Discovering DNA structure

History activity

Erwin Chargaff

• Worked with numbers of chemical molecules• Look at the molecules in your bag – These are VERY short DNA pieces

• Count up the number of Each molecule• Do you see a trend? What is it?

• Sugar – deoxyribose• Phosphate – phosphate• A = adenine• T = thymine• G = guanine• C = cytosine

Lord Alexander Todd

Structure of nucleotidesugar (deoxyribose)phosphatea base: one of each of A, C, T or G

Build as many nucleotides as you can = tape together - have to see letters/words

James Watson and Francis Crick

• Used others work to solve structure of DNA

• DNA is long and thin and has a repeating structure.

• - Rosalind Franklin and Maurice Wilkins

• DNA has a backbone made of sugar and phosphate.

• - Alexander Todd

• DNA is too wide to have just one backbone. • - Rosalind Franklin and Maurice Wilkins

• DNA is about [12 cm]* wide. • - Rosalind Franklin • (really, 20 Angstroms)

• The backbones of DNA are on the outside of the structure; bases are on the inside.

• - Rosalind Franklin

• Bases can bond to each other within the same molecule using hydrogen bonds (weak bonds).

• - J. M. Gullard and D. O. Jordan

James Watson and Francis Crick

• Solved true structure of DNA• Try to fit your nucleotides together – make

DNA

• So nucleotides match:• Guanine (G) bonds with Cytosine (C) • Thymine (T) bonds with Adenine (A)

DNA structure

• Describe it• Do you see how deoxyribose and phosphate

chains on outside are opposite?• A-T• G - C

DNA Structure

• Shape is called double helix• You made a backbone – it is actually twisted

around in a helix shape• Rosalind Franklin discovered “double helix” by

doing X-ray diffraction• Can you gently twist your DNA model?