D2.1 Cell Division and Mitosis [IB Biology SL/HL]

this video is for D 2.1 on cell and nuclear division this is standard level content and we'll be focusing on cell division and mitosis cell division is the process of creating new cells or you can think of it almost as copying cells if we're talking about mitosis so why do I need more cells well maybe because I need to grow multicellular organisms grow by adding new cells not by enlarging their existing cells but by adding new cells we also need to repair tissue whether that's from an injury or just um normal cell death and then

reproduction also requires cell division it's important to remember that new cells come from existing cells that's part of the cell theory so this process of cell division allows us to create new cells and allows for the continuity of Life remember theme D is all about continuity and change and we'll be talking about cell division processes that illustrate both of those so when I think about cell division I kind of like to start from the beginning right so a cell is doing its cell things we'll talk about that later and then it might go through nuclear

division dividing everything in the nucleus and then I have to actually split those new cells apart and that's going to happen here in a phase called cyto Kinesis so cyto canis is not the division of the nuclear material like the chromosomes this is the division of the cytoplasm so everything outside of the nucleus including all of the organel now in animal cells and plant cells that will look a little bit different we'll start with animal cells so when animal cells are ready to split their cytoplasm they are encircled by a ring called a contractile ring

that contractile ring is made up of two proteins called actin and myosin and that contractile ring starts to contract and get smaller and smaller and it forms something called called a cleavage Furrow so that's right here eventually that ring will get so small that the cells will pinch apart and those new daughter cells can separate in plant cells this involves building a new cell wall between the two um daughter cells so first we'll have a new cell membrane and that will come through the fusing of vesicles so vesicles will form in the center between where

the two daughter cells will split and will create the new cell membranes don't forget plant cells have cell walls so we also need to create that and that will happen with a microt tubu scaffold okay each new cell is going to build their own new cell wall you may see that referred to as a cell plate that's that beginning of that cell wall between those two plant cells and that eventually will lead to two entirely separate daughter cells so here I have an example of a parent cell cell and it's got nuclear material like chromosomes

we can think of and when that cell is getting ready to divide it's going to need to make a copy of its genetic material so we'll symbolize that with this so it has kind of like two batches of nuclear material usually the parent cell will divide equally into two daughter cells so that will look something like this okay this is an even division of the cytoplasm or equal division of the cytoplasm so I end up with two cells they're equal in size they each have nuclear materials and they would have about the same amount of

cytoplasm including the organel unequal division is not as common but it is possible and we'll talk about some examples of that if you get into like gamet production and things like that so again that nuclear material will make a copy of itself and when this parent cell um undergo cyto canis if it is unequal division of the cytoplasm then then we end up with something like this where one cell is not only bigger but contains more of the organel um than the other one this is possible as long as two things are true one is

that each cell receives um a nucleus and that each cell has at least one mitochondria that is important because cells don't synthesize their own mitochondria mitochondria rep replicate independently of the cell so if this cell did not have its own mitochondria it will never have any so that's why that is important budding in yeast is a great example of unequal division in cytokinesis budding is a form of asexual reproduction and we can see here there's some larger cells with little tiny baby cells coming off of them those are the buds and so the way that

this is working is that this parent cell has replicated the nucleus and this small Bud will receive the nucleus and just enough cytoplasm to have like maybe one copy of each organel and then a new cell wall will grow between them that small bud can then grow into um a bigger cell but again a great example of unequal division in cyto canes now oo Genesis is a great example of unequal division of the cytoplasm so oo Genesis is the process of producing eggs eggs are sometimes called oyes or when they are mature they're called oos

sites so we can kind of Link those terms together and they are made through a process called meosis we'll discuss meosis in a different video but for now it's important to understand that meosis has two rounds of cell division every time there is a split or a round of cell division there is an unequal division of the cytoplasm and one daughter cell receives more cytoplasm than the other they both receive a nucleus but one has many more of the organel and much more of the cytoplasm then during the second round the same happens again this

one that didn't receive much in the very beginning just splits again they're still really small but this one that was getting more of the cytoplasm when it under goes another round it is another round of unequal cell division so in meosis 2 there's another unequal division of the cytoplasm and this produces one viable oo site okay it's got a lot more than its fair share of that cytoplasm and then three cells called polar bodies and these polar bodies contain a nucleus but they won't be able to develop into mature oyes um because they lack the

organel that are necessary before cells can divide their cytoplasm they must replicate their genetic material okay that inside the nucleus so so the reason that they do that is so that when they split into two cells each cell may receive one nucleus that means that we won't end up with any a nucleate cells a means not so or without so we're talking about cells without a nucleus there are some cells that don't have nuclei for example your red blood cells red blood cells do not have a nucleus and that means if they don't have a

nucleus that they can't synthesize prot proteins there's nothing there to transcribe and then translate if they don't have proteins they're going to have a very limited lifespan so this is why red blood cells typically only live like a few months in our body and must continually be regenerated but for the most part in order for each daughter cell to end up with a nucleus we need to have that replication process happening before the division of the cytoplasm theme D is all about continuity and change right and and mitosis and meosis are great examples of both

of those processes so continuity is really well represented by mitosis and that is because the goal or the end product of mitosis is two genetically identical daughter cells so that genetic information has been conserved and passed along to the two daughter cells each of the cells produced by mitosis are diploid and that means that their chromosomes come in pairs and we denote that using this symbol 2 N that's for diploid again all cells are identical and have a complete genome if um an asexual reproducer is using a um a cell division process like this then

all of their genome would be passed on to their offspring when we get into meosis you'll find that that is very different theme D is all about continuity and change mitosis is an example of a cell division that really helps us to make sure that there is continuity in passing along genetic information from parent to offspring or from you know the original cell to the daughter cells mitosis produces two identical daughter cells and they are diploid diploid means that their chromosomes come in pairs and we use this um shorthand this abbreviation 2 n to talk

about diploid cells again all the daughter cells produced by mitosis are genetically identical and an asexual reproduction that means the full genome has been passed down to The Offspring meosis on the other hand is a great representative of change and that's because the daughter cells produced by meosis are not identical okay so meosis is a process used for creating gtes and it produces not two but four cells at the end of meosis and these four cells are all haid that means the chromosomes do not come in pairs and so we use this shorthand abbreviation little

n meosis Hales the chromosome number which we'll go through in a later video we're also going to find that there's a random assortment of genes and so this means that all of my gametes not only are they hloy but they are genetically unique and this is one of the things that gives um sexually reproducing organisms a lot of variation now again before cells can divide they must replicate their genetic material and that is a prerequisite for both mitosis and meosis prior to cell division DNA is elongated it's usually in a form called chromatin a little

Loosey Goosey um and there's a lot of reasons for that that involve transcription and translation and a cell's ability to function but during cell division we really need that DNA and that genetic material to be much more organized so two things are going to happen um not only is DNA replicated but it also condenses into chromosomes and those are the shapes that you typically associate um with our genetic material so before replication we might see a structure like this where we have a chromatid that's in blue and a centromere in the middle we'll talk more

about those a little bit later on we get this classic structure called a chromosome um when that replication process has taken place so because each arm of this chromosome is a replicate we call them sister chromatids okay and maybe I'll erase this here so you can see that a little bit better each arm is a sister chromed an exact replica and they are held together by these loops made out of a protein called cohesin so cohesin is a protein that kind of makes sure that those um sister chromatids stick together during a phase called anaphase

which we'll talk about later these chromatids are going to be pulled apart so they're going to be pulled in opposite directions and that's going to require the breaking of those cohesin Loops now we said two things have to happen DNA replication has to occur that's how we get those cister chromatids and this must condense into to that chromosome shape so the condensation of chromosomes means we're going from that Loosey Goosey chromatin form and condensing it and organizing it in a way that allows this immensely large molecule to move around through the cell keep in mind

that in one teeny teeny tiny little cell you have almost 2 meters of DNA it's very long and very skinny and so in order to get it to move around efficiently during mitosis we need to condense sat into chromosomes that is going to take place through a process called super coiling so this is when the DNA is wrapped around histone proteins which it always is but those histone proteins then start to link together and they condense on themselves and condense further and condense further and that process called super coiling that ends with this classic chromosome

shape once that genetic material has been condensed we need to start moving it to the poles the ends of the cells so that it can eventually become the two new daughter nuclei and that is going to be the job of something called the spindle microtubule if you've already studied cell structure you know that we have a structure inside of the cell called the cyto skeleton the cytoskeleton is made up of protein filaments called microtubules so we're going to recycle them we're going to disassemble that Sky cytoskeleton and then reassemble it into to this structure called

the spindle microtubule okay and I can kind of outline that here in Black so the spindle microtubules kind of look like these strings almost like puppet strings and they are going to link with a structure on the Centrum year called the kineticore the kineticore is going to act as a microtubule motor so you can think of it almost like as a crank okay moving these um chromatids to the opposite poles and the way that it does that is pretty clever it's actually going to remove um these little microtubule filaments like one at a time and

when it's doing that that's going to cause those microtubules to shorten as these spindle microtubule shorten and you can see that here those chromatids are then pulled apart into opposite ends of the cell mitosis is strictly the division of the nuclear material so in another part of the cell's life it's already replicated that genetic material and then it goes through mitosis which has four distinct stages followed by cyto canis the division of the cytoplasm so we'll talk about these four phases separately for each phase you need to know how to draw it how to describe

it and how to recognize it and when I say recognize I mean in these nice cute neat little like cartoon pictures but also in micrographs of real cells now we'll start with a part of a cell's life that's actually not part of mitosis mitosis is strictly prophase metaphase anaphase telophase this would happen prior to that so that's something called interphase during interphase DNA is in the form of Chromatin it has not condensed into chromosomes yet so you'll see in this micrograph it's pretty evenly spread out this stain is attached to DNA so this dark material

here is chromatin and and it's pretty spread out in this nucleus so that's chromatin um during interphase I would expect the cell to grow I would expect it to do normal cell functions like transcription and translation if it's a muscle cell it's doing muscle things if it's a liver cell it's doing liver things um all that good stuff and that at some point right before this cell is ready to start mitosis all of that DNA will have to be replicated you'll notice that as we move into prophase there are some pretty pronounced changes that are

happening inside of the cell so first of all this chromatin has condensed into chromosomes and I can see each chromatid here attached the spindle microtubules are starting to assemble and the nucleus is breaking down so this nucleus needs to disintegrate so that the chromosomes can move into daughter cells in a micrograph it's going to look something like this so I want to look for these clumps okay these black things here these are chromosomes and I can see some white space in between them classic sign of prophase in metaphase those chromosomes are going to line up

in the middle and you will almost never see that phrase that way on an exam because that would be too easy you may hear things like they line up in the center or on the equatorial plate something like that but these chromosomes have moved to the middle so that means that in prophase that nucleus has fully dissolved and the microtubules those uh spindle microtubules have attached to the centromere on each of these chromosomes okay so in a micrograph it's going to look something like this so This cell is kind of oriented to where the equator

is here here it's uh turned sideways you need to learn how to recognize cells in both ways but I don't have a clear defined nucleus and they're all aligned in the center as we move into anaphase those sister chromatids need to separate now a funny bit of naming here as soon as they or when they're connected they are called sister chromatids when they are separated each one is its own chromosome so the whole goal of anaphase or the main takeaway from anaphase is that those cister chromatids have been pulled apart and now each pull has

a chromosome so if we think of this Center area here as being the Equator the ends are called the poles and you can see in this picture that the sister chromatids have been separated and now these chromosomes are are moving towards the poles well when they're in metaphase those cister chromatids are connected by cohesin Loops so first those cohesin Loops need to be cut and then those chromat sorry those spindle microtubules as they shorten are going to pull these chromatids apart we can also see that here in the micrograph right all these chromatids or chromosomes

were together as a sister chromatids start to pull apart now I can see where each end of the cell is going to have a complete set of chromosomes and then we'll move into the last phase of mitosis which is telophase um I remember this t for telophase t for two um by the end of telophase I should have two new nuclei so in anaphase all the genetic material moves towards the poles once it is in those poles and it doesn't need to move around anymore those chromosomes can decondense back into chromatin and new nuclear membranes

form around them this happens simultaneously with cyto canis so you can see that here this is an animal cell and this cleavage Furrow is forming so cyto canis and telophase happen at the same time here's a great picture of early telophase and then very late telophase okay so each pole has um a full set of chromosomes and I can see the very beginnings of a cell plate here here it is kind of at the very end and I can see that this cyto canis process is quite far along so from start to finish here we

are okay interphase normal Cell live growth transcription translation okay and then prophase condensation of chromosomes right nuclear membrane dissolves spindle microtubules form and then we move into metaphase lining up in the middle the spindle microtubules are going to attach separation of those cister chromatids occurring in anaphase and then telophase and cyto canis happening simultaneously one of the skills that you're supposed to have um is preparation of microscope slides so I usually use an onion root tip um at that root tip um especially if you're a higher level student you should know that there's a meristem

down there a zone of cell division so there's a lot of mitosis going on this is where growth is happening for onion uh plants so they're really great to use you want to cut them in a really thin layer and then stain them so this stain is going to stick to the DNA um and really allow you to see this under a microscope you squash it under a slide and then you look at them and the goal here is to identify them I don't know how much you'll be assessed on microscope preparation um on exams

but it is a very good skill to have however I would expect to be um assessed on our ability to identify micrographs so I can help you here find some cells in different stages so um we have a lot of cells in interphase they look like this genetic material spread evenly out there's a lot that are in interphase um I won't Circle all of them so a good example of prophase I think is this cell right here you can start to see those clumps happening um let's see metaphase right next door all lined up in

the center there nice and neat for you to see aren't you lucky and then let's look for anaphase here's one in anaphase those sister chromatids are being pulled apart and then telophase um let's see here's a pretty good example of telophase okay this is not a separate cell just yet I can barely see that cell plate starting to form so again other examples in this photo as well but I hope that helps you here um identifying mitosis stages