so let's review the structure of DNA the simplest component of DNA is a nucleotide more precisely it's a deoxy ribonucleotide it has a backbone made of phosphate group attached to a five Prime carbon of a sugar that sugar is known as deoxy ribos these form the backbone of DNA or you can think of them like the sides of a ladder the code of DNA is determined by the nitrogenous bases and these could be adenine thyine guanine or cytosine each nucleotide is linked to another by what is known as a phosphodiester linkage and DNA has a direction DNA is read from a five Prime to thre Prime Direction what this means is that DNA is much read like you would read a book it starts with the five Prime end of the Strand that is the Strand that begins with the phosphate group and then DNA is read towards the three prime direction or toward toward the sugar end of the DNA not much was known about DNA until the 1950s we knew the DNA was a code of life but we had no idea how it worked all we really knew is it was made up of sugar phosphate and four nitrogenous bases it's about it much of the foundational work of DNA was done by James Watson and Francis Crick in 1953 Watson and Crick identified the structure of DNA throughout their collaboration Watson and Crick proposed three three characteristics of DNA it was known that the two backbones of the DNA molecule line up side by side however they could theoretically line up in two different ways they can line up in the same direction that is both sides of the ladder would be five Prime to three Prime in the same direction Watson and Crick however proposed that the two strands of DNA were anti-parallel this doesn't mean that they were perpendicular rather it means that they line up side by side but the direction of the DNA backbone goes in two different direction directions and if this were to be true Watson and criek proposed that the pairing of the nitrogenous bases was complimentary that is adenine bonds with thyine and guanine bonds with cytosine if DNA were parallel rather than anti-parallel it is theoretically possible that like nitrogenous bases would be paired with each other that would be an a with an A and A C with a c and so on but that's not what they proposed Watson and cric later shared a Nobel Prize for their Discovery their Discovery however depended heavily on the work of a single woman chemist Rosalyn Franklin whose research was used without her knowledge or permission even worse she wasn't even given credit for her work she was not authored on her Landmark publication identifying the structure of DNA and it was Franklin's photograph of the DNA molecule that allowed them to see the structure of DNA and this sparked a scientific re Revolution that was spearheaded by by Watson and Crick Watson said in regards to seeing this photograph for the first time my jaw fell open and my pulse began to race the photo showed For the First Time The Essential structure of DNA the double helix shape and it also indicated its method for replication she did not however know that these men were using her research upon which to base the article that appeared in the journal Nature she wouldn't share the Nobel Prize either however this is probably not because Franklin was overlooked but because she was dead the word is never given out posthumously and Franklin was diagnosed with ovarian cancer in 1956 at age 37 and died two years later likely because of her exposure to radiation during her laboratory work that exposed the true nature of DNA as scandalous as the situation with Rosalyn Franklin was James Watson and Francis Crick were truly revolutionary scientists Watson and CRI proposed that the two strands of DNA served as templates or patterns for the production of new strands of DNA in this way DNA could be copied over and over and over again life requires this they argue that these strands are copied according to a complimentary base pairing in other words a DNA molecule opens up and the nitrogenous base pairs match up with their complement A's with T's and c's with G's once the physical structure of DNA had been established the next major feat was to understand how it replicated and there are three possible ways DNA could replicate and these are known as the alternate hypotheses for DNA replication the hypothesis of semiconservative replication predicts that the parental DNA separates and each strand serves as a template in which a new copy is made from each of the old strands so in this picture the pink strands represent the parental DNA strands whereas the yellow strand represents the copied chromosomes the daughter chromosomes so in this pictures the pink strands separate a copy is made and then each new double strand of DNA has a single parental Strand and a single daughter strand in conservative replication the parental DNA is used as a template for the synthesis of a new molecule in other words both parental DNA strands are copied and we would end up with the two strands of DNA one with the original parental DNA and the other a complete copy of the parental double strand the other possibility for DNA replication is known as dispersive replication in this model the new DNA molecules are a combination of segments of Parental DNA and daughter DNA so how is DNA actually replicated we know based on the results of an ingenious experiment in 1958 Matthew meselson and Franklin stall designed an experiment that would would determine which of these hypotheses would be supported nitrogen is a major constituent of DNA nitrogen 14 is by far the most abundant isotope of nitrogen but DNA with a heavier isotope nitrogen 15 is also functional Meson and stall grew ecoli in heavy nitrogen nitrogen 15 what this did was create an ecoli strand with only nitrogen 15 within the DNA strands after growing ecoli in nitrogen 15 for many generations the heavy nitrogen ecoli was grown in a normal nitrogen medium nitrogen 14 then they follow the eoli for a few generations and could determine the ratio of nitrogen 14 and nitrogen 15 in each generation comparing this ratio would allow researchers to determine which of these alternate hypotheses would be supported let's see why so when the new generation of eoli came of age half of the DNA would be parental and half of the DNA would be a copy mesil in and wall could determine the amount of nitrogen 15 and nitrogen 14 and determine the percentage of Parental versus daughter chromosomes this would be true for all the alternate hypotheses of DNA replication for the first generation however after two generations these ratios would be different each of these three models makes a different prediction about the distribution of nitrogen 15 DNA and molecules formed after replication the semiconservative hypothesis predicts that each molecule after replication will contain one old and one New Strand this would mean that half of the second generation would be low density DNA those made entirely of nitrogen 14 and half would have an intermediate density of DNA and an intermediate density of DNA would have half of nitrogen 14 and half of nitrogen 15 the conservative hypothesis predicts that each double strand of DNA makes an exact copy of itself each generation if DNA were copied this way the nitrogen 15 and nitrogen 14 ratio would be different in the second generation 1/4 of the DNA would be high density that is DNA would be made of 100% nitrogen 15 DNA while 3/4 of the sample would be low density DNA that is made of only nitrogen 14 DNA the dispersive model predicts that each strand of the each new molecule will contain a mixture of old and new DNA if DNA were copied via dispersive replication each generation would consist of intermediate density DNA that is all the DNA would be made of parts of the nitrogen 15 and parts of the nitrogen 14 DNA so what do they find eoli were grown for several Generations in a medium with nitrogen 15 when DNA is extracted from these cells and centrifuged on a salt density gradient the DNA separates out to the point which the density equals that of the salt solution the DNA of the cells grown in the nitrogen 15 medium have a higher density than cells grown in a normal nitrogen 14 medium after that ecoli cells with only nitrogen 15 in their DNA were transferred to a nitrogen 14 medium and allowed to divide DNA was extracted periodically and was compared comped to Pure nitrogen 14 DNA and pure nitrogen 15 DNA after one replication the DNA was found to have close to the intermediate density as expected since conservative replication would result in equal amounts of DNA of the higher or in lower densities but no density of an intermediate density conservative replication was automatically excluded however this result was consistent with both semiconservative and dispersive replication semiconservative replication would result in a double stranded DNA with one strand of nitrogen 15 DNA and one strand of nitrogen 14 DNA while dispersive replication would result in a double stranded DNA with both strands having a mixture of nitrogen 15 and nitrogen 14 either of which would have appeared as DNA of an intermediate density the author's continued to sample cells as replication continued DNA from cells after two replications had been completed with was found to consist of equal amounts of DNA with two different densities one corresponding to the intermediate density of DNA of cells grown for only one division of in a nitrogen 14 medium the other corresponding to DNA from cells grown exclusively in a nitrogen 14 medium this was inconsistent with the dispersive replication which would have resulted in single density rather than the intermediate density of one generation cells but still higher than cells grown only in a n nitrogen 14 DNA Medium as the original nitrogen 15 DNA would have had been split evenly among all DNA strands the result was consistent with the semiconservative replication hypothesis meselson and stall showed that each parental DNA strand is copied in its entirety suggesting DNA is replicated via the semiconservative hypothesis however they didn't give a mechanis ISM the discovery of DNA polymerase cleared the way for the mechanism of DNA synthesis the discovery of the enzyme DNA polymerase led the way to understand more precisely the mechanism by which DNA is replicated it was discovered that nucleotides are only added to the three prime end of the nucleic Backbone in this way DNA is always synthesized from the five Prime to thre Prime Direction in other words DNA is read like a book from front to back never from Back to Front DNA is replicated in replication bubbles in the chromosomes and synthesis proceeds in two different directions since replication occurs only from the five Prime to thre Prime Direction and since DNA is anti-parallel synthesis occurs in both directions bacteria have a single origin of replications in ukar DNA is replicated at several origins of replication let's look at the details of how DNA is replicated DNA begins replication by being opened Unwound and primed for replication the DNA double helix is opened by an enzyme known as helicase this is a specialized enzyme that breaks the bonds between the two strands of DNA the DNA Helix is then stabilized this means that special proteins called single strand DNA binding proteins attached to the open DNA strands in order to prevent them from automatically rebonding try to imagine unwinding a slinky into a string of metal with nothing but your bare hands and you quickly notice that when you unwind the slinky would create some serious tension so you might try to employ some tools to make the process of unwinding the slinky a little bit more easy and that's what DNA does DNA uses a special protein to combat this problem this protein cuts and rejoins the DNA Downstream of the replication fork in effect relieving tension caused by the unwinding of the Helix in the last step of initiation the da polymerase is primed a special protein known as primase starts the process of replication by providing a three prime hydroxy group that can bond with a nucleotide in order to form the first phosphodiester Bond now DNA is ready to be replicated the reome is a complex molecular machine that carries out the replication of DNA it's composed of two DNA polymerase complexes one of which synthesizes the leading strand while the other synthesizes the lagging strand the leading strand is the strand of DNA being replicated continuously and it's this strand that is being continuously polymerized toward the replication fork all DNA synthesizes occur from five Prime to three prime and in the Leaning strand that happens continuously however the lagging strand grows in the direction opposite to the movement of the growing Fork it grows away from the replication fork and is synthesized discontinuously it's called a lagging strand because it's synthesized away from the Strand and lags behind the fork slowing down the process because the Strand is growing away from the replication fork it must be replicated in fragments because the primase that adds the RNA primer has to wait until the fork opens to be able to add the primer once the primase attaches the DNA polymerase begins to add bases to the three prime end of the primer and adds nucleotides away from the replication fork producing segments of DNA these fragments of DNA produced on the lagging strand are called okazaki fragments I have to say it like that the orientation of these original DNA on the lagging strand prevents continual synthesis as a result replication of the lagging strand is a little more complicated than replication of the leading strand this process repeats at the fork with further unwinding of the DNA what is left is a bunch of okazi fragments along the DNA template finally the DNA segments are attached to each other with DNA legasee okay we've seen how DNA is opened unw found and primed and how it's replicated in both the leading Strand and the lagging strand now we'll find out how this process comes to an end the end of the linear chromosomes and eukariotic organisms are known as the telr and at the end there's sort of a problem when the whole primer is removed it leaves a single strand section of DNA this is eventually clipped off shortening the chromosome let's see what it look like at the end of the tel Mir the DNA polymerase Contin continues along business as usual and slides off completing the leading strand just like a zipper without a stopper it just Zips off however once the last okazi fragment is copied however it can't just unzip like the DNA polymerase of the leading strand this leaves an unreplicated end this will eventually get cut off interestingly telr don't contain any genes rather they're just short repeating stretches of nucleotides in order that the DNA molecule doesn't get smaller and smaller and eventually lose all functionality these repeating nucleo tiddes are copied onto the end of the lagging strands by a special enzyme known as tase this prevents the lagging strand from getting shorter DNA is incredibly accurate for every billion base pairs that are copied only a single base pair is copied wrong however even though DNA is this accurate it has a built-in spell check mechanism special enzymes known as repair enzymes ZIP down the DNA and remove defective bases and repair them with their appropriate base pair this molecule is known as DNA polymerase 3 and this is how it works even though DNA is extremely efficient at copying itself and it has a built-in spell check mechanism with the DNA polymerase 3 it still makes mistakes and these are known as mutation most most are inconsequential some are detrimental but every so often a mutation comes around and actually provides an evolutionary advantage and now you know the secret of life you're welcome every cell in your body is produced by cell division before each cell divides it must copy its genetic material in a process called DNA replication understanding of DNA na replication comes largely from studies of ecoli bacteria that are found by the billions in your large intestine let's take a look at how DNA replication occurs in an ecoli cell as we zoom in we see the DNA at the origin of replication the two strands of DNA separate serving as templates for making new strands the result is a replication bubble the bubble grows in both directions forming two replication forks let's zoom in on one of them many proteins work together at the replication fork only some are shown here the DNA is Unwound and DNA polymerases shown in Orange build new strands of DNA original parental DNA strands are shown in dark blue newly formed DNA strands are shown in light blue because strands in a DNA double helix run in opposite directions the new strands must be made in different ways one New Strand the leading strand is built continuously the other New Strand the lagging strand is built in pieces first let's focus on the leading strand DNA polymerase builds a new strand of DNA by adding DNA nucleotides one at a time each new nucleotide must pair up with its complimentary nucleotide on the parental strand adding new nucleotides works the same way on both the leading and lagging strands each piece of the lagging strand begins with a short segment of RNA shown in red a clamp surrounds the RNA and attaches to DNA polymerase which builds the rest of the new piece as DNA when the piece is finished it is released from DNA polymerase how are pieces of the lagging strand joined together a different DNA polymerase removes RNA and replaces it with DNA however it cannot finish connecting the pieces an enzyme called DNA ligase joins the pieces together growth of the leading and lagging strands continues on both sides of the replication bubble until there are two identical DNA molecules although bacteria are very different from humans the process of DNA replic in bacteria is similar to what happens in your own cells