The Genetic Code Chapter 8 in your textbook DNA The Genetic Code Chapter 8 in your textbook

Discovery of DNA Experiments that identified DNA as the genetic material & determined its structure: 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