Chapter 12: DNA & RNA

Section 12.1 – Structure of DNA DNA – Deoxyribonucleic Acid; traits are determined by your genes, genes code for proteins, and genes are coded for in your DNA *Was discovered in the 1940’s to be the genetic material responsible for traits.

I.Scientists Discovering DNA A.Frederick Griffith (1928) 1.Worked with two strains of a pneumonia bacterium, one pathogenic and one harmless 2.When he mixed heat-killed remains of the pathogenic strain with living cells of the harmless strain, some living cells became pathogenic 3.Transformation – one strain of bacteria is changed into another strain; some “factor” was transferred from one strain to the other

Griffith’s Experiment

B.Oswald Avery (1944) 1.Repeated Griffith’s experiment, but added enzymes to heat-killed bacteria that would destroy carbs, proteins, lipids, & RNA. 2.When mixed with harmless bacteria, transformation still occurred (mice died) 3.Repeated and added enzymes that destroyed DNA, and when mixed with harmless bacteria, no transformation occurred (mice lived!) 4.Proved DNA was the transformation “factor”, meaning DNA stores & transmits genetic info.

Avery’s Experiment

C.Alfred Hershey & Martha Chase (1952) 1.Provided further evidence that DNA is genetic material by using bacteriophages (phages), viruses that infect bacteria 2.Used a phage known as T2 which infects E. coli 3.Radioactive Isotopes – used to tag DNA and protein Sulfur to tag protein Phosphorus to tab DNA 4.DNA was proven to be the genetic material of viruses versus protein

Phage infecting Bacterium

Figure Bacterial cell Phage Batch 1: Radioactive sulfur ( 35 S) DNA Batch 2: Radioactive phosphorus ( 32 P) Radioactive DNA EXPERIMENT Radioactive protein

Figure Bacterial cell Phage Batch 1: Radioactive sulfur ( 35 S) Radioactive protein DNA Batch 2: Radioactive phosphorus ( 32 P) Radioactive DNA Empty protein shell Phage DNA EXPERIMENT

Figure Bacterial cell Phage Batch 1: Radioactive sulfur ( 35 S) Radioactive protein DNA Batch 2: Radioactive phosphorus ( 32 P) Radioactive DNA Empty protein shell Phage DNA Centrifuge Radioactivity (phage protein) in liquid Pellet (bacterial cells and contents) Pellet Radioactivity (phage DNA) in pellet EXPERIMENT

I.The Shape of DNA A.A Winding Staircase 1.DNA is made of 2 parallel strands linked together and shaped like a spiral staircase. 2.The spiral shape is called a double helix. 3.Each strand is made of linked subunits called nucleotides.

B.The Parts of the Nucleotide 1.Phosphate group – a phosphorus atom surrounded by oxygen atoms, PO 3 2.Deoxyribose – a 5-carbon sugar molecule 3.Nitrogen-base – molecule containing nitrogen There are four possible nitrogen bases in DNA

C.Putting it together 1.The phosphates and deoxyribose sugars link together to form the “backbone” or outer strands. 2.The nitrogen bases pair up to form the “rungs” holding the 2 strands together.

II.The Information in DNA A.Nitrogen Bases 1.The four nitrogen bases are adenine (A), guanine (G), thymine (T), and cytosine (C). 2.Purines – made of 2 rings of carbon & nitrogen; adenine & guanine 3.Pyrimidines – made of 1 ring of carbon & nitrogen; thymine and cytosine

B.Base-Pairing Rules (Chargaff’s Rules) 1.A Purine on one strand is always paired with a Pyrimidine on the other strand. 2.More specifically: Adenine with Thymine (A with T) Cytosine with Guanine (C with G) 3.The base pairs are held together by weak hydrogen bonds and are called complementary base pairs.

C.Complementary Strands 1.The two strands are complementary because they fit together like a puzzle and their bases are identical. Ex: One strand has the following nucleotides: ATTCGGTACCCC The other strand would be what? TAAGCCATGGGG

III.Discovering DNA’s Structure A.Edwin Chargaff – in all organisms, the amount of adenine almost equaled the amount of thymine, and the amount of cytosine equaled the amount of guanine. Base-Pairing Rule!!

B.Rosalind Franklin – produced an X-ray (photo 51!!) that showed the shape of DNA 1.X-ray Diffraction 2.X-shape = helix 3.Angle of X = 2 strands 4.Other clues suggest N-bases are in the middle

C.James Watson and Francis Crick 1.Used the findings of Chargaff and Franklin, and their knowledge of chemical bonding to build an accurate, three-dimensional model of DNA 2.They were also able to propose a method of DNA replication based on its structure.

Recap of DNA Structure