Introduction to DNA (Deoxyribonucleic acid)

Questions at the beginning of the 20th Century How do genes work? What are they made of, and how do they determine the characteristics of organisms? Are genes single molecules, or are they longer structures made up of many molecules? How did we figure out the answers to these questions?

The discovery of the gene and DNA: Griffith Transformation Experiment Like many experiments in science, the discovery of the molecular nature of the gene was a complete accident. In 1928, British scientist Fredrick Griffith was trying to figure out how bacteria made people sick.

Griffith Transformation Experiment Griffith isolated two slightly different strains of pneumonia bacteria from mice. One strain that caused pneumonia and one strain that did not cause pneumonia. The pneumonia causing strain grew on nutrient agar plates in smooth colonies. The non-pneumonia causing bacteria grew on nutrient agar plates as colonies with rough edges. This is how Griffith distinguished the two types of bacteria.

Griffith Transformation Experiment When Griffith injected mice with the disease- causing strain (smooth) of bacteria, the mice developed pneumonia and died. When mice were injected with the harmless strain (rough), the mice didn’t get sick. Griffith wondered if the disease-causing bacteria might produce poison.

Griffith Transformation Experiment To find out, Griffith took a culture of these cells, heated the bacteria to kill them, and injected the heat- killed bacteria into the mice. The mice survived! This suggested that the cause of pneumonia was not a chemical poison released by the bacteria.

Griffith Transformation Experiment Next, Griffith mixed heat-killed, disease- causing bacteria with live harmless bacteria and injected the mixture into mice. By themselves, neither should have made the mice sick To Griffith’s amazement, the mice developed pneumonia and died.

Griffith Transformation Experiment When Griffith examined the lungs of the mice, he found that the lungs were infected with the pneumonia-causing strain. Somehow the heat killed strain of bacteria had passed on their disease causing ability to the harmless strain.

Griffith Transformation Experiment Griffith called this process transformation because one strain of bacteria (the harmless strain) had apparently been changed permanently into another (the disease-causing strain).

Griffith Transformation Experiment Griffith hypothesized that when the living, harmless bacteria and the heat-killed bacteria were mixed; some factor was transferred from the heat-killed cells into the live cells.

Griffith Transformation Experiment Those factors must contain information that could change harmless bacteria into disease- causing bacteria. Since the ability to cause disease was inherited by the transformed bacteria’s offspring, the transformation factor might be a gene.

Transformation of Bacteria

AVERY and DNA In 1944, a group of scientists led by Oswald Avery tried to repeat Griffith’s work. They did so to determine which molecule in the heat- killed bacteria was most important in transformation. If transformation required just one particular molecule, that might be the molecule of the gene.

AVERY and DNA Avery and his colleagues extracted all the “juice” from the inside of the heat-killed bacteria. They treated the extract with enzymes that destroy proteins, lipids, carbohydrates, and other molecules including the nucleic acid that makes up RNA. Transformation still occurred.

AVERY and DNA Since all of the molecules listed had been destroyed, they could not be responsible for transformation.

AVERY and DNA Avery and other scientists performed another experiment, this time using an enzyme that destroyed the nucleic acid that makes up DNA. This time, transformation did not occur.

AVERY and DNA This lead to Avery coming to the conclusion that DNA was transforming factor. Avery and other scientist discovered that the nucleic acid DNA stores and transmits the genetic information from one generation of an organism to the next.

Hershey-Chase Experiment In 1952, Alfred Hershey and Martha Chase studied viruses to further convince the scientist that genetic information is passed along from generation to generation by DNA.

Hershey-Chase Experiment They collaborated in studying viruses, nonliving particles smaller than a cell that can infect living organisms.

Hershey-Chase Experiment Bacteriophages: (bacteria eater) viruses that infect bacteria. –Composed of DNA or RNA core and a protein coat. –When a bacteriophage enters a bacterium, the virus attaches to the surface of the cell and injects its genetic information inside the bacteria.

Hershey-Chase Experiment The viral genes are inserted into the host DNA and tell the bacteria cell to produce hundreds of viruses. Gradually the viruses destroy the bacterium causing the cell to split open releasing the viruses.

Hershey-Chase Experiment Hershey and Chase reasoned that if they could determine which part of the virus, the protein coat or the DNA core, entered the infected cell, they would learn whether genes were made of protein or DNA.

Hershey-Chase Experiment To do this, they grew viruses in cultures containing radioactive isotopes. –Phosphorus – 32 –Sulfur - 35 Protein contains very little phosphorus and DNA contains no sulfur.

Hershey-Chase Experiment These radioactive substances could be used as markers. –If sulfur – 35 was found in the bacteria, the virus would have injected its protein coat into the bacterium. –If phosphorus – 32 was found in the bacterium, the virus would have injected its DNA into the bacterium.

Hershey-Chase Experiment The two scientists mixed the marked viruses with the bacterium and then waited a few minutes so the viruses had time to inject their genetic material. They separated the viruses from the bacteria and tested the bacteria for radioactivity.

Hershey-Chase Experiment Nearly all of the radioactivity found in the bacteria was from phosphorus – 32. Hershey and Chase concluded that the genetic material of the bacteriophage was DNA, not protein.

Classwork Get a Biology Text Turn to page 294 Answer questions 1 and 5