The Structure of DNADNA 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

1. Deoxyribose- The five carbon sugar in DNA. Forms the full name of DNA (deoxyribonucleic acid).

2. Phosphate Group- alternates with deoxyribose to form the sides of the “ladder”.

3. 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.

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

DNA Model

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