DNA Structure DNA is a double helix meaning that it has two stands that twist around each other The strands are made up of a 5 carbon sugar a phosphate group and a nitrogenous base Nitrogen Bases There are four types of bases Adenine Thymine Cytosine and Guanine These bases bind to their complimentary pair Adenine pairs with Thymine and Cytosine binds with Guanine There are two types of nitrogen bases Purine and Pyrimidine Purines are two ringed while pyrimidines are only one ringed Adenine and Guanine are purines Thymine and Cytosine are pyrimidines Purines and pyrimidines bind to each other which is why A binds to T and C binds to G To remember this think of Ag the chemical formula of pure gold So A and G are purines The bond between these bases is a hydrogen bond Between A and T there are three H bonds while there is only two H bonds between C and G This is also why DNA unravels so easily H bonds are weak The two strands of DNA run antiparallel to each other which means they run in opposite directions or one is upside down One strand is called a 5 pronounced 5 prime and has a phosphate group attached at the top because the 5th carbon that attaches the phosphate group is at the top The 3 pronounced 3 prime is the upside down strand that has a hydroxyl group attached at its 3rd carbon Which is on top instead of the 5th because it is upside down How the strands interact DNA Replication DNA synthesis is a semi conservative process meaning that half of the replicated DNA will be half new and half old There are two main part to replication Initiation and elongation Initiation getting ready to replicate An enzyme called topoisomerase relaxes super coiled DNA at a single place called the Origin of Replication The enzyme called DNA helicase unzips the DNA by separating the bonded nitrogen bases creating a replication fork This makes the two strands available to replicate Once the strands are separated they must be stabilized and this is done by a single stranded DNA binding protein or SSB An enzyme called primase then makes an RNA primer that binds to the strands which provides an extra OH group allowing them to be ready to replicate Elongation replication An enzyme called DNA polymerase III binds to the strand after the RNA primer and starts putting down complimentary nucleotides on the template strand the strand from the old DNA that is being used to make a new strand These complimentary pairs are put down in a 5 to 3 prime direction on a template strand running the opposite way This is an issue for the strand not going the opposite direction The two new strands being formed can either be called the leading or lagging strand The leading strand forms continuously The lagging strands are going the opposite way so they have to make their own 5 prime ends by starting as close to the opening of the strand as they can But since the Helicase is constantly opening more the strand ends up being fragmented because the bases have to start over further down the strand These fragments are called Okazaki Fragments Once all these new bases are matched Polymerase III proofreads the strands to make sure they are correct DNA polymerase I removes the RNA Primers and finishes off the strand DNA Ligase joins the two ends of the new DNA together and also bind the Okazaki fragments together DNA Repair Since DNA replication happens so fast many mistakes can occur Three mechanisms for repairing these mistakes are mismatch repair telomere repair and excision repair Mismatch repair This is done by DNA Polymerase III when it proofreads the complimentary pairs to make sure they are correct Telomere Repair An enzyme called telomerase extends the telomere region to allow an RNA primer and then shortens it after replication This prevents the telomere from shortening which eventually happens to every cell Excision Repair This occurs in the G1 part of the cell cycle and checks for mutations after cell division When a mutated part of DNA is found it is deleted that part is deleted and remade into the correct one