Frederick Griffith's R-Strain
The structure of DNA is a double helix, like a twisted ladder. The rungs each consist of two bases, either a pair of adenine and thymine, or a pair of cytosine and guanine, and the bases are shuffled, on different sides of the rings in a sequence. The sides of the of the 'ladder' are alternating sugars and phosphate groups. (Sugar-phosphate backbone) On one side of the ladder, the sugars are facing upwards, and on the other they are facing down. Each nucleotide has one deoxyribose sugar(without the extra oxygen attached to the second carbon), one base(adenine, thymine, cytosine, or guanine), and one phosphate group.
Cytosine and guanine are complementary to each other. Adenine and thymine are complementary as well. This is because cytosine and thymine are pyrimidines (they have one ring)and adenine and guanine are purines (they have two rings). It is also because of the electronegativity of oxygen and nitrogen, as well as the partial positivity of hydrogen. The oxygen and nitrogens in guanine attract the hydrogens in cytosine, and vice versa, to create three hydrogen bonds. The same with adenine and thymine, except only two hydrogen bonds are created. The backbone of the ladder is held together by phosphodiester bonds between the sugars and phosphate groups in the separate nucleotides. The fifth carbon forms a bond with an oxygen in the phosphate