DNA: The Genetic Material Ag Biology

Early 1950’s Most scientists were convinced that genes were made of DNA However, they weren’t sure what DNA looked like 2 young researchers at Cambridge University started to piece it together James Watson Francis Crick

Structure of DNA Double helix – two strands twisted around each other Each strand is made of linked nucleotides

Structure of DNA Nucleotides: subunits that make up DNA; each one contains 3 parts a phosphate group a 5-carbon sugar (called deoxyribose) a nitrogen containing base

Types of Nitrogen Bases Adenine Guanine Thymine Cytosine Classified as purines (bases made of 2 rings of carbon & nitrogen atoms) Classified as pyrimidines (bases made of 1 ring of carbon & nitrogen atoms)

Structure of DNA The sugar-phosphate backbones are like the side rails of a ladder The pairs of nitrogen bases are like the rungs of the ladder These bases face each other Double helix shape is held together by the weak hydrogen bonds between the base pairs

Erwin Chargaff Biochemist at Columbia University In 1949, he observed that the amount of adenine always equaled the amount of thymine (A=T) Also, the amount of guanine always equaled the amount of cytosine (G=C) However those amounts varied between different organisms

Wilkins & Franklin’s Photos X-ray diffraction used to study the structures of molecules In 1952, W & F developed high quality X-ray diffraction photographs of fibers from a DNA molecule It showed that DNA resembled a tightly coiled helix, composed of 2 or 3 chains

Watson & Crick’s DNA Model Based on prior findings, W & C were able to construct a model of DNA Used tin & wire models of molecules Used the double helix shape A purine on 1 strand is always paired with a pyrimidine on the other strand (A=T and G=C) These are known as the “base pairing rules” 2 strands are complimentary to each other