0:00 Let's talk about the cell cycle. 0:12 The cell cycle is defined as the stages through which a cell passes from one 0:18 cell division 0:19 to the next. 0:21 So it's the stages of which the cell grows and prepares for division. 0:27 When the cell divides, it divides into two identical cells. 0:31 This is the cell cycle. 0:34 To understand the cell cycle, we must remember what a cell is and what we find 0:40 in the cell. 0:41 So here is a cell and its membrane. 0:45 The cells are living structures that essentially make up organisms like our 0:51 cells. 0:51 Within a cell, we find the nucleus, which is the main base of the cell, you can 0:57 say. 0:57 Around the nucleus, we find the rough endoplasmic reticulum. 1:01 We also have the smooth endoplasmic reticulum, which I have not drawn. 1:07 The surrounding area outside the nucleus is known as a cytoplasm. 1:12 The cytoplasm contains many cell organelles such as the mitochondria, which is 1:18 the cell's 1:19 powerhouse. 1:20 It makes ATP, and also there's Golgi, which I haven't drawn. 1:26 But the main thing to take out of this is that in the cytoplasm, we also find 1:32 centrozomes. 1:34 Centrozomes are important in cell division because it actually helps in 1:41 separating the 1:42 cells with its microtubules. 1:48 Now within the nucleus, the main base of the cell, we find our genetic material 1:55 . 1:55 The genetic material is DNA, right? 1:59 But the DNA is packaged up to form chrome zones. 2:05 So here is one chrome zone. 2:09 This particular chrome zone is made up of two chromatids. 2:17 Here is another chrome zone with two chromatids. 2:24 So here I drew essentially one pair of chrome zones, or two chrome zones. 2:33 It actually has 23 pairs of chrome zones, which means that we have 46 chrome 2:41 zones, because 2:42 23 times 2 is 46. 2:47 Now within these chrome zones, if we zoom in, we have these proteins called 2:52 histones, which 2:53 actually help package our long DNA very nicely. 3:00 So here is our DNA wrapping around the histones, which make up the chrome zone. 3:07 Now this is important to understand that we have 23 pairs of chrome zones, 3:13 which means 3:14 46 chrome zones. 3:18 Keep that in mind. 3:21 Most of the cells in our body divide continuously, such as our skin cells, the 3:27 lighting of our 3:28 gut for you as well. 3:31 Some cells divide faster than others. 3:33 Some don't divide at all, like mature red blood cells. 3:37 Anyways, cells have to divide to replenish old ones or injured ones. 3:44 The cycle, the actual cycle through which it prepares for division and actually 3:50 divides 3:51 into two door to identical cells is known as the cell cycle. 3:57 The cell cycle is divided into two main phases, the interphase, which prepares 4:04 the cell for 4:05 division and then the M phase, where division, the division process actually 4:12 occurs. 4:13 The division process is known as mitosis. 4:18 Let's look at the interface phase first, which prepares the cell for division. 4:28 So at the beginning of interface, we have a one door to cell. 4:33 Let's zoom into the genetic material. 4:36 This cell also consists of chrome zones. 4:39 This chrome zone only has one chromatid. 4:43 So if you remember to what I drew here, this chrome zone had two chromatids. 4:50 Either way, this cell still has 23 pairs of chrome zones. 4:55 It just has one chromatid, which means that it also has 46 chrome zones with 46 5:07 chromatids. 5:09 Anyway, the interphase can be divided into three stages. 5:18 The first stage is growth 1 or G1. 5:23 The second is synthesis or S, and then growth 2, which is G2. 5:31 This daughter cell will enter the G1 phase, which actually the duration of this 5:39 phase 5:40 is about 10 hours, and this is where we have the cellular content excluding the 5:46 chrome 5:47 zones are duplicating. 5:49 So essentially, the organelles are duplicating. 5:52 However, the chrome zones are not. 5:55 And so this cell, at the end of G1, if we zoom into the genetic material, it 6:01 will still 6:02 have 46 chrome zones and 46 chromatids. 6:09 Or we can say it has 23 pairs of chrome zones. 6:12 And then this cell will enter the S phase, which goes on for about 9 hours. 6:18 And in the S phase, this cell will actually have DNA replication and chrome 6:24 zone duplication. 6:26 So here at the end of the S phase, this cell will have duplicated chrome zones, 6:32 as you can 6:33 see by the diagram. 6:34 So if we zoom into the genetic material of this cell, it would look quite 6:40 different. 6:41 It would still have, sorry, this is still one chrome zone. 6:48 And so this cell will still have 23 pairs of chrome zones. 6:53 It will just have double the chromatids, which means that the cell at the end 6:59 of S phase 7:00 will have 46 chrome zones, but it will have 92 chromatids. 7:08 And this is because the chrome zones have duplicated. 7:12 It doesn't mean the chrome zones are double in number. 7:17 Hope that makes sense. 7:20 Now after the synthesis stage, the cell will go on to growth 2, which goes on 7:27 for about 7:28 4 hours. 7:29 And this is where the cell grows and prepares for mitosis. 7:34 And it also checks for errors as well. 7:38 And essentially at the end of the G2 phase, the cell will still have 46 chrome 7:43 zones and 7:44 92 chromatids. 7:48 And then afterwards, the cell will enter the M phase. 7:52 It will enter mitosis where it divides into two daughter cells. 7:56 And as you can see, the cell divides into two daughter cells. 8:01 And the chrome zones only contains one chromatid, not two chromatids again. 8:08 I hope that makes sense, but it will end up each having still 46 chrome zones 8:15 or 23 pairs 8:16 of chrome zones. 8:17 Anyway, we'll look into more detail on that. 8:21 Now during the interface, before cell division, before mitosis, during the 8:26 interface, there 8:27 are two checkpoint areas where the cell will check for errors in the DNA, etc. 8:36 So these checkpoint areas occur at the before the end of G1 and before the end 8:44 of G2. 8:45 So when this cell enters the first checkpoint, which is called the G1 8:50 checkpoint, the cell 8:51 will check that nutrients are sufficient, that growth factors are present, that 8:57 the cell 8:57 size is adequate, and that there is no DNA damage. 9:01 Once all this is okay, the cell can enter the synthesis phase where they will 9:05 have DNA 9:06 replication, etc. 9:08 And then the cell will enter the G2 phase. 9:11 But before finishing the G2 phase, there's another checkpoint called the G2 9:16 checkpoint 9:17 where the cell will check if chromosome replication is complete and that there 9:24 is no DNA damage. 9:25 And once that's okay, the cell will enter the M phase. 9:29 It will enter mitosis where the cell with 46 chrome zones and 92 chromatids 9:38 will divide 9:39 to produce 2 daughter cells, 2 identical cells with 46 chrome zones and 46 9:48 chromatids each. 9:53 Now these daughter cells, they don't have to enter the growth phase straight 9:59 away and 9:59 in the synthesis phase, they can actually go into a cell arrest where they just 10:04 stay 10:05 around. 10:06 And this phase is known as the G0 phase where the cell cycle doesn't really do 10:12 anything 10:13 arrest. 10:14 Of course, after a while, it will enter the growth 1 phase, the synthesis phase 10:18 and then 10:19 the G2 phase where it will then enter mitosis and then it will produce 2 10:25 identical cells 10:26 again. 10:29 Mitosis in itself has a few phases within it. 10:36 This goes on for about an hour. 10:38 It can be divided into 4 main phases, mitosis. 10:43 The first phase is prophase, metaphase, anaphase and then cellophase. 10:50 The final phase is not actually part of mitosis and it is called cytokinesis. 10:57 This is where the process of where the cell actually divides and separates, 11:06 like so. 11:07 Let's learn a bit more about mitosis and what occurs within each of these 11:13 phases.
