![]() ![]() Likewise, A and G don't pair because they are both large purines. T and C don't pair because they are both small pyrimidines. The sizes of the four bases and the exact appendages of atoms on the hexagonal rings make it such that T and A pair and C and G pair. ![]() The base pairs in DNA need to be approximately the same length in order to fit within the double helix structure I'll be making later. The weaker the bond, the longer the bond length-recall the stronger covalent bond is represented by a 20-millimetre-long tube. Hydrogen bonds are represented by 30-millimetre-long clear tubes in the model. A hydrogen bond is a weak electrostatic attraction between a partially positive hydrogen atom and a partially negative atom such as oxygen or nitrogen. The next step is to join the bases into pairs using hydrogen bonds. Each purine has a pyrimidine-like hexagon with a pentagonal ring attached by sharing two adjacent corner carbon atoms. The T and C bases are called pyrimidines, and the bigger A and G bases are called purines. The appendages are composed of hydrogen, oxygen, carbon, and nitrogen atoms. I adorn the rings around the outside by connecting single atoms or simple molecules (of no more than four atoms) to most of the corners. These grey tubes represent strong covalent bonds formed by the sharing of electrons. I connect each atom to its two neighbors using 20-millimetre-long grey tubes that slip over the stubs on the atoms. Both have one ring in the shape of a hexagon, with four carbons and two nitrogens at the corners. Their main structures are flat rings built with carbon and nitrogen atoms-the nitrogen atoms make them bases because nitrogen can donate an electron pair. These bases are often referred to as T, A, C, and G. I'll be building 20 base molecules: five called thymine, five called adenine, five called cytosine, and five called guanine. A base is a molecule that tends to accept a proton or donate an electron pair (an acid, which we'll run into later as the "A" in DNA, is a molecule that tends to accept an electron pair or donate a proton). ![]() The first step in the model instructions, which are nine pages long and include illustrations, is to assemble the bases. There are only these five elements in DNA. The atoms are small plastic balls with stubs where bonds will be (you might recall that atoms have a nucleus of positive protons and neutral neutrons surrounded by shells of negative electrons, and electrons in the outer shells can sometimes be shared between atoms to form bonds). ![]() I feel excited, like I used to when I was a kid unpacking a new box of LEGO on the floor and previewing the construction steps. On a Saturday in January, 2018, I'm sitting on the carpet in our guest bedroom where there's room to lay out all the contents of the kit I bought online from the Molecular Models Company. Then I'll use another model to simulate how DNA copies itself and how the DNA code is used to produce proteins. I figure there's no better way for me to understand what DNA is than to build a ball-and-stick model myself. In this kind of model, the balls are atoms and the sticks are bonds between them. In 1953, James Watson and Francis Crick built ball-and-stick models to help discover the structure of deoxyribonucleic acid, or DNA. Now I'm striving to wrap my mind around the age-old inquiry: what is life and how does it work? Or more specifically: what is DNA and how does it work? Watson and Crick I found a general approach to learning that works well for me (more on this later.). A couple years ago, I pushed myself to fathom the essence of quantum physics. For some of the more impactful ideas, however, I want to at least understand the basics. I accept that I'm unlikely to ever fully understand in detail all the fantastic discoveries of modern science. Does this make me feel better? Yes, I guess so, a little bit. Perhaps general relativity will be second nature to future generations. A person two hundred years ago may not have been able to grasp the connection between electricity and magnetism. Despite the learnings of the ancient Greeks, some people two thousand years ago may have struggled with the idea of a round Earth. Science has always been working at the frontiers, beyond common knowledge. I'm not alone, and this isn't a new human condition. I'm not equipped to really comprehend the Big Bang 13.8 billion years ago, the warping of spacetime by a black hole, or force particles like the Higgs boson. I understand the scientific process all right, but the new discoveries often seem way out of reach. Sometimes I feel sad about how little I know of modern science. ![]()
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