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Current time:0:00Total duration:9:18

DNA repair 2

Video transcript

there are certain things that can cause damage to the structure of DNA and one example of that is UV rays and so before we get into the damage that UV rays cause let's just focus for a second on the key that I drew here on the left and it's just to help us remember which colors represent which nitrogen bases so yellow represents the nitrogen based thymine the orange bases represent cytosine the green bases represent adenine and the blue bases represent guanine and so back to our DNA that's being damaged if you look over here there are two themeing bases they're kind of stuck together and that's called a pyrimidine dimer so a dimer is simply when you have two molecules that are identical they're stuck to each other and pyrimidine tells us that it can be two things that are stuck together or it can be two saito scenes that are stuck together and UV rays cause the formation of pyrimidine dimers and so you can see that the pyridine dimer actually it's causing the sugar phosphate backbone to protrude or kind of stick outwards and not just that but because the backbone is sticking out words the bond between this cytosine and this Cline snapped so you can see that there's some structural damage that happened to the DNA and so what are some factors that can cause damage to DNA and I want to just make a clear distinction between a mutation and DNA damage so it's not the same thing a mutation is when you have a change in the sequence of DNA so for example if we had a piece of DNA that read a TCG and then something happened and it read a a CG so this a is in the wrong place that is a mutation but when we talk about DNA damage we're talking about damage to the structure of DNA but the nucleotides are actually in the correct order and so DNA damage can be caused by endogenous or internal factors and that means factors that originate within us within our own cells so for example there are certain byproducts of metabolism that can cause DNA damage or DNA damage can be caused by exogenous or external factors and those are factors that originate outside of us are outside of the organism that we're discussing so let's start with exogenous factors first so we spoke about one of them UV rays and there are a lot of exogenous factors that cause DNA - what we're just going to list a few gamma rays can cause DNA damage x rays and so that's why it's not healthy to be exposed to a lot of these rays and now let's talk about some endogenous factors so reactive oxygen species is an example of an internal factor that can cause DNA damage in reactive oxygen species are molecules that contain oxygen and they're highly highly reactive so there are a lot of different kinds of reactive oxygen species but we're just going to give two examples so for example a super oxide anion which is o2 with a negative charge so let's just draw that it's two oxygen atoms that are bound together but there is one extra electron and I'm actually going to draw the extra electron in a different shade of purple and so this whole molecule has a negative charge so that's a reactive oxygen species another example would be peroxides so peroxides are molecules that have two oxygens then on either end there's another atom so that AR can represent a different at you know different types of atoms so this is the general way that a peroxide looks you might have heard of hydrogen peroxide so this is hydrogen peroxide and so where are these reactive oxygen species in ourselves coming from so actually reactive oxygen species are a normal byproduct of the electron transport chain in the mitochondria there are a lot of reaction reactive oxygen species all over our cells but fortunately we have many enzymes that help protect against the damaging effect of reaction reactive oxygen species and you may have heard of the term anti oxidant and so and an anti oxidant is a molecule that also helps protect us against the damaging effect of reactive oxygen species you may have heard that certain foods are really healthy because they have a lot of antioxidants and that's true so vitamin C for example is an antioxidant vitamin E and there are many many different types of antioxidants but we're just going to give these two as an example and so now that we've discussed some of these sources of DNA damage let's go back to our damaged DNA see if there is a way to fix this so our cells can get rid of these pyrimidine dimers in a process called nucleotide excision repair and so the first step in nucleotide excision repair is an enzyme in endonuclease is going to remove the pyrimidine dimers and any other nucleotides that are kind of not the way they're supposed to be and so I just want to pause for a second and analyze that word so nuclease tells us that it's an enzyme that's able to cut out nucleotides and that prefix endo tells us that it's able to cut out nucleotides from within a DNA molecule that's in contrast to an exonuclease that can only take out nucleotides there at the beginning or end of edenia molecule but anyway the end a nucleus is going to cut out the dimer and any other nucleotides that are not properly arranged so let's just cut out all these nucleotides the next step is a DNA polymerase I'm just going to abbreviate that pol is going to come and bring the bring the nucleotides that belong there and then the last step is DNA ligase is going to make sure that those new nucleotides are attached properly to the nucleotides on either side and also to the nucleotide that's complimentary on the other strand and so that was a mouthful but let's actually just draw all of that so let's get rid of our backbone that's kind of protruding and just not right let's read to our backbone something like that or actually go a little bit closer and then DNA polymerase brings the correct nucleotides but remember it's the ligase that actually connects the nucleotides properly and so here's our corrected DNA but what happens if for some reason the nucleotide excision repair is not working properly and this repair mechanism is only one example there are many different types of DNA damage that can occur in many different types of our pier mechanisms what happens if for some reason one of the what a couple these are not working properly then we go to sell that has a lot of damage DNA and there are three things that can happen to a cell like this the first is it might go into this dormant state where it just ages and does not divide anymore that's called senescence the second thing that might happen to it is what's called programmed cell death or apoptosis and that basically means that the cell is going to commit suicide and die and the third thing that might happen is the cell might start to divide uncontrollably so I'm going to write unregulated cell division and this can cause cancer and so actually the skin cancer melanoma is an example of this melanoma that's an N melanoma happens when the nucleotide excision repair mechanism that we just discussed is not working properly and so you have this accumulation of pyrimidine dimers that damages the DNA very much and then the cell starts to divide uncontrollably