Structure of DNA

DNA has four kinds of Nitrogenous Bases Thymine (T) Cytosine (C) Adenine (A) Guanine (G) Pyrimidines Purines Figure 10.2B

Partial chemical structure Let’s look at DNA? Hydrogen bond Ribbon model Partial chemical structure Computer model Figure 10.3D

Hydrogen bonds between bases hold the strands together Each base pairs with a complementary partner A pairs with T G pairs with C

LE 16-8 Sugar Sugar Adenine (A) Thymine (T) Sugar Sugar Guanine (G) Cytosine (C)

The structure of DNA consists of two polynucleotide strands wrapped around each other in a double helix 1 chocolate coat, Blind (PRA) Figure 10.3C Twist

10.3 DNA is a Double Helix – Who helped to determine this? James Watson and Francis Crick….. 1962 Nobel Prize But who helped them? Figure 10.3A, B

Building a Structural Model of DNA: Scientific Inquiry After most biologists became convinced that DNA was the genetic material, the challenge was to determine how its structure accounts for its role Maurice Wilkins and Rosalind Franklin were using a technique called X-ray crystallography to study molecular structure Franklin produced a picture of the DNA molecule using this technique

Rosalind Franklin and Maurice Wilkins

Animation: DNA Double Helix Franklin’s X-ray crystallographic images of DNA enabled Watson to deduce that DNA was helical The X-ray images also enabled Watson to deduce the width of the helix and the spacing of the nitrogenous bases The width suggested that the DNA molecule was made up of two strands, forming a double helix Animation: DNA Double Helix

Erwin Chargaff, 1940, Columbia University Studied samples of DNA taken from different sources and found the amount of adenine was almost equal to the amount of thymine, and that the amount of guanine was almost equal to the amount of cytosine. The discovery of A=T and G=C became known as Chargaff’s Rule. 

Purine + purine: too wide LE 16-UN298 Purine + purine: too wide Pyrimidine + pyrimidine: too narrow Purine + pyrimidine: width consistent with X-ray data