12.2 Notes The Structure of DNA

Watson and Crick James Watson and Francis Crick determined that DNA is a double helix- 2 strands twisted around each other like a winding staircase. Each strand is made up of nucleotides- subunits of DNA. A nucleotide is made up of 3 parts. 1. phosphate group 2. 5 Carbon Sugar (deoxyribose) 3. nitrogen-containing base

A Nucleotide Base Sugar P O OH HO NH2 N OH O CH2 H OH Phosphate 2’ 3’ 4’ 5’ 1’ OH

Nitrogen Bases Adenine (A) Guanine (G) Cytosine (C) Thymine (T)

Purines Pyrimidines Uracil CH3 N O NH Thymine N Adenine NH2 NH2 O N NH (RNA) CH3 N O NH Thymine (DNA) N Adenine NH2 NH2 O N NH Guanine N O NH2 Cytosine

Base Pairing Guanine And Cytosine H O N Guanine - N O H Cytosine + + - - +

Base Pairing Adenine And Thymine - + Adenine CH3 N O H + - Thymine

Base Pairing Adenine And Cytosine H Cytosine - + N H - + Adenine

Base Pairing Guanine And Thymine CH3 N O H + - Thymine H O N Guanine + -

The Watson - Crick Model Of DNA T A G C C G G C T A 3.4 nm 1 nm Minor groove Hydrogen bonds connect the bases! Major groove 0.34 nm

Many Scientists Contributed to the Discovery of DNA’s Structure 1949: Edwin Chargaff at Columbia Univ, N. Y. Found that the amount of A = T Found that the amount of G = C 1952: Maurice Wilkins and Rosalind Franklin in London Developed high quality X ray diffraction photographs of the DNA molecule