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Now, a more comprehensive and detailed look at a strand of DNA. So again, we have this phosphate group, and again, it's a little more complicated than that but I'm gonna keep this to MCAT essentials. You have the five prime carbon here of this five carbon sugar, that converted DNA, okay. And you have an oxygen group here, so this is the five carbon, this is the four prime carbon, this is the three prime carbon, two prime carbon and one prime carbon.

Okay, and then the base comes here off of the one prime carbon, okay? And now we'll always say strands of DNA run five to three. So here you have five carbon, four carbon, three carbon and then the next nucleotide comes off here on the three carbon. So there is where we have the phosphate group that binds again to a five prime carbon.

The other four carbons, the oxygen there. And the next one will come off that three prime. Think you get the idea. All right, so that's a strand of DNA, and you can see why it runs five to three cuz you have five prime carbon coming down, and then three prime carbon, that's where the next one starts.

And now five prime to three prime, and so it goes, okay? Which carbon of the sugar does the base attach? Obviously the one prime carbon. Onto which carbon of the sugar does the phosphate group attach? Obviously here the phosphate is off the five prime carbon. Onto the hydroxyl group of which carbon does the nucleotide add?

So the nucleotide Is adding here of essentially the three prime carbon. And what type of bonds join nucleotides within a DNA strand? So, nucleotides are joined by covalent bonds and we're talking about the bonding here in one second. Okay so nucleotide refers to the entire phosphate, sugar and base. Where the nucleoside just refers to the sugar and the base.

So far as the bonding is concern between the sugar and the base connecting those two Is what's called an n-glycosidic bond. You just need to know that the bonding here between sugar and base. And then should also know that between nucleoside, so basically connecting the sugar base combination, is this that phosphate group that we talked about.

So basically you have this phosphate group and we talked about how you have this five prime carbon here. In this combination connecting this nuclear side the sugar based combo to this sugar based combo, this connection here is phosphodiester. And that's the bonding that you should know. That's that combination there.

So what does it mean when we call DNA a double helix with strands running antiparallel? Basically, you have a strand running in the five to three direction that has a complimentary strand that is then running five to three. And between here is where you have your bonds. Your Cs bonding with your Gs.

Your As bonding with your Ts. Okay? So what is the complimentary sequence then to five prime ATGCTCG3? Well, this is what we should do, we should write out five prime here, three prime here, and just match things up, TACGAGC.

The thing that I wanted to point out here, is on the MCAT, they'll have your four possible choices. And one of the wrong choices will be 5'TACGAGC3'. And obviously that's wrong. Their all the answer choice as usual will be as five prime and three.

So we need to put thing backwards again and say, 5'CGAGCAT3' is our correct answer. And be very careful not to get confused by that. It's something that every year you see students make that mistake where they take this strand here and they just write it exactly in that order and they forget that that's the 3 prime end.

So you have to flip this back around again to come up with this sequence. That's the correct answer, okay? So correlating DNA and ATP, basically what you should recognize is that ATP is the same type of five carbon sugar. And then the base coming off here. Adenosine.

And then ATP just has, instead of a single phosphate group, it has three phosphate groups. It's the same basic structure with all three phosphate groups are there is ATP. As you cut down and just have two of these it will be ADP, if you get rid of that and just have the one will be AMP. So it's the same type of format and it's worth recognizing and this is where the energy comes in ATP.

It's these covalent bonds that are attaching each phosphate group and as you cleave that off with ATP is hydrolyzed to ADP. Energy is released. And the same thing happens when ADP is hydrolyzed to AMP. Okay?

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