DNA: the Genetic Material Chapter 9.1 Identifying the Genetic Material

Chapter 9 Objectives Describe the three components of a nucleotide. Develop a model of the structure of a DNA molecule. Evaluate the contributions of Chargaff, Franklin, and Wilkins in helping Watson and Crick determine the double-helical structure of DNA. Relate the role of the base-pairing rules to the structure of DNA.

What is Genetics? Genetics – Study of heredity. Heredity – Transmission of genetic traits from parent to offspring. Trait – Attribute in an organism controlled by genes. I could do this all day people … mwuah ha ha ha ha! Gene – A section of a chromosome that codes for a protein or RNA molecule. Chromosome – Structure made of DNA & Protein on which genes are located.

AAAHH HHAAAA! Chromosome – Structure made of DNA & Protein on which genes are located. I know that for the most part you’re thinking to yourself, “Uuuhhhh, so what!” Well almost 100 years ago this was a ginormous issue. Biologists knew that chromosomes carried genetic material, but there was something that they didn’t know which became a rush against time & other Biologist to discover.

I know you’re all at the edge of your seat! The question was, what part of a chromosome actually contained the genetic material? Is it the DNA or is it protein? Biologists couldn’t exactly do genetic testing until they knew which one of these was actually carrying the genes. For years Biologists worked on finding the answer. A couple good experiments and a couple of accidents later an answer was found.

Frederick Griffith’s Experiments Griffith was a bacteriologist who was looking for a vaccine against a bacterium called Streptococcus pneumoniae. S. pneumoniae is the bacteria that causes pneumonia. Vaccine – A substance prepared from killed or weakened pathogens & introduced to a body to produce immunity.

Griffith’s 1928 Experiments Griffith worked with two types of S. pneumoniae. A. A strand of S. pneumoniae that was encased in a capsule made of polysaccharides. The capsule protects the bacterium from the body’s immune system. This helps the bacterium to become a virulent or able to cause the disease. These bacteria look smooth so they were called the “S” type.

Griffith’s 1928 Experiments cont. B. The other type of S. pneumoniae that Griffith used did not have a capsule, therefore it did not cause the disease. These bacterium appeared rough so they were given the name “R” type. Griffith did 4 experiments which accidentally strengthened the argument that DNA was the genetic carrier.

Four Blind Mice 1. Griffith first injected mice with the S bacteria. The S type bacteria was a virulent so it caused the disease and killed the mouse. 2. Griffith then injected the R bacteria that did not kill the mouse. The lack of the capsule caused the mouse’s immune to kill the bacteria.

Four Blind Mice Cont. 3. Griffith then heated up the S bacteria so that the DNA would die but the capsule would remain. Why do you think he did this? The “heat-killed” bacteria did not kill the mouse. What does that mean about the capsule?

Four Blind Mice Cont. 4. Just as a gag Griffith took the “heat-killed” S bacteria and the normal R type mixed them together and injected that into the mouse. What happened? The mouse died! What? Why? When Griffith looked at the blood of the dead mouse he noticed that the living R type had acquired capsules. The harmless R type had transformed into harmful S types.

Griffith’s Discovery Griffith discovered what is now called transformation. Transformation - is a change in shape or identity when bacteria take in foreign genetic material. This “gag” was the first real step in proving that DNA was the real genetic carrier. Next we find out how it happened!

Griffith’s Discovery of Transformation

Want more Proof? In 1944 a Biologist named Oswald Avery performed an experiment very similar to that of Griffith. Avery used DNA and protein destroying enzymes in his stock of S. pneumoniae. If DNA were the instructions for making the capsule what would happen to the bacteria if a DNA destroying enzyme was introduced?

Hershey and Chase In 1952 Alfred Hershey and Martha Chase performed an experiment that put a final nail in protein’s coffin. H & C used a virus called a T2 Bacteriophage (T2 phage) which injects it’s hereditary material into a bacteria. A T2 phage is made of a protein sheath that protects its DNA…that’s it.

Fun with Radioactive Material Step 1: They first grew T2 with E. coli in a medium that contained radioactive Sulfur (35S). Sulfur attaches to proteins. They also prepared T2 with E. coli in a medium that contained radioactive Phosphorus (32P). Phosphorus attaches to DNA. Step 2: The 35S and 32P were allowed to infect the E. coli.

Fun with Radioactive Material Cont. Step 3: The 35S and 32P with the E. coli were thrown into a centrifuge to strip them apart from each other. The lighter material at the top of the test tube was the virus, the heavier material was the bacteria.

H & C Finally! H & C noticed that the radioactive DNA material was with the bacterial material and the radioactive protein material was only found with the viral material in the top part of the test tube. What does that mean?