DNA Structure & Base Pairing: | ditki medical and biological sciences

DNA StructureOverview Here we will learn about about the structure of DNA. Sugar-Phosphate Backbone DNA has a fixed backbone that comprises units of: • Sugar (a deoxyribose) • Monophosphates These give each DNA strand directionality: 5' to 3' vs 3' to 5'. The Nitrogenous Bases Denote the 4 nitrogen bases. • The purines: Adenine & Guanine To remember the purines, think: Larger & think about ATP/GTP (adenine, guanine). The pyrimidines: Thymine & Cytosine (Uracil is in RNA, only*). To remember the pyrimidines, think: Smaller & think about the chemistry of UMP to dTMP (thymine) & CTP (cytosine). Base Pairing • Adenine pairs with thymine (via two hydrogen bonds). • Guanine pairs with cytosine (via three hydrogen bonds). Basic Organization To begin, let's draw the basic organization of DNA in a railroad track layout: Sugar-phosphate backbones • Show vertically-oriented sugar-phosphate backbones. Nitrogenous bases • Then, show that the sugars link with nitrogenous bases, which link via hydrogen bonds.uncoiled DNA With that organization as a background, we're ready to tackle the sugar-phosphate backbones and what is meant by 5' to 3' orientation. • First, let's draw DNA in an UNCOILED state and then in its COILED state.Sugar-Phosphate Backbone 5' to 3' Strand • Draw deoxyribose: • Draw a pentagon with an oxygen atom inserted at the top. - Label carbons 1' through 4', going clockwise from the oxygen atom. - Now add carbon 5' as an attachment to carbon 4'. • With the sugar in-place, next add a phosphate at the 5' end, which establishes the directionality of the strand of DNA – let's see how the rest, now, falls into place. • At the 3' carbon, add an oxygen. • Show that it forms a phosphate, which then connects to the next deoxyribose - This would continue along the strand. • We can cap the strand, though, at the 3' hydroxyl, because, we understand the pattern. • Draw the 5' (phosphate) to 3' (hydroxyl) orientation. - Note that we have not, yet,...

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• Sarah Zhang