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Transcript
DNA The Genetic Code Chapter 8 in your textbook Discovery of DNA • Experiments that identified DNA as the genetic material & determined its structure: 1. 2. 3. 4. 5. 6. Griffith Avery Hershey – Chase Chargaff Franklin & Wilkins Watson & Crick Griffith’s Experiment • Used mice and pneumonia bacteria – One strain, “S”, was harmful (pathogenic) – Other strain, “R”, was not harmful (non-pathogenic) • Discovered transformation – one bacteria was genetically changed by another bacteria Griffith, cont’d Animation • The harmless strain (R) mixed with the heat-killed pathogenic strain (S) killed the mice. Griffith, cont’d • Some “factor” was transferred from the S to the R bacteria, turning the harmless R into something deadly • At this point, scientists weren’t sure whether the transforming factor was DNA or protein Avery’s Experiment • Piggy-backed off of Griffith’s experiment to figure out what the “transforming factor” was • Used enzymes to isolate various components of the bacteria: DNA, proteins, lipids, and carbohydrates • Transformation only took place when the harmless bacteria was treated with DNA from the harmful bacteria Avery, cont’d • Discovered that DNA stores and transmits genetic information Hershey & Chase’s Experiment • Used viruses that infect bacteria (bacteriophages) • Used radioactive phosphorus to mark the DNA • Used radioactive sulfur to mark the proteins • This allowed them to trace where these molecules went during the viral infection process • Demonstrated that DNA was the genetic material -- video (2:05) Hershey / Chase, cont’d Chargaff • Chargaff determined that the amount of adenine is the same as the amount of thymine in all the organisms he studied (and that C = G) Franklin & Wilkins • Franklin & Wilkins did X-ray crystallography pictures of the DNA molecule that showed it to be a helical structure with at least 2 helices Watson & Crick • Using Chargaff’s rules, they knew that A=T and C=G • Using the X-ray pictures from Franklin & Wilkins, they knew it was a helical shape • They knew the structure had to provide a way to replicate very accurately • Video (4:04) Watson & Crick, cont’d • Determined the structure of DNA in 1953 – Double helix with bases on the inside and sugar and phosphate alternating on the outside DNA Basics • Deoxyribonucleic Acid • Double helix (twisted ladder) • Located in the nucleus of eukaryotic cells – Prokaryotes lack a nucleus, but still have DNA in their cells; prokaryotic (bacteria) DNA hangs out in the nucleoid region DNA is a Nucleic Acid • Organic compound made up of carbon, hydrogen, nitrogen, oxygen, and phosphorus. • Made of repeating subunits (monomers) called nucleotides The Nucleotide • Made of a sugar (deoxyribose), a phosphate group and one of 4 nitrogen bases Types of Nitrogen Bases Pyrimidines (1 ring) • Cytosine • Thymine Purines (2 rings) • Adenine • Guanine DNA Structure • Sides of the ladder are the sugar and phosphate group • The rungs (part you’d step on) are 2 of the nitrogen bases • Each rung is one purine paired with one pyrimidine – always C-G and A-T DNA Structure DNA Structure DNA Replication • The process of making identical copies of DNA • It is semiconservative – in each molecule, there is one old and one new strand • Occurs during the S phase of the cell cycle • It is necessary for cell division Steps of DNA Replication • Step 1: DNA helicase (an enzyme) unwinds the molecule • Step 2: DNA polymerase binds free-floating nucleotides to the exposed bases on the original strands • Step 3: The 2 new molecules wind up again • Animation