concludes our introduction to cell cycle regulation, and as we move forward in our course, we'll talk about these cell cycle checkpoints. So, I'll see you in our next video.

Cell Cycle Checkpoints

In this video, we're going to introduce the cell cycle checkpoints. And so recall from our last lesson video that these cell cycle checkpoints are very important for controlling or regulating the cell cycle, to make sure that errors do not accumulate, and to make sure that the cell does not prematurely enter the next phase of the cell cycle. And so these cell cycle checkpoints really act like stop signs where the cell can pretty much stop and make sure everything is okay before it moves on to the next phase. And so we're going to talk about 4 major checkpoints that control the progress of the cell cycle. And so if we take a look at our image down below, notice we're showing you the image of the cell cycle from our previous lesson videos. But notice here that we've included the 4 major checkpoints, which pretty much act like stop signs throughout the cell cycle, where the cell will literally stop at each of these checkpoints to make sure that everything is okay before it proceeds into the next phase. And so notice that we have these 4 checkpoints here at each of these positions. And so at each of these checkpoints, there are going to be very specific events that occur. And what's really important for you all to note is that the events of these checkpoints really don't need to be memorized. You don't need to memorize them if you understand the cell cycle itself. And so if you understand the events that occur at each phase of the cell cycle, then really you don't need to memorize these checkpoints because the checkpoints and the events that occur at the checkpoints are going to be dictated by the events that occur in the cell cycle. And so the very first checkpoint that we're going to talk about is the G1 checkpoint, which occurs right at the end of the G1 phase of interphase. And so notice that here is the G1 phase of interphase and right at the end of the G1 phase of interphase, there is the G1 checkpoint, which again is going to act like a stop sign for the cell where the cell is going to stop to make sure that everything is okay before it proceeds. And so at the G1 checkpoint, what we need to realize is that the G1 comes just before the S phase and the S phase is where the DNA is going to get replicated or duplicated or synthesized. And so before the DNA gets replicated or duplicated or synthesized, the cell needs to make sure that the DNA doesn't have any errors in it. And if the DNA does have errors in it, then the DNA needs to be fixed before the DNA gets replicated. Otherwise, the DNA, the errors that are in the DNA are just going to get replicated and that won't be something good. And so this G1 checkpoint, again, is going to be dictated by the events of the phases that come after. And so during the G1 checkpoint, again, which acts like a stop sign for the cell, the cell is going to stop and fix any damage or mutated DNA in preparation for DNA replication in the S phase. And so, of course, after the S phase, the DNA is going to get replicated and the centrosome is also going to get replicated. And that leads us to our second checkpoint which comes at the end of the S phase, the S checkpoint. And so if we go up here, we can see that the second checkpoint here is going to be the S checkpoint. And of course during the S checkpoint, what we need to realize is that the cell has just finished replicating or duplicating or synthesizing its DNA. And so what the cell wants to do is it actually wants to confirm the propriety of the DNA to make sure that the DNA was replicated completely and fully and properly. And so, the S checkpoint is going to confirm the proper replication of the genetic material and it's also going to attempt to fix any errors that may have occurred. And so that is what happens at the S checkpoint. Then notice that the 3rd checkpoint after that is the G2 checkpoint which comes at the end of the G2 phase. And what we need to realize about this G2 checkpoint is that the G2 checkpoint comes just before the M phase, mitosis. And so at the G2 checkpoint, the cell pretty much wants to make sure that it has all of the enzymes and proteins that it needs in order for mitosis to proceed properly. And so that's exactly what is going to happen here at the G2 checkpoint. At the G2 checkpoint, the cell again is going to stop because the checkpoints kind of act like stop signs, and the cell is going to stop and ensure that all of the enzymes and proteins that are needed for mitosis and cytokinesis are available. If they are not available, again, the cell is going to stop at this checkpoint to make sure that they are available. And then, of course, last but not least, we have our final checkpoint here which is the M checkpoint which comes right in the middle here at metaphase. And so, of course, during metaphase, we know that all the chromosomes are supposed to line up in the middle and that's exactly what the M checkpoint is checking for. So during the M checkpoint, which really is the metaphase checkpoint, the cell is going to stop and confirm that all of the chromosomes have actually aligned properly at the middle of the cell. And so it's going to check to confirm that all chromosomes are aligned and the spindle fibers are attached properly. And so, again, if any of these checkpoints are not working properly, then that can lead to an unregulated cell cycle, and ultimately, that can lead to the development of cancer. And so these checkpoints are very, very important to make sure that the cell cycle is under control and that it's regulated properly so that cell division occurs only when it's supposed to and that cell division occurs properly. And so this here concludes our introduction to the 4 cell cycle checkpoints, and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.

Checkpoints within the cell cycle:

• A. Are located within interphase and allow entry to G0.
• B. Are located within G1, S and G2.
• C. Are located within G1, M and G2.
• D. Are located within G1, M, S and G2.
• E. Are located within all the phases of the cell cycle.

The M phase checkpoint ensures that all chromosomes are attached to the mitotic spindle. If this does not happen, cells would most likely be arrested in ________.

a) Telophase.

b) Prophase.

c) G2.

d) Metaphase.

• A. Telophase.
• B. Prophase.
• C. G2.
• D. Metaphase.

Which of the following cell cycle checkpoints ensures that the genetic material is fully replicated before mitosis?

• A. G0.
• B. M.
• C. G1.
• D. G2.
• E. S.

Here’s what students ask on this topic:

What are the key checkpoints in the cell cycle and their functions?

The cell cycle has four key checkpoints: G1, S, G2, and M. The G1 checkpoint ensures DNA integrity before replication, preventing cells with damaged DNA from entering the S phase. The S checkpoint confirms accurate DNA replication, ensuring no errors are passed on. The G2 checkpoint checks for the presence of necessary proteins and enzymes required for mitosis, ensuring the cell is ready for division. Finally, the M checkpoint, occurring during metaphase, verifies that all chromosomes are properly aligned and spindle fibers are correctly attached, ensuring accurate chromosome segregation. These checkpoints prevent errors that could lead to cancer.

How do growth factors influence the cell cycle?

Growth factors are proteins that play a crucial role in regulating the cell cycle. They act as signals that promote cell division by binding to specific receptors on the cell surface, triggering a cascade of intracellular events that lead to cell cycle progression. Without these signals, cells remain in a quiescent state and do not divide. Growth factors ensure that cells only divide when necessary, contributing to tissue growth, repair, and maintenance. Their regulation is vital for preventing uncontrolled cell division, which can lead to cancer.

What role does the protein p53 play in cell cycle regulation?

The protein p53 is a critical regulator of the cell cycle, often referred to as the 'guardian of the genome.' It functions primarily at the G1 checkpoint, where it detects DNA damage. If damage is found, p53 can halt the cell cycle to allow for DNA repair. If the damage is irreparable, p53 can initiate apoptosis, or programmed cell death, to prevent the propagation of defective cells. This mechanism helps maintain genomic stability and prevents the development of cancer by eliminating cells with potential mutations.

What happens if cell cycle checkpoints fail?

If cell cycle checkpoints fail, cells can proceed through the cycle with DNA damage or other errors, leading to genomic instability. This can result in uncontrolled cell division and the accumulation of mutations, which are hallmarks of cancer. For example, if the G1 checkpoint fails, cells with damaged DNA may enter the S phase and replicate the errors. Similarly, failure at the M checkpoint can lead to improper chromosome segregation, resulting in aneuploidy. Therefore, the integrity of these checkpoints is crucial for preventing cancer and maintaining cellular health.

How does apoptosis contribute to cell cycle regulation?

Apoptosis, or programmed cell death, is a vital process in cell cycle regulation. It serves as a quality control mechanism, eliminating cells that have sustained irreparable damage or have accumulated too many errors. This process is often triggered by the protein p53 when DNA damage is detected at checkpoints like G1. By removing defective cells, apoptosis prevents the propagation of mutations and maintains tissue homeostasis. This mechanism is essential for preventing the development of cancer and ensuring the overall health of an organism.