The Structure of DNA DNA Has the Structure of a Winding Staircase Early 1950’s, James Watson and Francis Crick determined that DNA is a molecule that is a double helix. A double helix is two strands twisted around each other.

Each strand is composed of nucleotides. Nucleotides are the subunits that make up DNA. Made of 3 parts: phosphate group, deoxyribose (sugar) & a nitrogen base.

Nucleotide Components Deoxyribose- The five carbon sugar in DNA. Forms the full name of DNA (deoxyribonucleic acid). Phosphate Group- alternates with deoxyribose to form the sides of the “ladder”. Nitrogen bases- Adenine (A), Thymine (T), Cytosine (C), & Guanine (G).

Chargaff’s Observations In 1949, Erwin Chargaff’s data showed that for each organism he studied, the amount of adenine always equaled the amount of thymine. And, the amount of cytosine always equaled the amount of guanine.

DNA Model In 1952, Watson & Crick determined the structure of DNA with the help of data collected by Rosalind Franklin and Chargaff.

Base-Pairing Rules Adenine forms 2 hydrogen bonds with Thymine Cytosine forms 3 hydrogen bonds with Guanine The hydrogen bonds between the nitrogen bases keep the two strands of DNA together. Each strand is complementary to the other. Ex. TCGAACT is complementary to AGCTTGA