11.2 Structure of DNA

Actual structure of DNA was a complete mystery Big question: How did DNA hold genetic information and what did it look like?

1. Deoxyribose Sugar - pentagonal ring Inheritable genetic information is coded in the molecule called DNA (Deoxyribonucleic Acid). DNA is a Polymer - built from monomers called Nucleotides 1. Deoxyribose Sugar - pentagonal ring 2. Phosphate Group - Phosphorus atom surrounded by four Oxygen atoms 3. Nitrogenous Base - a single or double ring of carbon and nitrogen atoms with functional groups

Purines - double ring nitrogenous bases Adenine (A) & Guanine (G) Pyrimidines - single ring nitrogenous bases Thymine (T) & Cytosine (C)

Chargaff’s Rule: 1950 American Erwin Chargaff Analyzed different organism’s DNA: variation in number and amount of A, T, G, C Quantity of nucleotides in each organism he found A=T and G=C Chargaff’s Rule: Nucleotide number and concentration vary with each species. A=T and G=C within a specie’s DNA

1951 English Rosalind Franklin Studied molecules using X ray Diffraction Indicated a helix structure

1953 American James Watson and English Francis Crick Studying the structure of DNA Rosalind wrote to Watson and when he saw the X-Ray evidence he also believed DNA to be a Helix

Complementary Base Pairing Each nitrogenous base must pair (bond) with its complementary base each fitting like a puzzle piece Adenine (A) pairs to Thymine (T) Guanine (G) pairs to Cytosine (C)

Two long chains of nitrogenous bases bond with with their base pair and form a ladder structure "Backbone" are the phosphate and deoxyribose "rung" of the ladder and the Nitrogenous bases