this is the video for D 2.1 on cell and nuclear division and this is a higher level topic on control of cell division we're going to talk about several examples of cell proliferation which is a rapid increase in the number of cells and there are a few reasons um why cell proliferation might take place one of which being growth multicellular organisms like you and I grow larger by adding more cells if you've already investigated the concept of surface area to volume ratio then you know why and if not don't worry you will um but it's important to understand that our cells don't just grow in size we add on more cells and that's very easily seen here in this animal embryo where we start out as one cell that zygote that fertilized egg and we grow by adding and adding and adding cells via mitosis in Plants this is going to happen in a region called their meristem and we'll focus specifically on apical meristems apical meristems occur at the Chute that's the top of the plant and at the root the very bottom so apical meristems are going to be zones where there is a lot of cell division high rates of mitosis and if we're having a lot of mitosis that means a lot of growth so if it's happening at the Chute the plant is growing taller if it's happening at the root the root is extending downwards and one of the things that I highly suggest that you do is take a look at a root tip underneath of a microscope onion cells or onion plants tend to work really well here and you'll notice that there are a lot of cells undergoing my uh mitosis in this apical meristem compared to other parts of that plant the second reason for cell proliferation is cell replacement um a lot of our cells have a definitive life span and some of them can be relatively short like a matter of days no matter what their lifespan is if they have a pre-programmed lifespan or a cell death then we're going to need to have a process for replacing them a great example here is skin cells so on the very outer layer of our skin these are actually not living cells they are dead cells new cells are produced way down here in the dermis the very inner layer of our skin these new uh brand new skin cells eventually push their way to the top by the time they get here though they've lived their entire life and then once they reach the surface here they are dead cells so that means I'm going to continually need a new supply of skin cells um being pushed towards the surface and the third reason for cell proliferation is tissue repair repair is something that would follow an injury or a wound so if I have a wound um that means that I've had some kind of cell death and all of those cells that are dead or destroyed are going to need to be replaced and that means we're going to have to really kick up the rates of cell division of mitosis in order to replace those dead cells now this will happen faster in some areas versus others depending on how many cells can readily undergo mitosis so our skin tends to heal quite quickly because we already have a lot of cells that are here a available undergoing mitosis anyways they just have to get a little bit faster things like bones might require a little bit more time this video is all about control of the cell cycle and here we're looking at a diagram representing a cell cycle and so you'll notice it's separated into a couple of parts we have a very long phase here called interphase so interphase is where cells spend most of their life it is the longest phase for any cell and interphase itself is split up into three smaller phases so we have the G1 phase which is growth okay and during this time the cell is also just doing whatever it is that that cell is supposed to be doing transcription translation if it's a muscle cell it's doing muscle things if it's a mucus producing cell it's doing mucus things so that's all Happening Here in the G1 phase in the S phase s stands for synthesis we synthesizing DNA this is when DNA replication is going to occur and in the last bit of interphase we have the G2 phase which is preparation for mitosis so we're making enzymes that we need copying organel that kind of thing all three of these are part of interphase okay so that's a very long phase um some of you may have learned interphase as the resting phase the cell is not resting um the cell is doing lots of things it's just not actively dividing okay so I just want to clear that up after interphase um then we would move into mitosis you already probably know a lot about mitosis or hopefully and you know that that includes prophase metaphase anaphase and telophase and then the separation of the cytoplasm happens here in cyto canis once those cells have been separated each of the daughter cells starts its own cycle again and we're back in interphase so let's take a look at a cell and I'm going to focus in on the nucleus and inside the nucleus if we're in interphase all of the DNA is in that chromatin form that like Loosely organized DNA um it's not condensed into chromosomes that happens in prophase and because it's kind of loose it makes it possible to be transcribed and translated once those histones start to attach to each other it hides a lot of that DNA making transcription impossible okay so during interphase in the G one phase this cell is going to grow and so that means that we need to increase the size of the membrane so the ER is going to be producing vesicles that fuse with the membrane to allow it to increase in size we also need to increase the number of organel okay so if I think about the purpose of this I'm I need to have enough in here for two daughter cells that means I need to make um new organel and some of those are going to be made manufactured by the cell itself so like the ribosomes those are made by the nucleolus and I'm just going to make a bunch more if I'm a cell that's about ready to undergo mitosis other organel like the mitochondria and the chloroplast are self-replicating okay so they'll make their own copies but at the end of the day at the end of interphase we need a high level of metabolism because there's a lot going on okay so we're growing we're making new organel there's a high energy demand here the cell is not resting it's very busy so at this point in our story we've kind of gotten through interphase right and we're about ready or we think we're ready to go into mitosis but not so fast okay in order to proceed from one part of the cell cycle to the next you need to produce cyclin cyclin are proteins that control the cell cycle there is a different cycling produced at each checkpoint in the cell cycle and that cyclin is going to bind with proteins okay um that once that cyclon has bound with a protein it allows the cell to progress into the next part of the cell cycle so we're going to notice that cells produce cyclines at different phases okay so there's going to be one cyclin that's going to allow the cell to pass from G1 to S okay that's cycl e here in this graph then we'll see the increas in concentration of a different cycl in order to pass into G2 and a different cycl in order to pass into mitosis these cyclin act again as checkpoints making sure that the cell has done all the things that it needs to do in order to progress into the next stage so mitosis is good but uncontrolled cell division producing way too many cells is bad and cells have certain genes to ensure that mitosis is only happening when we actually need new cells and we'll talk about two different genes that help with this um one is called the Proto onco genes and then also tumor suppressing genes but before we get into this let's just quickly Define a tumor a tumor is uncontrolled cell division due to the mutation in one of these division controlling genes so when one of these is not um behaving correctly because of a mutation um we can get a tumor so this mass of cells that is dividing and dividing out of control these mutations can occur in one of three ways one is just random we know mutations are random it just happens sometimes sometimes there are heritable mutations okay like some of the um genes for breast cancer and the like and then exposure to to mutagens so it could be like chemical like certain toxins or radiation these can cause mutations in either the Proto onco jees or the tumor suppressing genes so all these words are going to start to sound similar in just a minute I want to be clear Proto onco jees these are great we like these these are genes that control the cell cycle and they make sure that cells are only dividing prolification is only happening when it's necessary these these can mutate and when they mutate they can turn into enogen enogen are bad they promote uncontrolled cell division and are genetically dominant and very active so they become a dominant Al and they override the non-mutated protooncogene and it leads to cell prolification that is out of control and the growth of a tumor in addition to the Proto onogen tumor suppressor genes um are also genes that can prevent cell prolification um and they correct errors due to DNA damage so they're very important if a mutation occurs um it's generally genetically recessive so you would need two copies of that recessive mutated tumor suppressor Gene in order to grow a huge tumor this is why the number of tumor cells is relatively rare compared to the number of normal cells in our body because this is genetically recessive most of these tumors are harmless so they're going to sit here you may have um seen that or heard that referred to as benign okay so just because they're there doesn't necessarily mean that they are causing problems but they are the result of mutations in those tumor suppressing genes now if you're not too distracted by my amazing drawing let's talk about some different types of tumors so a primary tumor is just a group of tumor cells that are sticking together so let's say you have or let's hope not but let's say someone has a primary tumor in their lungs as long as those tumor Stills stay in the lungs it remains a primary tumor sometimes however these tumor cells can break off and can travel via the bloodstream or the lymphatic system to other locations okay these secondary locations are called secondary tumors okay so here's the primary tumor and it traveled to a secondary location so we have the primary and we have the secondary what's really interesting about this is that these cells are already differentiated so if these are lung cells even if they travel to the brain they are still lung cells and if you were to open up this person's brain and remove this tumor you would find Lung cells in this person's brain kind of cool but very scary this process of spreading tumor cells is called metastasis so these are things tumors are things okay metastasis is a process that's represented here by this Arrow malignant tumors are special types of tumors that are capable of metastasizing not all primary tumors become secondary tumors sometimes primary tumors stay with where they're at only the malignant tumors are capable of metastasizing so this is a way of describing some types of primary tumors so what determines if something is malignant or not a lot of things but malignant tumors are more likely to occur I should just say m these they're more likely to occur in areas with high rates of cell division so things like our ovaries or things like the thyroid places where we are replacing cells a lot the more we have to undergo cell division the more likely it is that a mutation will take place and the more likely it is that that mutation will be in one of either the Proto onogen or the tumor suppressing genes one of the ways to determine how aggressive a tumor is is to calculate something called the mitotic index so the mitotic index is actually a number it will be between 0 and 1 and it's calculated by counting the total number of cells in a tissue sample okay and then finding out how many of those cells are in mitosis so that's prophase metaphase anaphase and telophase so basically the total number of cells minus interphase okay so the total number of cells in mitosis divide or I shouldn't say total the number of cells in mitosis divided by the number of cells and you'll get an answer between 0er and one the higher the mitotic index the more cells that are undergoing mitosis which means that there are faster rates of cell division if you're looking at an area of growth like let's say your bones versus my bones your bones will likely have a higher mitotic index because you're adding a lot more cells you're still growing I am not but if we're not looking at areas of growth if we're looking at tumors a higher mitotic index can indicate a more aggressive tumor so for example in this sample I'm looking at these cells most of them are in interface whereas in this sample I'm seeing a lot more cells in various stages of mitosis so something to think about here in calculating that mitotic index and one of the ways in which our cells control cell division again theme D is continuity and change continuity being can we get that DNA passed along to the Next Generation in those daughter cells change being well what are some of the ways in which cells division can go aise so please do keep an eye on that thematic approach