hello welcome back to part three of our look at cell cell biology we've kind of transitioned now the first couple of videos we've really been talking a lot about the cell membrane its structure and organization and how it engages in maintaining homeostasis and now what we're doing is we're going to be kind of uh going back a little bit actually we're going to be moving forward in our look at what are some of the processes that the cell can do and there's four main processes that we're going to be looking at dna replication protein synthesis mitosis and cell respiration this video that's going to that we're doing now is going to be focused on dna replication and within that we're going to kind of review the overall structure of dna as well a lot of this is going to be a review a lot of this we have already done in chapter 2 as well as in the lab but i think that refresher is going to be important so with that said let's go ahead and jump into looking at dna and this idea of dna replication um we're going to go back to our basic understanding of what a nucleic acid is because we know that dna is indeed a called nuclear nucleotides and we know that the nucleotides are only the very very basic components a sugar sugar and as in this case that sugar is going to be oxy ribose because we are dealing with dna right we're going to be dealing with a phosphate group and we're going to be dealing with one of four nitrogen bases either adenine cytosine orthogonal is right once we get to the idea of protein synthesis then we will go ahead and we'll dive a little bit more deeper into uh yourself and some things to keep in mind with this basic structure is again it's organized in monomers called nucleotides right and if you look right here you're gonna see that we indeed are using deoxyribose and we're going to look at that two prime carbon and we're going to say hey there is no sugar there or i'm sorry there's no oxygen there and so this is a molecule of ribose that is lacking one oxygen molecule and that's where it is of course we have our nitrogen base this is adenine it is a double rung structure even if you didn't know that was adenine you should be able to look at this and say okay there's two nitrogen rings so it's either going to be adenine or guanine just by looking at the structure you should have been able to narrow that down and of course we have a phosphate group that is located right there and these nitrogen bases if you recall are broken down into two major groups or classifications based off of the structure that they hold and so purines purines are double rung nitrogen structures all right and this is adenine and this is guanine and we know that adenine and guanine can never bind together all right so you can never have two purines that are bonded together this doesn't happen each of these purines must create a bond with one of these guys down here these single rung nitrogen structures and these single rung nitrogen structures are classified as pyrimidines okay this is again a review from chapter two we have three pyrimidines we have cytosine we have thymine and we have uracil and if you remember adenine can only form a bond with thymine guanine can only form a bond with cytosine and again uracil is rna so it's a little bit different of a circumstance but it is a pure pyrimidine because it is only a single nitrogen rung structure all right and taking this a step further adenine and thymine if you remember can only form double hydrogen bonds and guanine and cytosine will always form triple hydrogen bonds all right so you always have a purine with a pyrimidine you always have a purine with a uh pyrimidine and that's what you're seeing right here and so once again we've got guanine and cytosine and that's forming this triple bond we have adenine and thymine and that's forming a double bond here's a thymine and an adenine that's forming a double bond here's guanine and cytosine it is forming a double bond and if you remember once again from chapter two what kind of a bond is being formed over here and over here and over here if you remember these are phosphate groups that are forming bonds with these sugars and so therefore we refer to that as a phosphodiester linkage all right while these over here these are hydrogen bonds all right so we have hydrogen bonds holding the nitrogen bases together and we have phosphodiester linkages that are joining two nucleotides together at the phosphate group and the respected sugars in this case deoxyribose the other thing that i will mention here is that um the fact that adenine and thymine always only form double bonds and guanine and cytosine always only form triple bonds right we refer to that as the law of complementary base pairs right the law of complementary oops the law of complementary base pairs all right the law of complementary base errors says because of the hydrogen binding sites adenine and thymine can only form a bond guanine and cytosine can only form a bond adenine and cytosine cannot because adenine only has two room for two hydrogen bonds and cytosine needs three and so that limits it that puts an automatic limit on it now how do we we've got this we've got this this dna structure it's a it's a ladder structure it's a double uh stranded structure here's one strand here's the second strand both strands are linked by these hydrogen bonds all right how do we go from [Music] this to being able to copy and duplicate that well that's what we kind of need to look at next and one of the things that i want to remind you about here is that dna copying or dna replication is taking place during s phase of interphase right so dna replication is happening during s phase of interphase and it involves the direction of three enzymes to coordinate taking this strand of dna that you're looking at right here and making an identical copy of the strand and so to do that we have again three enzymes the first enzyme that i'm going to introduce is dna helicase dna helicase the job of dna helicase is to break the hydrogen bonds it breaks the hydrogen bonds so dna helicase comes in here and it unzips the hydrogen bonds so it only does that to a certain point right so you've got to pretend in here that we've actually got the right number of hydrogen bonds there we go all right dna helicase comes in and does what we call unzipping it unzips it breaks those hydrogen bonds but it only does so many base pairs at a time and where it stops is what we call a replication fork we refer to that as a replication fork so if i redraw this over here here's my adenine and my thymine but then these are all busted hydrogen bonds these guys are no longer linked that's a c c and then a g okay so thymine and adenine cytosine and guanine guanine and cytosine guanine and cytosine cytosine and guanine all right so that's the job of dna helicase that's the first enzyme the second enzyme is what we call dna polymerase and dna polymerase has two jobs one it matches new nitrogen bases up so it pairs back up the nitrogen bases so it looks here and it says oh i've got a c here i need a g i have a g i need a c i have a g i need a c i have a c i need a g i have a t i need an a and vice versa so what we end up with are these two partially formed new strands where one half of the strand is original and the other half is new all right we have one half of this newly formed strand of dna that's part of the original strand and then we have um the second strand that is of new nitrogen bases right so that's the first job of dna polymerase it's to match up these newly formed nitrogen bases the second job is to proofread it's the proofread so what do we what are we proofreading well dna polymerase usually makes a mistake one out of every 1 000 base pairs one out of every one thousand that's a lot that's a lot of mistakes right so maybe here he puts a c instead of a g maybe here he puts a g instead of an a dna polymerase is just making mistakes all right so dna polymerase goes through and will proofread the work that it did and when it proofreads it catches about 90 percent of the mistakes all right teaches catches ninety percent of the mistakes there's a million right so it reduces that rate of error from one in a thousand base pairs down to about one in a million base pairs all right so dna polymerase has a has a heavy job it's an important job but we're not done yet because notice there are no hydrogen bonds holding anything together and so now we need to come in and we need to put in hydrogen bonds well the enzyme that's responsible for reestablishing these hydrogen bonds is dna ligase ligase is a fancy word that means to join right and so dna ligase is going to join the nitrogen bases by putting in and re-establishing the hydrogen bonds so tna is going to get a double c and g is going to get a triple g and c gets a triple g and c gets a triple c and g gets a triple and then we can work our way right back up here like so now while dna ligase really while dna polymerase is doing its thing dna helicase comes back in it comes back to that replication fork and here's the replication fork it continues to break the hydrogen bonds going up and so this is this is a continual process dna helicase comes in it unzips the segment creates a replication fork dna polymerase comes in matches up the base pairs while it's proofreading those base pairs dna helicase is now con starting at the replication fork and it's unzipping more it's unzipping more dna ligase is now then coming in and it's reforming the hydrogen bonds in the first section dna polymerase moves up to the second section starts to match up the base pairs proofreads dna ligase can then go up and then continue to fill in those hydrogen bonds and so we've got this constant seesawing of action all right this is happening simultaneously right this is kind of all happening simultaneously as we get started but that is dna replication that is how we create new strands of dna during s phase of interphase and if i come back over here so now if i redraw this i'll draw it over here all right so now what we end up with we end up with a and t a and t tna tna c g g c g c cg and so you end up with two new strands of dna where one strand on each side is the parent strand and the other strand is new is new so every strand of dna has one original strand and one new strand and then when it replicates it's still that new one's going to have one old strand and one new strand and so this is the term for this is semi-conservation this is a semi-conservative process right because you're conserving half of the old genetic material but you're creating a new strand at the same time and then just to just to kind of make sure that you guys are good with the overall structure of this thing that we call dna all right we've already talked a lot about base pairings i'm not going to really talk about that but i do just want to point out to you the difference between chromatin chromatid and chromosome right so chromatin is what we find during interphase and it is a ball of genetic material in other words the the genetic material the chromosomes are all unwound right kind of think of like you go up in the attic to pull down christmas lights what do they look like somehow they're all balled up you didn't put them in that way but that's what they look like now all right and so chromatin chromatin kind of looks like this and this is exactly how it's duplicated in the cell during interphase it's not condensing it's not condensing at all it's it's copying that all right it is copying that all right then as we head into prophase prophase anaphase and the beginning of telophase early telophase i guess up here you could also include late telophase up here what ends up happening is all of that genetic material that was copied condenses it condenses all right so you've got to kind of imagine that you've got two balls of mess up here this ball condenses down into that this ball condenses down into that and we define those as being chromatids now these are identical chromatids and so this is where we get that term sister chromatids from and what do the sister chromatids do well during metaphase during metaphor during metaphase the sister chromatids join together and that is what we define as being a chromosome so what you when we use that term chromosome understand that technically the chromosome is only present during metaphase prophase anaphase and early telophase you've got chromatids and late telophase and interphase you've got chromatin it's all dna it's all genetic material it's just the level of organization that we are dealing with that's all that we're really dealing with here is that level of organization that is what differs all right now the only other thing that i will mention is that each of these sister chromatids have a protein that is on them that little red dot there all right and that little red dot uh is what we define as being a kinetochore and so that can that is a kinetochore so the red dot is what we define as being a kinetochore write that a little bit more legibly k i n e t o c h o it's a kinetochore and what does the kinetochore do well here's the kinetochore here all right here's the kinetochore here what it does is that drives the area where the attachments of the sister chromatids are going to happen and that creates a region that pinches in that is defined as being the centromere and so the centromere is the area that includes the proteins called a kinetochore that allows for the sister chromatids to join and that's it that's dna replication and an introduction into uh dna structure all right so um please take some time this week uh do some review of these first three videos before moving on to videos four through seven i have a really good understanding of what's happening in these first three videos because i think that will really help you moving forward for the the second half and uh with that said again keep studying keep reviewing keep chunking five-day study plan all right chunk that material don't cram it and uh any questions let me know um i'm always available you can always shoot me those emails to to ask questions and hey i'll catch you on the flip side