DNA
The Genetic Code
Chapter 8 in your textbook
Discovery of DNA
• Experiments that identified DNA as the genetic material & determined its structure:
1. Griffith2. Avery3. Hershey – Chase 4. Chargaff5. Franklin & Wilkins6. 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’dAnimation
• 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