welcome back everybody today we're going to be looking at dna replication and we're going to specifically focus on why dna replication needs to occur and we're going to look at the phases and steps now most importantly we need to focus on when exactly dna replication and where it's taking place now dna replication occurs during interphase and interphase is the phase in which a cell spends the majority of its time where it is growing and it's developing and specializing and this cell that you can see in front of you here is currently an interface and we know it is an interphase because the nucleus is still intact the dna is in the form of a chromatin network which is that long thread-like structure you'll notice the centrioles which are the structures that grow the spindle fibers are off to the side and they don't have any spindle fibers growing out of them and essentially this is the phase in which our cell spends the majority of its time now why does dna replication need to happen essentially it needs to happen because we need to make new dna and we need to make new dna because we need to make more cells and if we want to replace or repair a cell we have to make sure that all the dna in that cell is exactly the same as all its other sister cells so why is replication necessary well we need to produce another molecule that is exactly the same and we want it to be the same because any changes to the genetic code would result in mutations and mutations can sometimes be harmful we also needed to pass on our genetic code we want our offspring to have similar genetic code to us and therefore ensuring the survivability of our species and finally we need it for cellular replication we need to be able to produce lots of cells that are identical to each other in order to repair or replace any damaged ones now let's remember where interphase is in terms of the stages of mitosis interphase is the phase in which a cell spends its majority of its time and remember interphase is where our dna often finds itself in a chromatin network a long string-like structure and in order for us to progress into prophase into metaphase anaphase and finally telophase which results in two identical daughter cells we need to ensure that the dna that sits inside these cells is identical in other words we need to make a full copy of the set of chromosomes that we find in any one of our cells so to put it into perspective this graph or pie chart essentially shows us how much time a cell spends in each of its phases and you will notice here that the majority of the time spent in the cell cycle is actually an interphase and the g1 and g2 represent the growth phases of a cell where the s phase represents dna synthesis in other words this is the phase in which we synthesize new dna molecules which is what we are going to focus in on today you'll notice that meiosis is actually quite a small section of the overall cell cycle now in metric you're going to need to be able to describe the stages of dna replication in quite some detail and what i'm going to do now is explain each of the stages and i'm going to highlight some important keywords and add in a few extra pieces of terminology that you should be aware of so let's not forget that our dna strand is a double helix shape remember that that is two strands of dna that are wound around each other now when they're in this wound up state it's really difficult to get to the information that's on the inside which remember is the nucleotides the letters the a the t the g and the c and that's the genetic code and we need to be able to get into that so the only way to do that is to take this double strand and we need to unwind it in other words we need it to lie flat or like a ladder now as i just mentioned all the important information that we want to get to are these letters remember this is the genetic code and we want to copy the genetic code now we can't get to it if the code is still stuck together and so what we need to do is the two strands of dna are going to separate and their hydrogen bonds are going to break and as we can see here we can see at the lower part of this dna molecule that the bonds are breaking in between the two nitrogenous bases now who's responsible for breaking down those hydrogen bases is an enzyme remember enzymes catalyze reactions and so what this enzyme does is it reads the dna code and as it moves through it it separates the nucleotides from one another each of the resulting strands that you're left with are going to be used as a template in other words when this process is done you should have two separate stones with their nitrogenous bases that are exposed and each of these bases are going to be able to form a new complementary strand now that we have two separate strands we now need to allow our complementary base pairs to find their complementary partner so that they can form a new strand and floating in the nucleus itself are nucleotides and they're going to be attracted to their complementary base pairs let's not forget that adenine joins to thymine and guanine joins to cytosine and they're always found in equal proportions in other words if there are 50 adenines there will always be 50 thiamines if there are 20 cytosines there will always be 20 guanines now if this is our piece of dna that we are trying to copy inside of the nucleus in the nucleus floating around freely are dna nucleotides and they're complementary in other words floating around in the actual nucleus itself will be a complementary nucleotide so in this instance if i wanted a complementary nucleotide for my adenine there will be a thymine and attached to that thymine will be their sugar phosphate backbone there will also be a cytosine and its sugar phosphate backbone will connect with the thymines lastly we need a adenine to join our thymine and its sugar phosphate backbone connect as you can see this happens running all the way along as we make one long complete strand of dna please also remember that this is the entire strand of dna it is not just sections of the dna we are making full complete strands which means we need to read the whole dna strand now this process like many other processes is all controlled by enzymes and the names at this point are not very important to us so it's not really necessary for us to memorize them but all you do need to know is that a enzyme is currently attaching all of these nucleotides in the correct order and reading them to make sure that no mistakes were made so now that our enzyme has done some of the reading it ensures that all of our nucleotides have lined up correctly that they're joined together and this enzyme that i mentioned earlier is called dna polymerase now when the enzyme joins it together you will notice that we now have two strands strand one and strand two and they are identical to one another it's also really important to mention that hydrogen bonds form once again in between all of our nitrogenous base pairs because remember that is the chemical bond that holds the whole nitrogenous base sequence together you now have two completely identical strands of dna strand one and strand two both of them comprise of a original strand and i'm just going to use this black strand as our original strand and then i'm going to use a different color to represent the new strand that we have intertwined so essentially when you're done you'll have two strands each of them comprising of an original strand along with a complementary one that has just formed and lastly we can't forget we can't leave our dna simply as a ladder-like structure so we do need to rewind it back up into its double helix and an enzyme is responsible for doing this as well now that we've technically doubled the amount of dna it's important for the cell to do something with this doubled dna and in this example a cell would go on to divide and make two sister cells that are identical which is why we needed two sets of dna it's why we needed strand 1 and strand 2 because one of those sister cells is going to keep strand 1 while the other sister cell is going to inherit strand 2. now many matrix don't revise this section very much and often it is a topic that we confuse with a section about protein synthesis and i think the reason why we often confuse them as you'll see and progress is that they have similar components about them however they're very different and their products are very different and it's important to use your exam guideline booklet in order to guide you through what exactly you need to know and what exactly is the difference between these two processes thanks once again everybody and i will see you again [Music] bye