These bonded nitrogenous bases are called base pairs. Likewise, guanine (G) can only bond with cytosine (C) (G-C). Thankfully, when it comes to figuring out what part goes where, all you have to do is remember that if one nucleotide has an adenine base (A), only thymine (T) can be its counterpart (A-T). But they can’t be just any pair of nitrogenous bases. These two long chains are linked together by the nitrogenous bases via relatively weak hydrogen bonds. Now, it is time to make ourselves one of these famous double helices. And you’ve probably figured this out already, but this is called the 3’ (3 prime) to 5’ (5 prime) direction. And it ends at the bottom with a phosphate. It begins up top with a free end at the sugar’s 3rd carbon and the phosphates connect to the sugars’ fifth carbons all the way down. Now on the other strand, it’s exactly the opposite. It always ‘points’ from 3 prime to 5 prime. I’ve always thought of the deoxyribose with an arrow, with the oxygen as the point. This creates a pattern called 5 prime and 3 prime. One strand begins at the top with the first phosphate connected to the sugar molecule’s 5th carbon and then ending where the next phosphate would go, with a free end at the sugar’s 3rd carbon. In other words, if you look at each of the sugar–phosphate backbones, you’ll see that one appears upside down in relation to the other. These sugar–phosphate bonds run down each side of the helix but, chemically, in opposite directions. In DNA, the sugars and phosphates bond together to form twin backbones. And like any good structure, we have to have a main support. They’re like an intertwined, microscopic, double spiral staircase. It’s important to note that in living organisms, DNA doesn’t exist as a single polynucleotide molecule, but rather a pair of molecules that are held tightly together. But all the really significant stuff, the genetic coding that makes you YOU, is found among the four nitrogenous bases: adenine (A), thymine (T), cytosine (C) and guanine (G). DNA gets the first part of its name from our first ingredient, the sugar molecule, which is called deoxyribose. Now, before we actually put these tiny parts together to build a DNA molecule like some microscopic piece of IKEA furniture, let’s first take a look at what makes up each nucleotide. Link them together and you have yourself a polynucleotide. In DNA, these small units are called nucleotides. Structurally they’re polymers, which means that each one is made up of many small, repeating molecular units. Well, nucleic acids are the fourth major group of biological molecules and, for my money, they have the most complicated job of all. Now if you can make your mind do this, remember all the way back to Episode 3, where we talked about all of the important biological molecules: carbohydrates, lipids and proteins. And so is its cousin, which we’ll also be talking about, ribonucleic acid, or RNA. These chromosomes are packed together tightly with proteins in the nucleus of the cells.ĭNA is nucleic acid. In which case every body cell, or somatic cell in you right now, has 46 chromosomes, each containing one big DNA molecule. I’m going to go out on a limb and assume you’re human. And it does the same thing for pretty much every other living thing. It’s a 6 billion letter code that provides the assembly instructions for everything that you are. DNA is what stores our genetic instructions-the information that programs all of our cell’s activities. Of course you know I’m talking about deoxyribonucleic acid, known to its friends as DNA. Laid end to end, the DNA in those cells would stretch to the sun. Now consider that there are probably 50 trillion cells in my body right now. So multifariously awesome that if you took all of it from just one of our cells and untangled it, it would be taller than me. It’s so complex that we didn’t even know for sure what it looked like until about 60 years ago. It’s pretty much the most complicated molecule that exists, and potentially the most important one. It’s double hel-exciting! You really can tell, just by looking at it, how important and amazing it is. HANK GREEN, Narrator: It’s just beautiful, isn’t it? It’s just mesmerising.
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