DNA The Genetic Material What Features do all people share? 2 legs 2 ears What features are different among people? Shape of nose What Makes Me Unique? Think of the crime solving shows youve seen. Detectives search for clues that will help them solve the mystery. Geneticists are detectives looking for clues in the mystery of inheritance. Discovery of Genetic Material Once Mendels work was re-discovered in the 1900s, scientists began to search for the molecule involved in inheritance. Scientists knew that genetic information was carried on the chromosomes in eukaryotic cells, and that the two main components of chromosomes are DNA and protein. For many years, scientists tried to determine which of these macromolecules (nucleic acid DNA- or proteins) was the source of genetic information. Griffith The first major experiment that led to the discovery of DNA as the genetic material was performed by Fredrick Griffith in Griffith studied two strains of the bacteria Streptococcus pneumoniae, which causes pneumonia. He found that one strain could be changed into another form. One of the forms had a sugar coat and one didnt. The sugar coated strain causes pneumonia and is called the smooth strain (S). The non-coated strain does not cause pneumonia and is called the rough strain because, without the coat, the bacteria colonies have rough edges(R). Mouse Experiment Honestly, Im not quite getting the flow of his experiment (which is on page 326, if youd like to try to figure it out) so Im just going to give you the results. The smooth bacteria caused the mouse in the experiment to die. The bacteria were converted by a transforming substance from a rough to a smooth form of bacteria, which killed the mouse. Oswald Avery, 1944 Avery identified the molecule that transformed the R strain of bacteria into the S strain. Avery isolated different macromolecules, such as DNA, protein and lipids, from killed S cells. Then he exposed live R cells to the macromolecules separately. When the live R cells were exposed to the S strain DNA, they were transformed into S cells. Avery concluded that when the S cells in Griffiths experiment were killed, DNA was released. Avery Continued Some of the R bacteria incorporated this DNA into their cells and this changed the bacteria into S cells. Averys theories were not widely accepted, but scientists continued to experiment to determine whether proteins or DNA were responsible for the transfer of genetic material. Hershey and Chase Hershey and Chase published results of experiments that provided definitive evidence that DNA was the transforming factor. These experiments involved a bacteriophage. Two components made the experiment ideal for confirming that DNA is the genetic material. First, the bacteriophage used in the experiment was made of DNA and protein. Second, viruses cant replicate themselves. They must inject their genetic material into a host to reproduce. Their experiment showed that genetic material in the form of DNA, not proteins, transformed information. DNA Structure In the 1920s, the biochemist PA Levene determined the basic structure of nucleotides that make up DNA. Nucleotides are the subunits of nucleic acids and consist of a 5-carbon sugar, a phosphate group, and a nitrogenous base. The two nucleic acids found in living cells are DNA and RNA. Erwin Chargaff Chargaff analyzed the amount of adenine, guanine, thymine, and cytosine in the DNA of various species. Chargaff found that the amount of guanine is almost equal to the amount of cytosine and the amount of adenine nearly equals the amount of thymine. This is known as Chargaffs rule: C=G and T = A. DNA Structure DNA is often compared to a twisted ladder. The rails of the ladder have alternating deoxyribose and phosphate. The pairs of bases (cytosine-guanine or thymine-adenine) form the steps of the ladder. The rungs in different models contain a variety of bases that represent genetic code. With the exception of identical twins or triplets, each organism has a unique genetic code. Double Helix DNA Activity Divide into groups. Each group gets 20 strips of paper with 4 different colors. Label each color with the letters A, T, G and C. Tape the strips in any order you wish. Compare Strips Just like DNA, there are many differences in the order these traits can occur. Questions 1. Which carries the genetic trait, DNA or proteins? 2. What shape is DNA in? 3. What are the two nucleic acids found in living cells? 4. Each organism has a __________ genetic code.