hi guys in this video we'll be looking at epigenetics chromatin remodeling dna methylation and then we'll finish with a summary so we've mentioned in other videos how gene expression can be regulated by various factors but they can also be regulated by chemical modifications to the chromosomes so remember the chromosomes are the way that we package dna inside the nucleus and their massive regions of dna all coiled up and we can make these chemical changes to the chromosomes to alter whether a gene is expressed or not and by expression we mean whether a gene is turned on i.e the product is made or whether it's turned off and the product is not made so these chemicals can help control which genes are turned on and off at a certain time and these kind of changes are known as epigenetic changes this is not the same as a mutation they're not like mutations they do not change the base sequence of the dna so in a mutation what we see is a change in the nucleotide sequence but this is not happening in this case it's not a mutation instead what it is is the addition of chemical tags added to the dna or onto associated proteins which we call histones so we think of dna as existing just on its own but actually it doesn't normally interact with proteins known as histone proteins these have various different roles which aren't so important here but they help to kind of pack up the dna and the dna kind of winds around the proteins and this is all happening in the nucleus so you can see that we can have these chemical tags added to the dna or we can have them added to those proteins these epigenetic changes tend to occur during development of an organism but importantly they are also caused by environmental factors so for example they can be influenced by the foods that we eat and the chemicals that get digested they can be influenced by smoking and other habits and also the amount of exercise and activity that we carry out in our daily life whenever a cell divides to make daughter cells either in mitosis or meiosis many of these chemical tags get passed on to the next cell so whenever there is a cell dividing into two new daughter cells if we take mitosis as an example then half of that dna is going to be passed on and half of it's going to be new so there's always going to be some of these original chemical tags being passed on to the next generation and this might be important because it keeps some genes turned on and off during particular processes but more importantly what this means is that epigenetic changes are heritable i.e they can be passed down from one generation to the next generation so as an overall definition epigenetics refers to changes in the dna which alters the expression of genes without changing the base sequence of the dna itself these changes can be caused by the environmental factors and they are heritable so when we consider all of the chemical tags attached to the dna and attached to the histones this total complex is known as the epigenome of a cell so remember the genome is kind of the total amount or the total order of bases for all the genes in the nucleus the epigenome is simply the same thing but we're referring to the epigenetic part so the chemical tags on all of the dna and all of the histones that is the epigenome and you can see how the histones kind of associate with dna for most of its length actually all the way across the genetic material in the nucleus so those tags can be on both the dna and the histones so the epigenome is therefore this complex result of the lifetime accumulation of various signals that the cell has received from its environment so as various things from the environment impact on our cells they might change the makeup or the composition of those chemical tags and then therefore change which genes are turned on and off and this gets passed on to the next generations of cells related to the epigenetics we can have remodeling of the chromatin as well so the epigenetic changes we've been talking about regulate transcription of the genes and they do this by remodeling the chromatin the chromatin is basically the complex formed by the dna and the histones so we normally say that the nucleus is just where the dna exists and that's true but actually it's not just dna on its own it's dna and it's very closely associated with these different proteins and they're called histone proteins and as they interact the total complex of all the dna and all of the histones is called chromatin so the nucleus actually contains chromatin not just one or the other and actually it's the remodeling of this chromatin which alters whether a gene is turned on or not so a signal from the environment causes chemical tags to be added to the histones or the dna and this changes how tightly packed the chromatin is so if you can imagine the dna as it is this blue line here wrapping around all these histones you can hopefully appreciate that it can exist in several ways we can either have it very loosely packed whereby we've got these kind of beads on a string which are quite loose and floating around on the other hand we can have it quite tightly packed whereby the dna is tucked away nice and tight wrapped around the histones and there's not very much space for any access and this is altered by the chemical tags which are present on either the dna or the histones so it's these chemical tags as the epigenome which alter the state between these two and these states determine whether a gene can be switched on or off so generally speaking when the chromatin is very tightly packed i.e the histones in dna is wrapped up very tightly the dna that's wrapped around it might not be accessible to the rna polymerase enzyme or the transcription factors so here we've got tightly packed chromatin and you can see that dna is very closely associated with these histones and there's not very many spaces for it to be accessed so because it's not very accessible rna polymerase cannot bind and the transcription cannot occur so the gene is not expressed so remember if we're talking about a general gene represented by this red area we have to have the enzyme rna polymerase binding because from this enzyme we make our mrna and also normally the rna polymerase needs transcription factors to help bind itself to that gene so hopefully you can understand that if the histones and the dna are tightly packed together there's no space for these things to come in and start binding to that gene and so that gene will not be expressed so tightly packed chromatin means it's less accessible and the transcription factors and rna polymerase are just not able to bind and whenever we see tightly packed chromatin around a region this is known as heterochromatin so we named the region of dna and histones which are tightly packed together heterochromatin now let's consider the opposite scenario when the chromatin is loosely packed the dna is exposed and it's now accessible to the rna polymerase and the transcription factors so now it's very loosely packed and you can see that it's still associated with those histones which is very important but the dna now has these regions where it's very accessible and it's a very loose arrangement and therefore in this scenario the rna polymerase enzyme is allowed to bind to the dna and the gene can then be transcribed so now we've got the rna polymerase which has space to bind and also all of those transcription factors can have space to bind to encouraging the rna polymerase binding the gene will be transcribed and the product will be made so loosely packed chromatin genes can be transcribed we call tightly packed chromatin heterochromatin but loosely packed chromatin is known as euchromatin so when we see regions like this we describe the area as euchromatin so therefore overall these chemical tags control which genes are switched on or off by regulating the formation of euchromatin or heterochromatin so certain chemical tags for example these green ones might bind to particular parts of the chromatin and cause it to be euchromatin where it's loosely packed at which point several genes will be transcribed and then in different scenarios or in different areas we would have other chemical tags the red ones which cause tight packing of the chromatin which is known as heterochromatin and therefore genes are too inaccessible to be transcribed so these can be in different regions of the chromatin based on different environmental influences on these chemical tags and in different stages of an organism's life so now that we've covered how chemical tags affect the chromatin let's use the example of methylation of dna to see how this works so eukaryotic cells can use various methods but one of the methods they use to regulate gene expression by these epigenetic changes is by methylation of the dna so methylation is basically addition of a particular chemical group known as a methyl group and it's simply adding a chemical group to particular points on the dna to alter whether a gene is expressed or not so in methylation methyl groups get added to dna at a very specific location called a cpg sites that's big c little p big g sites this is basically wherever cytosine is found next to guanine in the dna chain so if we look at the dna chain here we've got g c c and we've got c g g and wherever we have a g and a c together in these two sites for example we call this whole site a cpg site and so the methylation occurs at this point specifically these methyl groups are added to the cytosine base by an enzyme called dna methyl transferase so when we look back at the diagram you can see that the methyl groups are actually attached to the cytosine not the guanine and you can see that here we have the cytosine base originally and then you can see that the change is on this part here where we've added a methyl group so a methyl group is always in the form of ch3 carbon and three hydrogens so the enzyme which carries this out is known as dna methyl methyltransferase methyl because we're adding a methyl group and transferase because it's transferring the methyl group from somewhere else to this place whenever we have this dna methylation it always inhibits transcription so it stops that gene being transcribed and this happens via one of two ways either the transcription factors can no longer be able to bind to the dna and so they prevent the rna polymerase from binding so here's the first scenario we've methylated the dna so here are our methyl groups and they physically block the transcription factors from binding and therefore they block the rna polymerase from binding too so they just don't allow the gene to be transcribed by physical blockage the other way that they might stop a gene being transcribed is that the methyl groups can attract proteins that condense the chromatin making the genes inaccessible for transcription so what we've got in this case are methyl groups being added to dna which in this case are those chemical tags and what they do is they attract other proteins to come along and basically carry out various processes to tightly pack up the chromatin in this area so they kind of just store it away and by doing so they form heterochromatin and we've said that heterochromatin is very inaccessible to enzymes and transcription factors so the gene expression goes down if we want a gene to be suddenly turned on again we can have methyl groups being removed from the dna and this process is called d-methylation so by cutting off those methyl groups we're simply removing them in d-methylation and this means that d-methylation has the reverse effect of methylation the chromatin is more loosely packed and the genes are accessible for transcription so it does two things the transcription factors and the rna polymerase can now bind but also those proteins which were tightly packing the chromatin now leave because those chemical tags are no longer attracting them and now we've got loosely packed chromatin which is the euchromatin and because it's loosely packed expression can occur because the rna polymerase and transcription factors can access those genes hey guys i hope you enjoyed the video if you are looking for an amazing a level biology resource join me today in my series of engaging bite size video tutorials just click the snap revise smiley face and together let's make a level biology a walk in the park