Deoxyribonucleic Acid or DNA Notes - DNA (Chapter 12-1) Deoxyribonucleic Acid or DNA Chapter 12-1

How is genetic information passed from cell to cell? By early 1900, scientists knew that… genetic information is passed from parents to offspring genetic information is passed in the form of genes But in early 1900, scientists did not understand… what a gene actually was how a gene was responsible for carrying genetic information More research and experimentation was necessary

Griffith & Transformation 1928 – Frederick Griffith wanted to learn how certain types of bacteria cause pneumonia He worked with two strains (types) of pneumonia bacteria Disease-causing bacteria (smooth colony) Harmless bacteria (rough colony) He injected mice with the different strains to see what would happen

Griffith & Transformation Results – Inject mouse with… disease-causing bacteria = mouse dies harmless bacteria = mouse lives heat-killed, disease-causing bacteria = mouse lives mixture of heat-killed, disease-causing bacteria and live harmless bacteria = mouse dies Conclusion Transformation – something is transferred from dead, harmful bacteria to live, harmless bacteria, which changes them into disease-causing bacteria Could this “something” be a gene? - Probably

Avery and DNA 1944 – Oswald Avery repeats Griffith’s work to see which molecule was most important in transformation Treated heat-killed, disease-causing bacteria with different enzymes that destroyed different types of molecules (lipids, proteins, carbs, nucleic acids)

Avery and DNA Results – Destroy the… Conclusion proteins = transformation occurs carbs = transformation occurs lipids = transformation occurs RNA = transformation occurs DNA = transformation DOES NOT occur Conclusion DNA is responsible for transformation Genes are probably made of DNA

The Hershey-Chase Experiment 1952 – Alfred Hershey & Martha Chase try to figure out which part of a virus, protein or DNA, enters a bacteria cell to infect it Grow viruses in culture containing radioactive phosphorous (32P) and radioactive sulfur (35S) Protein contains sulfur but little phosphorous DNA contains phosphorous but no sulfur Allow viruses to infect bacteria

The Hershey-Chase Experiment Results – When the viruses infected the bacteria… nearly all the radioactivity in the bacteria was from 32P, the DNA marker Conclusion Genetic material of a bacteriophage virus is DNA Genes are made of DNA

Structure of DNA Genes are made of DNA, but how can DNA… carry information? determine the characteristics of an organism? be replicated easily?

What did scientists already know about DNA in 1952? DNA is a polymer A polymer is a large molecule made up of many individual, smaller molecules called monomers, that are bonded together The monomer of the DNA polymer is the nucleotide DNA is a long chain of many nucleotides bonded together

Nucleotides: A Closer Look A nucleotide is made up of 3 basic parts: A phosphate group A deoxyribose sugar molecule A nitrogen containing base molecule (either adenine, thymine, cytosine, or guanine) All DNA nucleotides have the same phosphate group, deoxyribose sugar molecule, and a nitrogen base Since there are 4 different bases (A, T, C, G), DNA nucleotides come in 4 varieties These nucleotides can be strung together in any order

It took even more research and experimentation to figure out what DNA actually does and how it does it Erwin Chargaff DNA from any organism contains equal amounts of A & T, and of C & G Rosalind Franklin & Maurice Wilkins Take X-Ray diffraction pictures of DNA “X” pattern indicates that DNA molecules are helical – they contain 2 strands of nucleotides that are twisted around one another

Chargaff’s Ratios

The Discovery of DNA’s Structure 1953 – James Watson & Francis Crick Used Chargaff’s information & Franklin’s images to inform their model building Built a double helix model with two strands wound around one another Model paired up “A” nucleotides with “T” nucleotides, and “C” nucleotides with “G” nucleotides

Watson Builds a Model

Base Pairing in DNA DNA strands are held together by weak hydrogen bonds between the nitrogen bases of each strand A always bonds with T C always bonds with G

The Double Helix

Watson & Crick’s Model Answers the Crucial Questions The genetic information is carried by DNA and is determined by the order of the A,T,C,G nucleotides DNA can be copied easily by splitting in half, and each half forming new, complimentary pairs