12 1 DNA Slide 1 of 37

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2 Griffith and Transformation Griffith and Transformation In 1928, British scientist Fredrick Griffith was trying to learn how certain types of bacteria caused pneumonia. He isolated two different strains of pneumonia bacteria from mice and grew them in his lab. Griffith made two observations: (1) The disease-causing strain of bacteria grew into smooth colonies on culture plates. (2) The harmless strain grew into colonies with rough edges. 2 of 37

3 Griffith and Transformation Griffith's Experiments Griffith set up four individual experiments. Experiment 1: Mice were injected with the disease-causing strain of bacteria. The mice developed pneumonia and died. 3 of 37

4 Griffith and Transformation Experiment 2: Mice were injected with the harmless strain of bacteria. These mice didn t get sick. Harmless bacteria (rough colonies) Lives 4 of 37

5 Griffith and Transformation Experiment 3: Griffith heated the diseasecausing bacteria. He then injected the heat-killed bacteria into the mice. The mice survived. Heat-killed diseasecausing bacteria (smooth colonies) Lives 5 of 37

6 Griffith and Transformation Experiment 4: Griffith mixed his heat-killed, disease-causing bacteria with live, harmless bacteria and injected the mixture into the mice. The mice developed pneumonia and died. Heat-killed diseasecausing bacteria (smooth colonies) Harmless bacteria (rough colonies) Live diseasecausing bacteria (smooth colonies) Dies of pneumonia 6 of 37

7 Griffith and Transformation Griffith concluded that the heat-killed bacteria passed their diseasecausing ability to the harmless strain. Heat-killed diseasecausing bacteria (smooth colonies) Harmless bacteria (rough colonies) Live diseasecausing bacteria (smooth colonies) Dies of pneumonia 7 of 37

8 Griffith and Transformation Transformation Griffith called this process transformation because one strain of bacteria (the harmless strain) had changed permanently into another (the disease-causing strain). Griffith hypothesized that a factor must contain information that could change harmless bacteria into disease-causing ones. Transformers! 8 of 37

9 Avery and DNA Avery and DNA Oswald Avery repeated Griffith s work to determine which molecule was most important for transformation. Avery and his colleagues made an extract from the heat-killed bacteria that they treated with enzymes. The enzymes destroyed proteins, lipids, carbohydrates, and other molecules, including the nucleic acid RNA. Transformation still occurred. 9 of 37

10 Avery and DNA Avery and other scientists repeated the experiment using enzymes that would break down DNA. When DNA was destroyed, transformation did not occur. Therefore, they concluded that DNA was the transforming factor. Avery and other scientists discovered that the nucleic acid DNA stores and transmits the genetic information from one generation of an organism to the next. 10 of 37

11 The Hershey-Chase Experiment The Hershey-Chase Experiment Alfred Hershey and Martha Chase studied viruses nonliving particles smaller than a cell that can infect living organisms. Bacteriophages A virus that infects bacteria is known as a bacteriophage. Bacteriophages are composed of a DNA or RNA core and a protein coat. 11 of 37

12 The Hershey-Chase Experiment When a bacteriophage enters a bacterium, the virus attaches to the surface of the cell and injects its genetic information into it. The viral genes produce many new bacteriophages, which eventually destroy the bacterium. When the cell splits open, hundreds of viruses burst out. If Hershey and Chase could determine which part of the virus entered an infected cell, they would learn whether genes were made of protein or DNA. They grew viruses in cultures containing radioactive isotopes of phosphorus-32 ( 32 P) and sulfur-35 ( 35 S). 12 of 37

13 The Hershey-Chase Experiment If 35 S was found in the bacteria, it would mean that the viruses protein had been injected into the bacteria. Bacteriophage with sulfur-35 in protein coat Phage infects bacterium No radioactivity inside bacterium 13 of 37

14 The Hershey-Chase Experiment If 32 P was found in the bacteria, then it was the DNA that had been injected. Bacteriophage with phosphorus-32 in DNA Phage infects bacterium Radioactivity inside bacterium 14 of 37

15 The Hershey-Chase Experiment Nearly all the radioactivity in the bacteria was from phosphorus ( 32 P). Hershey and Chase concluded that the genetic material of the bacteriophage was DNA, not protein. The Components and Structure of DNA DNA is made up of nucleotides. A nucleotide is a monomer of nucleic acids made up of a five-carbon sugar called deoxyribose, a phosphate group, and a nitrogenous base. 15 of 37

16 The Components and Structure of DNA There are four kinds of bases in in DNA: adenine guanine cytosine thymine 16 of 37

17 The Components and Structure of DNA The backbone of a DNA chain is formed by sugar and phosphate groups of each nucleotide joined together in any order. Chargaff's Rules The percentages of guanine [G] and cytosine [C] bases are almost equal in any sample of DNA. So G and C pair up! The percentages of adenine [A] and thymine [T] bases are almost equal in any sample of DNA. So A and T pair up! 17 of 37

18 The Components and Structure of DNA X-Ray Evidence Rosalind Franklin used X-ray diffraction to get information about the structure of DNA. She aimed an X-ray beam at concentrated DNA samples and recorded the scattering pattern of the X-rays on film. 18 of 37

19 The Components and Structure of DNA The Double Helix Using clues from Franklin s pattern, James Watson and Francis Crick built a model that explained how DNA carried information and could be copied. Watson and Crick's model of DNA was a double helix, in which two strands were wound around each other. Watson and Crick discovered that hydrogen bonds can form only between certain base pairs adenine and thymine, and guanine and cytosine. This principle is called base pairing. 19 of 37

20 DNA Double Helix The Components and Structure of DNA 20 of 37

21 12 1 Continue to: - or - Click to Launch: 21 of 37

22 12 1 Avery and other scientists discovered that a. DNA is found in a protein coat. b. DNA stores and transmits genetic information from one generation to the next. c. transformation does not affect bacteria. d. proteins transmit genetic information from one generation to the next. 22 of 37

23 12 1 The Hershey-Chase experiment was based on the fact that a. DNA has both sulfur and phosphorus in its structure. b. protein has both sulfur and phosphorus in its structure. c. both DNA and protein have no phosphorus or sulfur in their structure. d. DNA has only phosphorus, while protein has only sulfur in its structure. 23 of 37

24 12 1 DNA is a long molecule made of monomers called a. nucleotides. b. purines. c. pyrimidines. d. sugars. 24 of 37

25 12 1 Chargaff's rules state that the number of guanine nucleotides must equal the number of a. cytosine nucleotides. b. adenine nucleotides. c. thymine nucleotides. d. thymine plus adenine nucleotides. 25 of 37

26 12 1 In DNA, the following base pairs occur: a. A with C, and G with T. b. A with T, and C with G. c. A with G, and C with T. d. A with T, and C with T. 26 of 37