01The nucleotide
Each nucleotide = a five-carbon sugar + a phosphate group + a nitrogenous base. In DNA the sugar is deoxyribose (missing an –OH on carbon 2'); in RNA it is ribose.
02Bases: purines and pyrimidines
Purines have two rings: adenine (A) and guanine (G). Pyrimidines have one ring: cytosine (C), thymine (T) in DNA, uracil (U) in RNA.
In DNA, A pairs with T (2 hydrogen bonds) and G pairs with C (3 hydrogen bonds). A purine always pairs with a pyrimidine, keeping the helix a constant width. More G–C pairs = more hydrogen bonds = more heat needed to separate the strands.
03The backbone and its direction
Nucleotides link by phosphodiester bonds between the phosphate on one sugar's 5' carbon and the –OH on the next sugar's 3' carbon. So every strand has a direction: a 5' end (free phosphate) and a 3' end (free –OH).
DNA's two strands run antiparallel — one 5'→3', the other 3'→5' — held together by hydrogen bonds between bases. Nucleotides can only be added to the 3' end, which is why this direction matters so much in Unit 6.
04Notebook box
DNA vs RNA: deoxyribose vs ribose · T vs U · usually double- vs single-stranded.
Worked example: A DNA sample is 30% adenine. What % is guanine? → A = T = 30% → A+T = 60% → G+C = 40% → G = 20%.