Bozeman science: Development timing and coordination

hi it's mr. Andersen and welcome to biology essentials video number 24 this is on regulation of timing and coordination in development and so it's really about development figuring out how we go from that first I go for that first fertilized egg to an embryo and then eventually to a baby and eventually to you and the magic of that we're starting to kind of unlock in other words how do we go from this one cell with its DNA to all the various cells inside this baby that do different things even though they have the same exact DNA inside them and so is really about new discoveries pretty exciting and so we start though by talking about seeds and how seeds germinate seeds can remain dormant for a long period of time but eventually they come to life and so how do we go from those cells inside that that really small embryo fight inside that small cita the seeds of a tree how do we go from an acorn to a tree that's what development is all those specific stages and so the first step is cellular differentiation in other words each of the cells have to figure out what kind of cell they're going to become they're going to express specific proteins and those proteins as a result of which ones they express determine what cell it's going to be cells eventually become embryos and cells inside the embryo are going to induce other cells to become cells like them and we've been able to show through genetic transplantation and the use of transcription factors how that actually works cell death we're starting to find is just as important as cell growth example I'll talk about is how the fingers and toes form and the importance of micro RNA and regulating that and then finally we're going to talk about the body genes the homeotic genes and how they place different body parts where they should go and the study of mutants has unlocked a lot of that and so let's start with seed germination and so if you take a sunflower seed and plant it it probably won't grow and the reason why is that too much salt on it but if you take a normal sunflower seed and plant it it will grow into a sunflower and what it'll have in order to do to do that you have to give it two things first thing you have to do you have to increase the temperature you have a a perfect kind of a window for the temperature of the seed but you know this if you grow plants the other thing that you have to do to activate a seed is you have to add water to it but if you do those two things if you have the correct temperature in the correct water on the seed is going to germinate and so what we're going to get is a beautiful sunflower eventually but the cells here and the root are going to be different than the cells here in the leaves and the cells here in the shoot and so how cells determine that is going to be development it's the stages by which those cells eventually go and become an organism and so first thing I want to talk about is cell differentiation cell differentiation is how cells become the cell that they're going to be or the cell that they're going to actually act as and so these are colonies of stem cells human stem cells and they can just kind of grow them on this lattice of these fibroblasts of mice but those cells inside us will eventually specialized they'll become a cell for example these are nerve cells and once you become that cell you can never go back to being a stem cell again in other words once you've made that choice to become a cell you can't go back when we can kind of do that in the laboratory but in life you don't really do that with a few exceptions for sure um and so how two cells become cells well they have tissue specific proteins in other words if this is a stem cell and this is really simple it's it's just got one chromosome pair and this is inside its nucleus but how do we go from that first cell to a red blood cell for example or a neural cell or this might be just a run-of-the-mill skin cell for example how do we become that well the DNA in each of these cells is identical and so how could you form all these myriad of cells if the DNA is X exactly the same well what we've discovered is there are going to be cells in here or genes inside here let me label these ones red and again this is really simple and so when a cell decides it's going to be a red blood cell what we think happens is that all of the parts of the DNA that don't express or don't express proteins to make red blood cell all of that DNA is going to wad up or crumple up and so all of this DNA isn't used its wadded up and the only DNA that can actually make proteins are going to be the genes for a red blood cell and so even though the DNA in this red blood cell and this neural cell are exactly the same all the neural saw the neurons genes aren't going to be expressed I could give a different color for them there might be like these blue jeans that just make the neuron but since they're wadded up they're not going to be expressed however in a neural cell all the red blood cells would be wadded up those jeans and then the neural cells are actually going to express and so when cells make this choice as to what cell they're eventually going to become we call that cell differentiation and that happens almost from day one so I'll start to figure out what cells are going to be let's take a look at that and how that might work and so that one fertilized egg that ones I go is going to copy itself through mitosis and make a bunch of cells and so now we call those stem cells these cells can become any Cell that they want to and in us what it'll do is it'll actually form a sphere a blastula then it folds in on itself and causes it form something called a gastrula and the part on the folded inside becomes your digestive parts and the outside eventually becomes your arm the nerves and the ins in the middle part is going to become like the bones and the muscle and so how does cells determine what cell they're going to be what we're starting to figure out is there are what are called transcription factors in other words cells are going to give off proteins and those proteins are going to determine what type of cell you're going to become and so I made it really simple we call these the top and the bottom transcription factors in other words this cell right here just as a result of being on the top of these stem cells is going to express this transcription factor will call that the top transcription factor what's that going to do well it's going to bind with the DNA inside the cell and it's going to cause it to activate that the specific proteins for a top kind of a cell it's also going to give off more transcription factors to the cells adjacent to it and so if I animate that here for a second so what we'll have is will have the transcription factor or choose me the the transcription factor right here produced by this cell it's a yellow cell will say and that's going to create transcription factors that are going to move to adjacent cells and those adjacent cells now are going to express more transcription factors so they're going to determine what cells they should become and so these cells know that hey we're on the top these ones are on the bottom these ones are on the middle and so there's this cascade of these transcription factors that kind of play out how we go from that first stem cell to the actual embryos let me give you an example of one all of us are born female but there's one specific gene it's called the sry gene stands for the sex-determining region why gene because it's found on the Y chromosome and what happens is in males there's an X chromosome and a Y chromosome but this one gene will be given off and it will flood adjacent cells which will also give off the sry gene and then you become a boy and if there is no sry gene then you become a girl and so you could for example the X Y and you become a boy but you could also be X X Y and you peak about a boy or you could be X X X X Y we'll talk more about those later and you'd still become a boy because you have that sry gene likewise if you're XX you don't have the sry gene you don't become a boy a cool study was done to show the importance of induction in other words this idea that these cells can induce the cells adjacent to it or the tissues adjacent to it to become something these are going to be these cave fish that are found in Mexico and there was a study done where they took the lens of because these fish started living in caves and lost the I we call these the eyeless cave fish and what they did is they took a similar fish that are found above a cave level and they actually have eyes they just pulled the lens out of this fish and they put it in the lens of one of these cave fishes and it event it developed a normal eye they then took the lens of this cave fish put it in the embryo of a normal fish and nothing happened and so there was something about the cells inside that lens that we're actually triggering the formation of all the other parts of an eye the retina the optic nerve all of that function correctly through this induction and so this is called the transplantation and it's one of those studies that showed the importance of where you are or location location location now not only differentiation but cell death is super important as well so these all the parts of this embryo of a human are going to develop through differentiation and induction but you'll notice that right here between the fingers and between the toes the cells are actually dying and that process called apoptosis is actually really really important and so not only the growth of cells but the death of cells is important in forming a new organism a great example has been studying the drosophila Drosophila this would be an embryo of drosophila and this would be an adult and um this will eventually become this but a pop ptosis or cell death is super important in that regulation and so there are a number of different genes that that scientists identified and i love the names here they're the hid the grim and the Reaper genes and these three genes in drosophila cause the cells to die in other words they make proteins that cause cells to die another important thing that I should talk about is micro RNA micro RNA is going to be a single strand of RNA it's found in all organisms and what it does is it actually disrupts RNA so it disrupts RNA so can actually make proteins and so what scientists found is that if we dish rupted the RNA in drosophila in other words we got rid of the are micro RNA the hid the grim and the reaper genes went crazy and they took this embryo and turned it from something that was eventually going to turn into a fly and turned it into something that was nothing to died in other words these death genes go crazy unless we have the micro RNA actually regulating them when we put the RNA back in their tempering these death genes and then we get development and so again development is growth of cells but it's also death of cells and the micro RNA we're finding is really important in regulating this timing of the development and the last thing I want to talk about our homeotic genes homeotic genes are found in us they're found in a fruit fly they're found in the mouse but we've studied them in fruit flies first and what we find is that they're a series of genes in the chromosome of fruit fly that tell the fruit fly where to put the body parts now these genes actually lead to a number of other genes there's a cascade of these genes causing all these other things build an eye or build an antenna or build a leg but these ones are super crucial for where it goes that's what homeotic genes do and an important set of those are called the hox genes that's what I have pictured here um and so where did we learn about homeotic genes what we learned about them from mutants and so there is a mutation called the ultra by thorax remember a head thorax and abdomen of a fly or an insect what the ultra by thorax gene does is it actually causes a duplication of this thorax and so a fruit fly normally looks like this but if you have a mutation or a change in this ubx gene so it can't function what you get is not one thorax with two wings but you actually get to throw axes or if there is a a mutation in this antenna pedia gene what you'll get is one of the legs of the fruit fly will actually grow out head the fruit fly and so that's kind of science fiction stuff it's creepy but what what it tells us is that inside fruit flies in there by inside us there are a series of genes that tell us where the body plan where the parts go and so as embryo starts to develop into a fetus and eventually into you homeotic genes are super important and kind of laying down the landscape of where things go and so that's development those are some the cutting edge discoveries that we've made and I hope that's helpful