Interphase - Online Tutor, Practice Problems & Exam Prep

Interphase

So now that we briefly introduced the Cell Cycle in our previous lesson videos, in this video, we're going to begin to discuss the first major phase of the cell cycle, which is interphase. Recall from our previous lesson videos that interphase is a non-dividing stage where the cell is not going to be dividing. Instead, interphase is a stage of cell growth and organelle and enzyme production in preparation for cell division. Interphase typically makes up the vast majority of the cell cycle, and that's because interphase takes quite a long time with respect to the M phase, which is a relatively short period of time because it's a relatively fast process. The dividing phase occurs relatively quickly in comparison to interphase, which takes a long time. Once again, in this video, we're focusing mainly on interphase, the phase that makes up the majority of the cell cycle and takes a long time. Recall from our previous lesson videos that within interphase, there are smaller subphases. The subphases of interphase include four subphases and these four subphases are based on specific events that occur inside of the cell. The very first subphase here we have is G₁. The G here stands for growth, and this is because in the G₁ subphase of interphase, the cell is going to be performing its normal functions. If it's a heart cell, it's going to be doing whatever a heart cell does. If it's a liver cell, it's going to be doing whatever a liver cell does and so on. In the G₁ subphase of interphase, the cell is going to be performing its normal functions. In addition, it's also going to be growing and producing organelles, enzymes, and proteins. Look at the image down below, here, you'll find the G₁ phase. This is where the cell cycle begins is right here, at this position. You can see this would be a cell that is first born and this cell that is first born is going to enter interphase the G₁ phase, which is the cell growth. The cell is going to be doing what it normally does and it's going to be growing and producing enzymes and organelles. Notice that after the G₁ phase, we're actually going to skip over the G₀ phase here for now and we're going to transition straight from the G₁ into this phase here which is the S phase. The S phase, the S is symbolic for the S in synthesis. This represents DNA synthesis or DNA replication, and that's exactly what the S phase is characterized by. The DNA is going to be synthesized or replicated producing a replicated chromosome with 2 sister chromatids. Just as we discussed in our previous lesson videos. In addition to the DNA being synthesized or replicated in the S phase, there's another cytoplasmic protein called the centrosome. Not to be confused with the centromere of a chromosome, of a replicated chromosome. The centrosome is a cytoplasmic protein that we'll get to talk more about its function as we move forward in our course. But the centrosome is going to be important for the M phase for mitosis later down the line. The centrosome is going to get replicated during the S phase as well. It's also replicated during the S phase in addition to the DNA. In this image of the S phase, you can see it's characterized mainly by DNA replication. But in addition to DNA replication, a structure called the centrosome is also going to be replicated. You'll see at the end of the S phase, there are 2 of these centrosomes. Also notice that the unreplicated chromosomes have been changed to replicated chromosomes over here in the nucleus after the S phase DNA replication. After the S phase, the next phase is the G₂ phase. The G₂ is the third phase here, and G₂ also stands for growth. It's just a continuation of G₁ in a way because the cell is going to continue to grow and produce new proteins. Because G₂ is the last phase just before the M phase, G₂ is also going to be preparing for the M phase. It's going to be creating new proteins that are required for the M Phase essentially prepping and preparing itself for the phase that follows. You'll notice here in this image, that at the end of the G₂ phase or in the G₂ phase the cell is going to continue to grow so it's characterized by cell growth and also preparation for division, essentially preparing itself for the M phase. Notice that the size of the cell is much larger than it was originally, so the cell has grown in size here at the end of interphase. Really, G₂ is the phase just before the M phase, The only phase that we subphase that we have not yet talked about is this G₀ phase here, which is not a non-dividing phase where the cell is not dividing. So, essentially a cell would go from initially starting in the G₁ phase, but it would only transition into the S phase DNA replication if the cell is going to commit to dividing. But if the cell does not want to commit to dividing, then it will not go into the S phase. Instead, if the cell does not want to commit to dividing, then the cell would go into the G₀ subphase, which is a non-dividing phase where cells do not divide. It will not divide. It will not continue through the cell cycle as normal. Some cells will be in the G₀ phase for a permanent period of time, whereas other cells will enter the G₀ phase for a temporary period of time, and then they can also return back to the G₁ phase and then continue to divide. This is almost like stepping aside and saying a cell basically saying that they don't want to continue through the cell cycle just yet. They would rather remain in a non-dividing state. This here concludes our introduction to interphase and the subphases of interphase, and we'll be able to get practice applying these concepts and breaking down this image here as we move forward in our course. So I'll see you all in our next video.

Cells spend the majority of their time in which one of the following phases of the cell cycle?

• A. Interphase.
• B. Prometaphase.
• C. Metaphase.
• D. Anaphase.

The first phase in the cell cycle (G1) corresponds to ___________.

• A. The phase between DNA replication and the M phase.
• B. The beginning of mitosis.
• C. The phase in which DNA is being replicated.
• D. Normal growth and cell function.

Which pair includes a phase of the cell cycle and a cellular process that occurs during that phase?

• A. G1 phase, DNA replication.
• B. G2 phase, preparation for mitosis.
• C. S phase, cell division.
• D. M phase, cell growth.

Centrosomes & Mitotic Spindles

In this video, we're going to briefly introduce centrosomes and mitotic spindles. Recall from our last lesson video that during the S phase of interphase, it's characterized by DNA synthesis where the cell is going to replicate their DNA, but they also replicate another structure called the centrosome. The centrosome is not to be confused with the centromere, which again is the waist position of a replicated chromosome where the 2 sister chromatids come together. The centrosome is different than the centromere. The centrosome is a cytoplasmic protein complex that forms or organizes what are known as the mitotic spindle. The mitotic spindle is very important for mitosis which occurs, of course, during the M phase of the cell cycle. The mitotic spindle is really just microtubule proteins of the cytoskeleton that coordinates division of the chromosomes, essentially dividing the DNA, into the 2 daughter cells.

Here in our example image, we see that the centrosome is going to be replicated in the S phase and the centrosome is ultimately going to be responsible for forming the mitotic spindle during mitosis. Over here in this image down below, notice on the left, we're showing you a single cell that has only 1 centrosome. Notice that a single centrosome actually consists of these 2 smaller proteins here. We have one single centrosome right here, and we have the nucleus of the cell, and within the nucleus, we have these unreplicated chromosomes. Notice that, after the S phase, remember the S phase is the subphase of interphase where DNA synthesis is going to occur, DNA replication. But in addition to DNA replication, the centrosome is also going to be replicated. So now there are 2 centrosomes. There was one over here and now there's a second centrosome over here. And, we have these centrosomes here, but also in addition to the centrosomes being replicated, of course, the DNA is also going to be replicated. We convert the unreplicated chromosomes here into replicated chromosomes. Notice they each have these X-shaped chromosomes here indicating that they are replicated, whereas over here they are not X-shaped, they are more linear like what you see here.

Ultimately, these centrosomes are going to be very important during mitosis because during mitosis, they are going to form what are known as the mitotic spindle. Here you can see the mitotic spindle are these structures that are projecting off of the centrosome and they are forming these microtubules that are going to be important for ultimately moving the DNA around and splitting the division of the chromosomes. This introduction to centrosomes and the mitotic spindles is a bit of foreshadowing for what you are going to see as we move forward in our course, setting the stage for deeper discussions on mitosis. So, this here concludes our brief introduction to centrosomes and the mitotic spindles, and we'll be able to get some practice as we move forward in our course. I'll see you all in our next video.

What is the name of the microtubule-organizing center found in animal cells as an identifiable structure present during all phases of the cell cycle?

• A. Spindle.
• B. Centromere.
• C. Centrosome.
• D. Kinetochore.

What is the role of the spindle fibers during mitosis?

• A. They separate the chromosomes.
• B. They break down the nuclear envelope.
• C. They replicate the DNA.
• D. They condense the chromosomes.