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Video transcript

you've probably heard of stem cells by now and you probably know that every cell in our body whether it's a muscle cell or a nerve cell or a skin cell or a red blood cell or any other type of cell really they all came from a common group of stem cells during development so all of these really really specialized cells like this muscle cell here with its little contractile proteins and this nerve cell here that can send signals and and this waterproof skin cell here and and this red blood cell that carries our oxygen all of these came from these stem cells up here which were completely unspecialized so how does something like this happen well it's it's actually pretty interesting let me first give you an analogy here so just imagine a library right like the one you used to go to when you're a teenager or something like that and the one that you hopefully still go to it has all the books you can imagine right but depending on which books you borrow and which books you read you are changed you end up knowing a totally different subset of stuff compared to someone who read different books in you right but all the books that you both read are still in this one library and there's actually a really similar system with our genes and with our DNA so recall that inside the nucleus of each cell is your DNA this is our library this is our set of genetic instructions for for building our entire body and within our DNA library here we have our books which which are segments of our DNA that we call genes and genes give ourselves specific instructions on how to make different kinds of proteins and having different proteins around will that changes the way our cells look and it changes the way ourselves act so it gives ourselves really different abilities so well I mean with the exception of the red blood cells which lack nuclei every single somatic cell in your body contains the exact same DNA yet this muscle cell here right it looks and it acts differently to this neuron here and that's because they're each reading different books in our DNA library there they're using different genes to make their proteins and just a bit of terminology here when a cell is actively using certain genes it's said to be expressing those genes and a gene being expressed is said to be turned on and one not being expressed as turned off so just keep that in mind so why am I telling you all this well because in the end it all relates to how our stem cells all the way up here end up differentiating into our specialized cells down here so the bottom line is in order to differentiate to for example specialize into our muscle cell here this stem cell up here turned on its muscle cell genes so here's its DNA and I'm highlighting its muscle cell genes that it turned on right now and it also turned off some other genes so by turning on its muscle cell genes now proteins get made within the cell that changes how the cell looks see now it's a bit elongated right this muscle cell here in it and it also changes its functions now our muscle cell has contractile proteins in it to help it be a nice useful muscle cell to help us move around right and our neuron here our stem cell turned on it's become a neuron genes here right and it turned off some other ones and then the cells started producing all the proteins that needed to turn into a neuron like the proteins that would make it a long gate like this and grow out these little spiky things up here called dendrites ok and let me also say that remember our stem cell up here was pluripotent it could turn into any of our somatic adult body cells but once it specialized into these mature cell types these can't go on to differentiate into other cells de and they actually can't de differentiate either they can't go backwards up to stem cells naturally at least in us humans so so so these cells stick around to form our bodies so by now you must be wondering well what determines what genes in a given cell are turned on or off in other words how the heck does the cell know it's time to specialize into a different cell type well it turns out that cells decide what they're going to grow up to be based on cues they get and these cues can be from their internal environment or the cues can come from their their external environment their outside environment so let me just show you two major ways this can happen here these cues so in the development of lots of different organisms us humans included we start out with one cell right the zygote and our zygote has as these little proteins called transcription factors floating around in a cytoplasm and also the precursors of these transcription factors are there two little bits of mRNA so two things to note first transcription factors will activate certain genes and turn them on that's what transcription factors do and second notice that all these little transcription factors are clustered around in one area and and this is important because when this zygote starts to divide where do all these transcription factors end up well like you see here they only end up in the cells that divide it off in that original region where they all were clustered around right so these cells up here don't have any or don't have much and these cells down here have a whole heap of transcription factors so now you can imagine that different genes will get activated in these different cells and that'll determine what each of these cells specializes into because now they're going to make different proteins so this mechanism here is pretty appropriately called asymmetric segregation of cellular determinants it's this big mouthful here but you can if we break it down here you can see asymmetric because it really just refers to how these transcription factors are not symmetrically distributed among the daughter cells here and these um this cellular determinants bit is just referring to the transcription factors or their precursors so that's one way that cells can be made to specialize in two different things just having different transcription factors around but the second way to specialization that I'll mention is is called inductive signaling or just induction and induction is kind of like really strong encouragement almost like peer pressure where one cell are actually usually a group of cells can induce another group of cells to differentiate by just using some signals and the signals could be passed a few different ways so they could be passed by diffusion they could be released from one group and just diffuse over to the other group where they'll bind receptors on the other groups and cause the cells over there to differentiate or the induction could be done by direct contact between cells right you can see little you can see these little surface proteins on each of these cells binding each other that's direct contact or you could have signals pass through gap junctions which are little connections are actually I should say connects ons between cells that are connected and and that could induce the cell to specialize the cell over here and I call this a connects on because in cellular biology these proteins that make up part of a gap Junction are collectively called a connects on anyway induction is absolutely key in forming lots of our body parts like our limbs are formed by partially through induction and our ears and our eyes and lots more of our body parts are formed through induction in development and embryological development so induction is really important in cell specialization and so on that note I'll just remind you remember the goal here with the cytoplasmic determinants those transcription factors I talked about earlier and then all these signals that you get in induction remember the goal is to get cells to change their gene expression right to flick on or flick off certain genes which which ultimately is what causes cells to differentiate into other more specialized cells
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