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Class Notes Discovering DNA Questions/Main Idea: Briefly describe Frederick Griffith’s pneumonia experiment. Name: _______________________________________ Period:_______________________________________ Date: _______________________________________ Notes: pgs 287-294 1928 – Frederick Griffith conducted experiments with mice to study the effects of pneumonia bacteria. Griffith isolated two strains of pneumonia bacteria – one caused the lung disease (pneumonia) and the other did not. Mice injected with the disease causing bacteria died. Mice injected with the harmless bacteria lived. Mice injected with the heat-killed disease causing bacteria also lived. Griffith then mixed the live harmless bacteria with heat-killed disease causing bacteria. Mice injected with this combination died. How did Griffith explain the results from his experiment? Based on his results, Griffith hypothesized that when the harmless and heat-killed bacteria where mixed, some factor was exchanged between them, making the live harmless bacteria deadly. What is transformation? Is the process in which one strain of bacteria is changed by the gene(s) of another bacteria. Describe how did Oswald In 1943, Avery, with fellow scientists, conducted an experiment Avery’s modify Griffith’s similar to Griffith’s, except they used enzymes to selectively destroy experiment. molecules one at a time. When they injected harmless bacteria with only lipids, carbs, or proteins: transformation did not occur. When they used the nucleic acids (DNA): transformation did occur, the bacteria became lethal. This helped to determine that DNA stores and transmits genetic information. Briefly describe Alfred Hershey and Chase (1950) conducted experiments with bacteriophages Hershey & Martha (viruses that attack bacteria) to determine if genetic information is Chase’s experiment. carried on proteins or DNA. They used radioactive elements to ‘mark’ DNA and protein. Only the radioactively-labeled DNA was found in bacteria cells. What did their Hershey & These findings further supported the conclusions of Avery’s experiment, Chase conclude from their that genetic material is DNA and NOT protein. experiment? Explain Chargaff’s rules. Chargaff determined that in any sample of DNA: The # of adenines (A) = the # of thymines (T) The # of cytosines (C) = the # of guanines (G) Thus in DNA, the bases A and T pair together, and C and G pair together. How did Rosalind Franklin used x-ray diffraction to create pictures of DNA’s molecular Franklin contribute to the structure discovery of the DNA structure? Who discovered the James Watson and Frances Crick determined the structure of DNA in structure of DNA? 1953 using their data and the work of previous scientists. Describe the structure of DNA is shaped like a double helix, with strands held together by the weak DNA. hydrogen bonds formed between the bases A-T and C-G. On the diagram: Circle and label a nucleotide. Label the sugar and phosphate molecules. Label the bases that are not already labeled Label a base pair. Label the sugarphosphate backbones. Label the hydrogen bonds. Why and when does DNA DNA must be replicated (copied) in order to insure that during cell replication occur? division, each daughter cell receives a complete copy. DNA replication occurs in the nucleus during S phase of the cell cycle, before chromatin (DNA wrapped around proteins) condenses into chromosomes. Describe the 4 steps of Step 1 – Helicase unzips the strand of DNA by breaking the weak DNA replication. hydrogen bonds between base pairs. Step 2 – DNA polymerase inserts the appropriate bases. Step 3 – A new sugar-phosphate backbone is built. Step 4 – The sequence is proofread by DNA polymerase. Why is DNA replication considered semiconservative? Because each side of the parent strand serves as a template for the 2 new DNA strands. Summary: Base Pair Practice: For each example below, please give the correct complementary strand of DNA. 1. T T G C T A G 3. A C C G T C A AACGATC TGGCAGT 2. T A G C G C T ATCGCGA 4. G C T A T G T CGATACA DNA Replication: For DNA to replicate, an enzyme, helicase must first ‘unzip’ the hydrogen bonds between complementary base pairs. The site where the unzipping begins is referred to as a replication fork. The two template strands of DNA are kept separate by single stranded binding proteins. New strands of DNA are built from the template strand, using DNA polymerase (enzyme) to bring in the nucleotides. Nucleotides on the leading strands are brought to the template strand in a continuous fashion. Nucleotides on the lagging strand are brought in segments known as Okazaki fragments. The Okazaki fragments must then be bonded together using the enzyme DNA ligase. After replication is complete, DNA polymerase then acts a spellchecker, checking for any mutations in the new strands of DNA.