Discovering the Structure of DNA

What is DNA? DNA = deoxyribonucleic acid Stores, transmits and copy all information Located in the cell’s nucleus

What do you know about DNA? Codes for proteins nucleic acid Monomer is a nucleotide The three parts of a nucleotide: – 1. Phosphate group – 2. Sugar (deoxyribose) – 3. Nitrogen base

Nitrogen bases nitrogen two types: purine- 2 carbon rings pyrimidine. -1 carbon ring

More about nitrogen bases DNA has 4 nitrogen bases: – Thymine (T) – Adenine (A) – Cytosine (C) – Guanine (G) A G - purines C T - pyrimidines.

YouTube: DNA Structure of DNA

A collaborative effort! Early 1900s – known: information is passed from cell to cell. – Unknown: what carried the information? scientists thought a protein others that it was a nucleic acid. Three major experiments helped shows nucleic acid carried cell information: – Griffith – Avery – Hershey-Chase

Frederick Griffith studied pneumonia bacteria 1928 isolated two strains of pneumonia injected them into mice

Griffith got lucky? – Something transferred from heat-killed bacteria – What happened? Griffith’s conclusions: Transformation = process by which one strain of bacteria changes the gene(s) of another bacteria

Avery DNA was transforming bacteria 1943 Uses pneumonia ( just like Griffith) Showed gene is made of DNA Scientists were slow to accept the results

Hershey and Chase bacteriophages to see if information is carried on proteins or DNA 1950 Further supported Avery’s experiment that genetic material is DNA

Discovery of the structure of DNA Many scientists contributed to determining the structure of DNA – Erwin Chargaff – Rosalind Franklin – James Watson & Francis Crick

Erwin Chargaff Base pairing rule 1950 percentages of bases in DNA 10% A = 10% T 40% C = 40% G This is Chargaff’s Rule!

Rosalind Franklin x-ray photography to try to find DNA structure 1952 Her “Photo 51” revealed DNA’s structure Died of cancer in 1958

Watson and Crick Credited with finding the structure of DNA) 1953 Watson got a sneak peak at Franklin’s x-ray photos and used them with other evidence Did not give credit to Franklin ages/watson_crickjpg

DNA structure Twisted ladder made of nucleotides Sides- phosphate/sugar Rung- nitrogen bases

What bonds with what? – A bonds with T – G bonds with C – Bonds sides- covalent- strong rungs - hydrogen - weak

Your turn...the structure of DNA On the diagram: – Circle and label a nucleotide. – Label the sugar and phosphate molecules. – Label the bases – Label a base pair. – Label the sugar- phosphate backbones. – Label the hydrogen bonds. nucleotide P S A C A A T T C G G Base pair Sugar /P backbone G Sugar /P backbone Hydrogen bonds